This chapter will deal with the aspects of physics that pertain to the structure of the atom, the nature of its component parts, and the predictable behavior of the several atomic components. It will also glance very briefly at the means man has developed, or is now developing, for the liberation of the energy available in the atomic nucleus.
Within the limits allowed by security, subsequent chapters will trace the military uses of nuclear energy.
12A2. Why study the atom?
Readers of this text may not remember, and may find it hard to imagine, what life was like before the wonders and perils of atomic power became a daily theme for cartoonists, headline writers, and microphone orators.
The average citizen is likely to think of the atomic age as dating from the destruction of Hiroshima. Actually the bomb that burst there was not a true beginning; it was simply a world-shaking announcement of an already accomplished fact. Behind the Hiroshima explosion lay a half-century and more of research, speculation, and calculation. Directly behind it, at a distance of almost three years, lay the event that might better be used to mark the dawn of the atomic era.
This earlier event made no headlines, for it was a strictly guarded military secret. It happened in the late autumn of 1942, almost a year to the day after the attack on Pearl Harbor. A group of physicists, chemists, and mathematicians, hand-picked for a special Government project, had for some time been building a mound, shaped somewhat like a huge doorknob, in a shielded area under the unused stadium belonging to the University of Chicago.
The bricks in the mound were carbon. Every second brick had a lump of uranium-or, when economy demanded, uranium oxide-imbedded in it. As the work progressed, detecting instruments were set up and movable cadmium strips and rods were incorporated in the
structure. Arrangements were made for observation and control from beyond the safety shields.
On 2 December a number of these scientists-acting under the general direction of the Italian-born nuclear physicist E. Fermi and under the group leadership of W. H. Zinn, H. L. Anderson, and V. C. Wilson-met to test the truth or falsity of the hypothesis that their research (and that of other specialists) had forced upon them. While designated men read the indicating instruments, others removed all but one of the cadmium strips, then pulled slowly on the remaining strip. With mingled feelings, including horror, they saw that their predictions had been correct.
Energy, in measurable and controllable amounts, was being generated in the mound. The source of that energy lay deep at the center of one special kind of uranium atom. The quantities of energy available were fantastically out of proportion to the size of the source. For better or for worse-or for better AND for worse-the atomic age had become a reality.
Since the end of World War II, as every reader of this text is undoubtedly aware, the military and industrial applications of atomic energy have become a major concern of the United States, its potential allies, and its potential enemies. Much military thinking has been revised, and much more is in the process of revision. Regardless of his specialty, no military man can afford to ignore the atom. It is hoped that these chapters will prepare prospective naval officers to make further studies and to follow new developments.
This chapter will take up, first of all, the nature of matter. It will start with a brief summary of the "conventional" ideas that were taught early in the twentieth century. Many of these ideas are still valid; they have been supplemented, rather than superseded, by the results of atomic research. The major emphasis, of course, will be on the atom and its component particles.
The second major division of the chapter will deal with radioactivity. This phenomenon gave scientists some of their most significant clues as to the nature of the atom. Because of their bearing on health and safety, the radioactive by-products of atomic fuel and atomic explosives are likely to be an increasing
concern of industrialists, community leaders, and military officers.
The latter part of the chapter will be concerned with nuclear reactions. These reactions are the real source of the power that is popularly called atomic energy.
B. Nature of Matter
12B1. Conventional ideas
About 1900 a chemist, if requested to explain the material world in non-technical language, would have spoken somewhat as follows, stressing certain facts which, as has already been implied, are STILL VALID and are STILL FUNDAMENTAL to an understanding of more recent discoveries.
ELEMENTS. All material is made up of one or more elements. These are substances that cannot be broken down into other and simpler substances by any chemical means. Ninety-two elements are found in nature, some of them in very small amounts.
Iron, mercury, and oxygen-existing at normal temperatures as a solid, a liquid, and a gas respectively-are typical elements. By heating, a solid element can be changed to a liquid and even to a gas. By cooling, a gaseous element can be changed to a liquid and even to a solid.
The smallest portion of any element that shares the general characteristics of that element is called an ATOM, which is Greek for INDIVISIBLE PARTICLE.
MIXTURES. Elements may be mixed without necessarily undergoing any chemical change. For example, if finely powdered iron and sulfur are stirred and shaken together, the result is a mixture. Even if it were possible to grind this mixture to atom-size particles, the iron atoms and the sulfur atoms would remain distinct from each other.
COMPOUNDS. Under certain conditions, however, two or more elements can be brought together in such a way that they unite chemically to form a compound. The resulting substance may differ widely from any of its component elements. For example, drinking water is formed by the chemical union of two gases, hydrogen and oxygen; table salt is
compounded from chlorine, a gas, and sodium, a metal.
Whenever a compound is produced, two or more atoms of the combining elements join chemically to form the MOLECULE that is typical of the compound. The molecule is the smallest unit that shares the distinguishing characteristics of a compound.
ATOMIC WEIGHT. Hydrogen is the lightest element. Experiments have demonstrated that the oxygen atom is almost exactly 16 times as heavy as the hydrogen atom. Chemists express this truth by saying that oxygen has an atomic weight of 16. Continued experimentation has determined the atomic weights of the remaining elements.
TABLE. Figure 12B1 is a standard table of the elements. The atomic weight of each element appears below its name. The vertical columns represent family groups. All members-from lightest to heaviest-of a family behave like one another in forming (or in refusing to form) chemical compounds with other families.
To meet current needs, the foregoing discussion of matter must be supplemented (but not replaced) by an analysis of the atom. In 1900, only a few advanced scientists had become convinced that the atom is not INDIVISIBLE after all. By 1920, the complexity of the atom had become common knowledge. The analysis of the several atomic components was still incomplete; but much of the theory of the 1920 period is still recognized as valid, and must be understood before later and more detailed studies can be undertaken.
12B2. Atomic structure, early interpretation
The upper half of figure 12B2 shows the hydrogen and the oxygen atom as they were
Figure 12B1.-Standard table of the elements.
pictured in the average science classroom during the 1920's.
NUCLEUS AND ELECTRONS. As shown in these sketches, the atom is not solid at all; in fact, it consists largely of empty space. At the center of each atom is a small amount of substance, extremely heavy for its size, called the NUCLEUS.
In the hydrogen atom a single very light particle, called an ELECTRON, travels in an orbit around the nucleus and spins meanwhile on an axis of its own. Very roughly speaking, this electron is to the nucleus as the earth is to the sun. (It is often called an ORBITAL or PLANETARY electron, to distinguish it from another kind that was later identified.)
The electron bears a charge of static electricity, of the type that physicists have arbitrarily called negative. A single positive
charge on the hydrogen nucleus balances the negative charge on this electron. Thus, in its normal or "unexcited" state, the hydrogen atom as a whole is electrically neutral.
Helium, the element next heavier than hydrogen, has two electrons that travel in a single orbit. Two positive charges on the helium nucleus counterbalance the negative effect of the two electrons.
Lithium, the next heavier element, has three electrons. Only two of these travel in the comparatively small area near the nucleus; the third has a much larger orbit. The lithium nucleus has three positive charges.
If time and space permitted, the atoms of all the elements could be examined one by one. For present purposes, however, the oxygen atom (fig. 12B2, upper right) will be an adequate example.
Figure 12B2.-Changing interpretation of the atom.
The oxygen atom has eight electrons, and therefore eight positive charges on its nucleus. The first two electrons are in the inner orbit, as is normal in any atom above hydrogen. Three outer orbits, all equidistant from the nucleus but each lying in a separate plane, contain the other three pairs of electrons, as shown in the drawing. We can think of the six outer electrons, all revolving and spinning, as tracing a hollow shell like an ultra-light tennis ball. Scientists customarily speak of electrons as being located IN SHELLS.
Under normal conditions, there is a limit to the number of electrons a given shell can contain. After this limit has been reached, the next heavier element in the table (fig. 12B1) starts a new shell with a much longer diameter. Uranium, the heaviest NATURAL element, has 92 electrons arranged in seven concentric shells.
CHEMICAL IMPLICATIONS. The outermost shell of any atom concerns the chemist in particular. Two atoms with completely filled outer shells won't unite to form a molecule; this is why some substances cannot be combined chemically. An atom with vacant spaces in its outer shell can fill those spaces by sharing certain electrons of other atoms; in this way molecules are built up.
In the oxygen atom six electrons are located in an outer shell that has room for eight. For this reason the oxygen atom can be made to share the electrons of two hydrogen atoms, thus creating a water molecule as shown in figure 12B3. (For the sake of simplicity, the figure shows all the electrons as though they were in one plane; this is not really correct.) As long as they remain chemically united, the combining atoms have, in effect, a single outer shell that gives the molecule its distinctive character.
One fact about the NUCLEUS is important to the chemist. The number of positive charges is a key to the chemical identity of any element. Hydrogen is hydrogen because its nucleus has one positive charge; uranium is uranium because its nucleus has 92 positive charges. (This fact will be more understandable after subsequent studies.)
ELECTRICAL IMPLICATIONS. Some atoms can, under one condition or another, be made to give up an outer electron rather easily.
Figure 12B3.-A molecule is formed by a union of outer shells.
This FREE or STRAY electron takes its negative charge with it as it moves away, seeking some atom with a vacant space in its outer shell. Electric current consists of free electrons in motion.
A molecule or atom that has temporarily lost an electron (and consequently bears an unbalanced positive charge) is called a POSITIVE ION. A molecule or atom that has temporarily gained a surplus electron is a NEGATIVE ION.
Ionization does not alter the nucleus, and therefore does not change one element to another. As soon as conditions permit, an ionized particle reverts to its balanced or electrically neutral state.
INFORMATION STILL LACKING. Like a 1900 lecture on matter, the foregoing 1920 interpretation of the atom is correct in the statements it makes. Its fault lies in a lack of complete understanding of the atomic nucleus. Research scientists acquired this understanding bit by bit, over many years. Even now, some details need further clarification. Between the two world wars, however, the results of many nuclear studies began to fit together like a jigsaw puzzle. By 1940 the men of science, but not the general public, had become aware of the far-reaching implications of nuclear physics.
12B3. Atomic structure, present interpretation
The lower part of figure 12B2 shows the hydrogen and oxygen atoms as they are now understood. The planetary electrons look much the same as in the upper half of the drawing. The hydrogen nucleus has gained nothing except a new name; now, if one desires, he may refer to this simple nucleus as a PROTON. The oxygen nucleus, however, has lost its early simplicity. It is now a tight cluster of small pellets. There are sixteen pellets, some of which are hidden by perspective.
PROTONS. Eight of the particles in the oxygen nucleus are positively charged. These are the protons. They are equal in number to the planetary electrons, but they are vastly heavier. It is really an atom's complement of protons that determines what element it represents and how it reacts in chemical experiments. In any normal atom, the protons and the planetary electrons are equal in number, just as they are in oxygen.
NEUTRONS. In addition to the eight protons, the oxygen nucleus contains eight uncharged particles, each equivalent (for all practical purposes) to a proton in size and weight. These heavy but uncharged nuclear particles are the neutrons; they are tremendously important in nuclear physics.
With the single exception of hydrogen in its simplest form, all atomic nuclei contain neutrons as well as protons. The lighter elements tend to have approximately equal quotas of the two kinds of particles; the heavier elements have more neutrons than protons.
The covering word NUCLEONS is often used when one wishes to refer to BOTH neutrons and protons; similarly, nuclear physics is sometimes called nucleonics.
SOME FACTS ABOUT WEIGHT. Because electrons are extremely light, their total effect on the weight of any atom is negligible. But neutrons and protons are both heavy. The atomic weight of an element, as measured in the chemical laboratory, reflects the total number of nucleons in the atom, not merely the number of protons. This explains the fact (puzzling to early chemists) that oxygen, with only eight times the number of positive charges, is almost sixteen times as heavy as hydrogen.
The subsequent article on isotopes will account for another puzzling phenomenon;
namely, the fractional components of the atomic weights shown on conventional tables like figure 12B1.
LOOKING AHEAD. Electrons, protons, and neutrons are not the only particles that occur in atoms. Nevertheless two important topics-isotopes and the nuclear physicist's symbols-can and will be introduced before the other atomic particles are listed.
As already mentioned, the atomic weights shown in figure 12B1 were determined by chemical processes. Therefore the present tendency is to call them CHEMICAL ATOMIC WEIGHTS. The chemical atomic weight usually differs by a small fraction from some whole number; in a few instances the fraction is large. The presence of the fraction defied explanation until the neutron was discovered and studied.
Nuclear studies revealed that atoms which have the same complement of protons (and therefore are chemically identical) may vary in their complement of neutrons. The variant forms of any given element are called its ISOTOPES.
Hydrogen has three isotopes. The most abundant hydrogen isotope has one proton and no neutron, as shown in figure 12B2. One isotope of hydrogen-called deuterium or, more popularly, heavy hydrogen-has one proton and one neutron. The third isotope, tritium, has one proton and two neutrons. See figure 12B4.
When isotopes occur in nature, the ratio of one isotope to the other(s) tends to remain constant in all samples of a given element. Because a laboratory sample contains all the natural isotopes, and because all the neutrons in the sample enter into the measurement of the chemical atomic weight, this weight has a fractional component.
Isotopes will be mentioned frequently in later parts of this chapter and volume.
12B5. Nuclear symbols
Before the discovery of the neutron, scientists identified any atom by a one-letter or two-letter symbol representing its chemical name-H for hydrogen, Cl for chlorine, Na for sodium (whose latinized technical name is natrium), and so on. The nuclear physicist accepts these time-honored letter symbols; when speaking in
Figure 12B4.-The three hydrogen isotopes.
general terms he refers to any of them as SYMBOL X.
To make precise reference to a given atom, the nuclear physicist (1) precedes symbol X with a numerical subscript called SYMBOL Z and (2) follows symbol X with a number (often, but not always, written as a superscript) called SYMBOL A. His identification of an atom, then, takes the form zXA or zXA or sometimes, when Z is clearly understood from the context, simply XA or XA.
SYMBOL Z. The subscript Z is called the ATOMIC NUMBER; it tells how many protons the nucleus contains (and simultaneously, of course, how many planetary electrons the atom has in its normal state). For hydrogen Z is 1; for oxygen it is 8; for uranium it is 92.
SYMBOL A. The final identifying symbol is an ATOMIC MASS NUMBER representing the nucleons; that is, the sum of the protons and the neutrons. For the most common hydrogen isotope A is 1; for deuterium it is 2; for tritium it is 3. For the most abundant isotope of uranium, A is 238.
To find the number of neutrons in any fully identified atom, subtract Z from A.
Note: The reader may sometimes see the term ATOMIC WEIGHT used for symbol A. It is preferable, however, to restrict this older term to its original sense of CHEMICAL ATOMIC WEIGHT and to use ATOMIC MASS NUMBER or simply MASS NUMBER for symbol A.
The accompanying table shows the interpretation of a few nuclear symbols.
Interpreting Nuclear Symbols
Total Nuc- leons (A)
Common hydrogen 1H1
Deuterium 1H2 or 1D2
Tritium 1H3 or 1T3
12B6. Other nuclear particles
Observations of nuclear radiation (the phenomenon taken up in section C of this chapter) and other nuclear reactions have shown that an atomic nucleus does not always consist solely of protons and neutrons.
In the first place, the neutron itself is not indivisible. If set free from the nucleus, it breaks down or DECAYS sooner or later into a proton, an electron, and an uncharged particle called a NEUTRINO. (The neutrino need not concern the student further in this course.)
The electron liberated by neutron decay is called a BETA PARTICLE to distinguish it from a planetary electron. The equal and opposite charges on its component proton and beta
particle give the neutron as a whole its electrically neutral character.
Under some conditions, an excited nucleus emits positively charged beta particles. These are called POSITRONS or POSITIVE ELECTRONS.
A large particle consisting of two protons and two neutrons is emitted in some reactions. This unit, which is essentially a free helium nucleus, is called an ALPHA PARTICLE. If and when an alpha particle acquires two electrons, it becomes a helium atom.
A DEUTERON is an emitted particle consisting of a proton and a neutron. It is, therefore, the same as the heavy hydrogen nucleus.
A TRITON consists of a proton and two neutrons. It is the same as the nucleus of tritium, the third hydrogen isotope.
Another term defined for purposes of orientation (but which should not further concern the student at this time) is the MESON, or as it is sometimes called, the mesotron. The meson is a short-lived particle that may sometimes be found with either a positive, negative, or zero charge. The mass of the meson is also variable, sometimes being 200 or 300 times the mass of an electron.
Additionally, there is the GAMMA RAY, which while being a form of electromagnetic radiation, sometimes behaves as a nuclear particle. The gamma ray originates from the atomic nucleus, carries no charge, and is highly penetrating. The X-ray can be considered very similar to the gamma ray except that the wavelength of the former is longer. Gamma radiation is the most significant of the radiological hazards of nuclear explosions.
Later sections and chapters of this text will assume that the reader is familiar with the names and definitions of these several nuclear components. Because exploration of the nucleus is still going on, a time may come when the present definitions will have to be revised and new definitions added.
12B7. Expanded table
The discovery of a great multiplicity of isotopes (some natural and some man-made) soon overtaxed the capacity of the conventional table shown in figure 12B1. The student of atomic phenomena now refers to a much more comprehensive table in which all known NUCLIDES (separate species of atomic nuclei) are arranged in an orderly number by ascending
values of mass number and neutron complement.
Figure 12B5 shows accurately (but necessarily to very small scale) the shape of this table as a whole. Small excerpts from the table, enlarged to show the notations concerning individual nuclides, appear in figures 12B5, 12B6, and 12B7. Sections C and D of this chapter will refer to these excerpts.
In this comprehensive table, each block represents a nuclide. The value of Z (that is, the number of protons and therefore the chemical nature of the substance) is determined by the vertical distance from the base line to the given block. This arrangement places isotopes in horizontal rows.
The value of A-Z (that is, the number of neutrons) is determined by the horizontal distance of the block to the right of the extreme left line. The atomic mass number (symbol A) is printed after the chemical letter symbol at the top of the block.
12B8. Chemical versus nuclear reactions
This topic will receive only brief notice at this time. The underlying theory will receive some attention in section D of this chapter. The practical importance will become evident in later chapters.
CHEMICAL. When two atoms unite chemically to form a molecule-or even when several types of molecules break apart and their atoms recombine in new molecules-the planetary electrons in the outer shells are the only atomic particles involved in the transformation. The several nuclei remain as they were before.
Chemical reactions are frequently marked by the release of usable amounts of kinetic energy, as when gasoline burns or TNT detonates. The quantity of energy thus produced is small, in proportion to the number of molecules involved in the reaction.
NUCLEAR. When an atomic nucleus gains or loses one or more PROTONS, the parent element is changed or TRANSMUTED to another element. (This is the feat the medieval alchemists tried in vain to accomplish.) When a nucleus gains or loses a neutron, it becomes a different isotope.
These nuclear reactions are always accompanied by a release of kinetic energy. The ratio of energy release to the number of atoms
Figure 12B6.-Excerpt from the medium-weight area of the table.
involved is almost incomparably greater than in a chemical reaction. Hiroshima demonstrated the military importance of the atom
as a source of energy. Mankind has just begun to utilize the non-military possibilities.
Figure 12B7.-Excerpt from the heavy-element area of the table.
Among the several types of research that led to our present understanding of the atom, few were more significant than the patient, physically hazardous pioneer studies of radioactivity.
By radioactivity we mean the spontaneous emission of particles and photons by certain unstable atomic nuclei.
NATURAL RADIOACTIVITY. Of the 300-odd nuclides found in nature, about 40 are naturally radioactive. With very few exceptions, these naturally radioactive substances are isotopes of the heavy elements at the upper right end of the table of the nuclides. Figure 12B7 is an excerpt from the area where natural radioactivity is comparatively common. It is not surprising that the complex nuclei of the heaviest elements should be unstable; that is, should tend to disintegrate or DECAY.
The early studies of natural radioactivity were conducted on radium, from which the phenomenon receives its name. Radium occurs, in extremely small amounts, in uranium deposits. We know now WHY radium is found associated with uranium; it is formed at one step of a long process by which the unstable isotopes of uranium disintegrate until a stable end product, lead, is reached.
There are three families of naturally radioactive elements-the uranium, thorium, and actinium groups. Each family has a separate isotope of lead as its final product. Natural lead is a mixture of these three isotopes, plus a fourth one.
INDUCED RADIOACTIVITY. Very few of the elements lighter than polonium (Z = 84) have naturally radioactive isotopes. By nuclear reactions, however, man has produced a large number of unstable isotopes not found in nature. The great bulk of these lie in the medium-weight area of the table of the nuclides. Figure 12B6 is a representative excerpt showing artificially radioactive nuclides.
Artificially produced radioactive isotopes have become very useful to medicine and industry. It is probable that still wider uses will be found for some of them.
12C2. Radioactive series decay
As has been mentioned, the three families of naturally radioactive elements disintegrate or decay in a series of steps, until a stable end product is formed. At any step, either an alpha or a beta particle is emitted from each reacting nucleus.
Artificially produced radioactive substances decay in a similar (but frequently much shorter) series of steps until some stable end product is formed. The next paragraphs will describe the types of radiation.
ALPHA RADIATION. As previously noted, alpha particles are essentially helium nuclei. When emitted, these particles travel fast (2,000 to 20,000 miles per second). Because they are large, as compared with other emitted particles, and because they contain protons that are capable of attracting stray electrons, alpha particles are unlikely to penetrate far into barrier substances.
Even if the barrier substance normally contains very few stray electrons, the alpha particles tend to PRODUCE some by colliding with atoms and knocking off planetary electrons. By capturing two free electrons, the alpha particle becomes stabilized as a helium atom. Meanwhile, of course, the formerly normal atoms that have lost electrons remain ionized until they can replace the loss.
BETA RADIATION. The emitted beta particle is a fast-moving free electron. It is more penetrating than the much larger alpha particle. When finally captured as a surplus free electron, the beta particle makes a negative ion of the atom (or molecule) it has joined.
GAMMA RADIATION. The loss of an alpha or a beta particle leaves a radioactive nucleus with an excess quantity (quantum) of energy, which it emits almost immediately as a gamma ray. This extremely high-frequency radiation travels at the speed of light. Like X rays, which they closely resemble, gamma rays are invisible, are capable of darkening photographic plates, and have great penetrating power.
When a gamma ray passes through a normal atom, it is likely to cause a planetary electron to be expelled. This, of course, converts the atom to a position ion. The ray-produced ion and the electron it has lost are called an ION
PAIR. This term will recur in article 12C5, on RADIATION UNITS.
Not all gamma rays cause ionization immediately. In the following reactions, studied under laboratory conditions, the original gamma rays merely start processes that are capable, later, of producing ions. (1) High-velocity gamma rays have been used to bombard certain atoms and thus produce nuclear fission, a topic discussed in the final section of this chapter. (2) Sometimes a gamma ray, in passing close to an atomic nucleus, undergoes conversion to two free electrons-one negatively and one positively charged. (This is an instance of the conversion of energy to mass. See section D of this chapter.) The positive electron (positron) soon collides with a normal electron, with the result that their two masses are converted to energy in the form of two opposite-direction gamma rays.
Another possibility is that a gamma ray may not cause any emission, but may simply give some planetary electron an abnormal amount of energy. As a result, the atom containing this electron becomes "excited" and may emit short-lived flashes of visible light.
The gamma ray, then, is more versatile and potentially more dangerous than the alpha or the beta particle.
PRACTICAL SIGNIFICANCE. Though the topic will be taken up again in greater detail, it is well to note at once that nuclear radiation has great practical significance. Any ionizing process has an effect on the various types of complex molecules in the human (or other animal) body. Depending on a variety of circumstances, the effect can be slight, fairly serious, extremely serious, or fatal.
Before the beginning of the atomic age, radiation hazards and precautions were the concern of medical officers and hospital corpsmen trained in X-ray techniques. Now that nuclear power and nuclear weapons are increasingly common, radiation has become the concern of all officers and men. The remainder of this section will discuss, very briefly, some of the topics that any junior officer should understand in preparation for later and more intensive studies of radiation problems.
12C3. Half life
WHAT IT IS. A student of radioactivity must understand clearly what is meant by HALF
LIFE. This term is always included in the complete description of a radioactive nuclide.
The half life is the time required for half the atoms in any given mass of the substance to enter into the decay process. The half life is a CONSTANT for the given substance, regardless of the size of the sample.
Assume that the block shown at the extreme left in figure 12C1 is composed entirely of the radioactive protoactinium isotope known as Pa231 The half life of this particular isotope is 34,000 years. At the end of 34,000 years, therefore, half of the original protoactinium atoms will have undergone transmutation to other elements. The rest will still be protoactinium. If they were separated from the transmuted atoms, the protoactinium atoms would now constitute the second block in figure 12C1.
The second block will not decay completely in the second 34,000-year period. Unquestionably, it has only half as many atoms as the original block; but by that very fact it offers only half the original opportunity for atomic reactions to take place. As before, half of the atoms (a quarter of the original protoactinium atoms)will remain unchanged at the end of the half life.
During the third half life, the number of protoactinium atoms will again be cut in half-and so on. In general terms, then, x pounds of any radioactive isotope will decay in accordance with the following series:
No matter how far this series is extended, its final term, x/n will represent only half of the protoactinium atoms that were intact at the beginning of the given half life. If x/n atoms have been lost this time through decay, an equal number still remain to start the next half life.
In other words, the sum of this series APPROACHES x, the original number of atoms, but (in theory at least) never quite reaches it. There is always a fraction, equal to x/n, representing the atoms that are still intact.
PRACTICAL APPLICATIONS. Half life has important bearings on safety. It is obvious, first of all, that any isotope with a long half life is potentially dangerous if it is produced in large amounts. When such an isotope is
Figure 12C1.-The principle of half life.
once formed-whether by nature or in an atomic power plant or during a nuclear explosion-it will contaminate its surroundings for a long time to come.
For some of the artificially produced radioactive isotopes, the half life is measured in hours, minutes, or even seconds. These isotopes are extremely active; that is why they decay so fast. They are, therefore, highly dangerous for a short time after they are formed, but the danger diminishes rapidly after the first few half lives have been completed. After a few days (or, in some instances weeks) too few of these radioactive atoms are left to do much harm.
Another, and very different, application of the half-life principle is in determining the approximate age of the earth. This has been done by measuring the proportion of stable lead in uranium deposits and computing the number of centuries of uranium decay required to form this lead. The earth's age, as determined by this method, is 2.5 billion years. The figure agrees rather closely with those arrived at by other recent calculations.
12C4. Half thickness
As has already been mentioned, radioactive substances emit particles and rays that produce ionizing reactions in previously normal atoms or molecules. Under competent control and with proper safeguards, radiation can be harmless, as when a hospital corpsman takes a chest X ray. It can even be a power for good, as when a surgeon trained in radiology destroys cancerous cells without damaging
much of the patient's normal tissue Out of control and without safeguards, radiation can be one of the great hazards of our time.
Article 12C2 explained that alpha particles have low penetrative power. Ordinary clothing, or even unbroken skin, will prevent them from entering the body from the outside. The only way one can suffer much ionization damage from alpha particles is by eating, breathing, or otherwise taking into the system, some radioactive isotopes that will become lodged in the body and remain there through a series of half lives.
Beta particles traveling in air are effective within an approximate 10-foot range of their source. Denser substances, like wood or even water, limit their effective range to about a thousandth of its value in air. Moderate clothing gives substantial (though not complete) protection against beta radiation.
It is, however, the gamma rays produced by all radioactive decay, and the free neutrons that characterize man-made nuclear reactions, that are the grave threats to health and life. Nuclear power plants must be shielded to protect the operators and other personnel from these radiation hazards. The waste products of these power plants must be disposed of with great care. Civilian populations must be instructed in the hazards of radiological warfare and the means of coping with these hazards.
The term HALF THICKNESS (or half value layer thickness) is commonly used in specifications for materials and structures intended to be used primarily as shields against radiation. The half thickness of any substance is the thickness necessary to reduce the intensity
of the given radiation to half. As figure 12C2 shows, the half thickness varies from one shielding substance to another. It also varies with the type of radiation (neutron or gamma) that is under consideration.
12C5. Radiation units
The student of radiation phenomena soon becomes obliged to think in terms of the standard radiation units described in this article.
CURIE. First of all, the student is likely
to notice that substances vary widely in the
degree of radioactivity they exhibit. If isotope
x is more radioactive than isotope y, more x atoms than y atoms will decay in a given time interval. The unit called the curie establishes the activity (that is, the decay rate) of radium as the standard with which the activity of any other substance may be compared.
By using a formula that takes into account the number of atoms per gram and the value of the half life in seconds, scientists have determined that the activity of radium is equal to 3.7 x 1010 nuclear disintegrations per gram per second. This value becomes the unit of comparison.
Figure 12C2.-Typical examples of relative half thickness.
A CURIE of ANY radioactive isotope, therefore, is the amount of that isotope that will produce 3.7 x 1010 nuclear disintegrations per second.
In the manufacture of radioactive isotopes for medical and industrial purposes, the curie is often too large a unit for convenient use. Frequently, therefore, the MILLICURIE and even the MICROCURIE are used instead.
At the opposite extreme, the curie is too small a unit for convenient measurement of the high-order activity produced by a nuclear explosion. For this purpose the megacurie (1,000,000 curies) is used.
ROENTGEN. The roentgen was established for the benefit of the medical profession. This unit measures the amount of X-ray dosage given to a patient. Because gamma rays behave like X rays, the roentgen is usable in measuring gamma-ray dosage.
A ROENTGEN is the amount of X or gamma radiation which, in passing through a cubic centimeter of standard air, produces an electrostatic unit (equivalent to 2.083 X 109 of the ion pairs defined in article 12C2.) The absorption of one roentgen by one gram of air results in the release of about 87 ergs (small energy units used in delicate problems in mechanics). This fact will be mentioned again.
In evaluating the effect of radiation on a clinical patient or on a victim of some type of accidental exposure, the radiological expert must know (or at least estimate) the answers to questions like the following: Was the exposure to radiation brief or extended, single or repeated? How many roentgens (or fractions thereof) were received at each exposure? Was the radiation received over the whole body or only on a limited area?
It is impossible, in this brief article, to make a detailed study of the effects of the many possible amounts and combinations of X or gamma radiation dosage. The reader will, however, have a clearer idea of the roentgen if he remembers the following key facts:
1. Twenty-five roentgens or less, received in a single dose and not repeated, have no effect that can be detected by clinical processes.
2. Single doses larger than 25 roentgens (or small, frequent doses amounting to a total of more than 25 roentgens) have increasingly more serious consequences.
3. If the entire body is exposed to 450 roentgens, the chances are 50 to 50 that the victim will die within a month.
4. If the entire body receives 700 or more roentgens, the victim will be almost certain to die.
Some of the doses given in radiation therapy are very small; they are measured in MILLI-ROENTGENS (1/1,000 roentgen). Other therapeutic doses are very large, but they are directed at a small portion of the body, and therefore destroy few cells except the malignant ones at which they are beamed.
REM. The roentgen, as a unit of radiation dosage, is defined with respect to X and gamma radiation, and should be applied only to those two types. We have seen, however, that alpha and beta particles also initiate ionizing reactions. The various other nuclear particles produced in the laboratory, the reactor, or the exploding nuclear weapon are likewise capable of causing ionization.
The rem is the correct term to use in measuring dosages of these various other ionizing particles. A REM (roentgen equivalent man or mammal) is the quantity of ionizing radiation of ANY type which, when absorbed by man or some other mammal, produces a physiological effect equivalent to that produced by the absorption of one roentgen of X or gamma radiation.
REP. The roentgen measures the strength of the radiation field-the dose to which the patient or victim is exposed. For one reason or another (partial shielding, or variations in penetrative power, for example) the full value of an exposure dose may not always be received by a living organism. Therefore the rep (roentgen equivalent physical) was established to measure the strength of the radiation absorbed within the body.
The rep was originally defined as the absorbed dose of ANY nuclear radiation (gamma rays, beta particles, neutrons, and so on) that would result in the absorption, within animal tissue, of 87 ergs per gram. This figure, as will be recalled, represents the amount of energy released by one roentgen in a gram of air.
Continued experimentation showed, however, that exposure to a dose of one roentgen actually caused animal tissues to absorb more than 87 ergs of energy per gram. After some fluctuation, the value of the rep was defined,
as of 1957, as an absorbed dose of 93 ergs per gram of tissue.
RAD. Because the value of the rep was keyed to that of the roentgen and varied as more exact data became available, a need was felt for some INVARIABLE unit of absorbed dosage that would be roughly equivalent to the rep. Accordingly, in 1953, the International Commission on Radiological Units adopted the rad as the desired unit.
A RAD is the absorbed dose of any nuclear radiation that results in the liberation of 100 ergs of energy per gram of absorbing material.
PRACTICAL COMPARISONS. For gamma radiation, which is generally the major threat to health and life, the relationships between the dosage units are simple and easy to remember. For gamma radiation, the exposure dose in rems is approximately equal to the absorbed dose in reps or rads, and for soft tissue this, in turn, is roughly equal to the exposure dose in roentgens. In other words, one can receive 20 rems, reps, or rads without ill effect; he has little chance of recovery if he receives 700 of any of these units.
As the accompanying table shows, however, the rem and the rep are not numerically equivalent for ALL types of nuclear radiation. The student may find this table a convenient means of reviewing, and fixing in his mind, the units he is most likely to meet in further studies of radiation dosage.
12C6. Utilizing ionization phenomena
Much has been said, thus far, about the ill effects of the ionizations produced by nuclear emissions. These same ionizations, however, have been made to serve useful purposes. They have, for example, enabled laboratory scientists to follow and photograph the paths taken by emitted particles. They have also enabled
design engineers to construct a number of radiation detectors for safeguarding personnel and evaluating damage. Only a few examples will be mentioned.
CLOUD CHAMBER. The cloud chamber is one means of showing the traces left by radiation products. (The products themselves are too small and too fast-moving to be shown directly.)
The cloud chamber is essentially a hydraulic cylinder. Before operation, a piston head compresses a volume of gas, saturated with vapor, at one end of the cylinder. When ready to trace emitted particles, the operator quickly moves the cylinder head to enlarge the gas-filled area. He also sends light through the
gas. As the gas expands, it cools; the cooling of the intermixed vapor causes visible droplets to collect on any ionized gas particles.
DETECTORS. Most of the radiation detectors in current use are really ion detectors. Many detectors, including the Geiger counter, are characterized by a gas-filled cylinder in which the entering gamma rays (the most penetrating of the three radiation products) ionize the gas molecules. An electrical device in the detector measures and indicates the degree of ionization.
Certain small film-type detectors, however, do not have ionization chambers. Instead, they show directly the effects of beta particles and gamma rays on photographic film.
D. Nuclear Reactions
12D1. Mass-energy relationship
CONVENTIONAL IDEA. Conventional physics has a law of the CONSERVATION OF MATTER. According to this law, the total mass of the material universe remains always the same, regardless of all the rearrangements its component particles undergo. Likewise, conventional physics has a law of the CONSERVATION OF ENERGY. This law states that one form of energy can be converted to another form, but the total amount of energy in the universe neither increases nor decreases.
Mass and energy, to the physicist of the old school, were separate entities, as indeed they appear to be.
EINSTEIN'S IDEA. In the early 1900's Einstein made (and supported mathematically) a revolutionary statement. He said that there is an exact equivalence between mass and energy. Mass can be converted to energy and, conversely and even more strangely, energy can sometimes be converted to mass. (Brief mention of these two types of conversion was made in the paragraphs on gamma radiation in article 12C2.)
According to Einstein, it is not mass or energy as a separate entity, but rather the total mass-energy of the universe, that remains constant. The relationship between mass and energy is expressed in the equation:
E = MC2,
E = energy, in ergs, generated in any reaction;
M = mass, in grams, lost in any reaction;
C = speed of light (3 x 1010 centimeters per second, equivalent to 186,000 miles per second).
CHEMICAL IMPLICATIONS. Even the ordinary chemical processes, Einstein declared, involve the conversion of mass to energy. In order for a chemical reaction to take place, enough energy must be made available, through a conversion of mass, to break the binding forces that have been holding the molecules together.
We have seen that only the electrons in the outermost shells are involved in molecule formation. A comparatively small amount of energy, therefore, is sufficient to free the atoms from one molecular formation and recombine them in another. The mass loss (M) in such a reaction is this small amount of energy (E) divided by the square of the speed of light.
This loss of mass is too minute to be measurable by any laboratory instrument yet devised. For example, the energy produced by the burning of a pound of coal represents a mass loss of 1/3 billionth of a pound. The rest of the coal particles continue to exist as measurable combustion products.
NUCLEAR IMPLICATIONS. As the next article will explain, the forces binding the
atomic nucleus together are vastly stronger than the forces binding atoms into a molecule. To break these forces, a comparatively large amount of matter must be converted to energy.
Not even the nuclear physicist has succeeded in converting to energy more than a small fraction of any mass large enough to be seen by the unaided eye. In the fissioning of uranium, as in the Hiroshima bomb, about a thousandth part of the total mass of radioactive substance is changed to energy; the remainder is transmuted to fission products.
Nevertheless, fission is a much more efficient source of kinetic energy than combustion. Figure 12D1 makes some comparisons that speak for themselves.
12D2. Binding energy; mass defect
The protons in an atomic nucleus are all positive; and one positive charge, as every student of basic electricity knows, repels another. The neutrons, as long as they remain within the nucleus, are electrically neutral. Why, then, does the nucleus hold together?
For one thing, the neutrons serve as buffers to reduce the effectiveness of the repulsive forces between protons. For another thing, some natural force, comparable to gravitation but many times stronger, binds the nucleus into a unit. This super-gravitational force or BINDING ENERGY is thought to be similar, in its effect, to the surface tension that binds a large number of water molecules into a raindrop.
The entrance of a new particle into a nucleus, or the exit of a particle from a nucleus, must be accompanied by enough energy to break the binding force; in short, the travelling nuclear particle must act like a small bullet. The energy to break the binding force involves a conversion of mass. The loss of mass by conversion is called the MASS DEFECT.
In nuclear reactions the mass defect, though small, is large enough to be measured by laboratory procedures that are beyond the scope of this chapter. These measurements have shown the truth of a seemingly fantastic statement: with the single exception of 1H1, any atomic nucleus weighs less than the sum of the weights of the nucleons composing it; these nucleons lost a small fraction of their mass when they combined.
The binding energy of the nucleus varies from one element to another. The elements in the middle-weight portion of the table of nuclides (fig. 12B5 and 12B6) are most strongly bound together and therefore are least promising as sources of usable energy.
The simple nuclei at the hydrogen end of the table have the lowest binding power. The complex nuclei at the uranium end of the table tend, by virtue of their very complexity, to lack stability; that is why some of them are naturally radioactive. It is the elements at these two extremes of the table that are the most likely sources of usable energy.
Nuclear energy can be released by combining light nuclei to form heavier and more strongly bound ones. This is the process called FUSION.
Figure 12D1.-Nuclear energy as compared with chemical energy.
Energy can also be released by splitting heavy nuclei into intermediate (and, again, more strongly bound) ones. This is the process called FISSION. These are the two reactions with which nuclear power engineering is concerned.
12D3. Nuclear fission
PIONEER STAGES. During the late 1930's scientists were conducting several types of "atom-smashing" experiments. One of these experiments involved the use of the neutrons from the deuterium (heavy hydrogen) atom as high-velocity bullets to bombard small quantities of uranium. This experiment produced a result that even the specialists were reluctant to believe until all other possible explanations had been tried and discounted. Some of the uranium atoms had been split into almost equal parts, to form new atoms of barium and krypton.
The physicists could explain this phenomenon in only one way. The heavy uranium nucleus taxes its binding energy almost to the breaking point, much as an oversize dewdrop taxes its surface tension. If all conditions are favorable, a slight, sudden stab against the dewdrop, or the impact of a single neutron against the uranium nucleus, suffices to split either into two nearly equal parts.
The startling experiment was repeated a number of times, and the energy liberated by the reaction was carefully measured. The energy per atom proved to be about 5,000,000 times that of burning coal. Here, then, was a discovery that might have tremendous practical importance, provided the fission reaction could be sustained and controlled.
FISSIONABLE MATERIALS. An intensive search for fissionable materials revealed three substances with practical possibilities as nuclear "fuel." They are as follows:
U235, a uranium isotope constituting 0.7% of natural uranium,
Pu239, an artificial isotope of an element plutonium that is itself (for all practical purposes) man-made,
U233, an artificial isotope of uranium, derived in a reaction involving thorium.
NEUTRON PRODUCTION. Free neutrons are the major tools of the nuclear physicist. They have the proper size and weight to invade
the atomic nucleus, and their electrically neutral character keeps them from being repelled by the protons. For large-scale nuclear fission operations, man needs an abundant supply of neutrons.
For laboratory purposes, the physicist can bombard the atoms of a light element (boron or beryllium, for example) with alpha particles or gamma rays from certain radioactive isotopes, or with charged particles from a cyclotron or other accelerator. All these methods result in neutron emission.
In the practical production of radioactive materials, he secures free neutrons by the NUCLEAR REACTIONS THEMSELVES. Studies of radioactive series decay have shown that some fissions result in the freeing of at least one neutron. Under proper control, the free neutrons can be put to work.
CONTROLLING THE NEUTRON. In any nuclear reaction except a planned explosion, both the production rate and the speed rate of free neutrons must be kept under control. This is an essential safety precaution.
When emitted from their atoms, some neutrons travel fast-at about 1/20 the speed of light. For some purposes, such as the splitting of a heavy but firmly bound nuclide like U238, the high kinetic energy of FAST neutrons is required. In other nuclear processes-including the fissioning of unstable heavy nuclides such as U233, U235, and Pu239-a much lower neutron speed is desirable; fast neutrons would simply pass through these nuclides, exciting them but failing to produce fission.
By control methods that will be mentioned shortly, it is possible to decrease the speed of neutrons to about 1/10,000 of the maximum possible value. At this low speed, the kinetic energy of the neutrons is about equal to that of a gas under standard conditions. For this reason they are called THERMAL neutrons.
The term SLOW neutrons includes thermal neutrons, but is much less restricted in meaning.
To slow down fast neutrons, the designers of nuclear reactors use substances or devices called MODERATORS. Good moderating materials are elements from the low end of the table of the nuclides, and compounds formed from these elements. Moderating substances include (but are not limited to) hydrogen, carbon, beryllium, ordinary water, heavy
water (in whose molecule deuterium replaces common hydrogen), and paraffin.
When a free neutron enters a moderator, it collides with (but does not penetrate) one nucleus after another, losing energy with each collision. Eventually it leaves the moderator at a greatly reduced velocity.
Moderating materials can be designed to serve as REFLECTORS. These are layers or structures that turn stray neutrons back toward the parts of the reactor where they will serve a useful purpose.
Substances that allow free neutrons to enter but tend to hold them captive are called ABSORBERS. These substances are used in the safety shields and control rods that are required in all designs for nuclear reactors. If unavoidably present where they are not desired, absorbing substances reduce efficiency.
NEUTRON REACTIONS. Not all emitted neutrons behave alike. Frequently they cause non-fissioning reactions, typical examples of which are sketched in figure 12D2. The moderators and absorbers recently described are deliberately used to produce non-fissioning reactions. It is possible, however, for such reactions to occur even within fissionable substances.
In ELASTIC SCATTER (also called ELASTIC COLLISION) a neutron or other particle touches or nearly touches the target nucleus, then bounces away. No nuclear energy is released, though the colliding particle may transfer some of its kinetic energy to the nucleus.
In INELASTIC SCATTER, part of the energy of the collision EXCITES the target nucleus and causes it to give off gamma radiation. The bombarding particle may
merely touch the target nucleus, or it may actually pass through it as shown in the central part of figure 12D2.
The reaction called PARTICLE EJECTION ON BOMBARDMENT resembles inelastic scatter, with the difference that one particle enters the nucleus and a different particle leaves it. Gamma radiation accompanies this reaction.
In CAPTURE a neutron (or, rarely, some other particle) enters the target nucleus and stays there. This reaction, once again, excites the target nucleus and produces gamma radiation. By the capture of a neutron, the nucleus changes from one isotope to another.
When the bombarding particle splits the target nucleus into two smaller nuclei, as shown in figure 12D3, the reaction is, of course, FISSION. Though omitted from this drawing for the sake of simplicity, the planetary electrons of the fissioning nucleus are divided between the product nuclei when the new atoms are formed. The result, then, is the production of two lighter and often more stable atoms.
Since fissionable substances have a high ratio of neutrons to protons, their transmutation to medium-weight substances is usually accompanied by the liberation of at least one
Figure 12D2.-Some non-fissioning reactions.
Figure 12D3.-A representative fission reaction.
spare neutron. This neutron is welcomed by the physicist, for it becomes a tool for possible use in producing the NEXT fission. It is the emission of free neutrons that makes possible a self-sustaining chain reaction.
CHAIN REACTIONS. The two neutrons liberated by fissioning in figure 12D3 may behave in any of the various ways that have just been summarized. If conditions are especially favorable to fissioning, they may both produce fissions. Under slightly different conditions, one may cause fissioning and one may be captured. As long as any fissioning reaction can be traced back, step by step, to the original fission, the process is a CHAIN REACTION.
The term CHAIN REACTION is not the exclusive property of the nuclear physicist. It may be used to describe ANY chemical or physical process in which the products of one stage (sometimes called a GENERATION) act to produce the next stage.
Chain reactions fall into three classes: nonsustaining, sustaining, and multiplying.
A NONSUSTAINING (or CONVERGENT) chain reaction comes to a dead stop sooner or later. In this reaction, too few products of the various stages are effective in producing new stages. The process, therefore, cannot continue very long.
The nonsustaining chain reaction shown in the upper part of figure 12D4 starts with a first generation of three emitted neutrons. (For the sake of clarity in this drawing, the free neutrons are the ONLY fission particles shown; the presence of all the other fission products is to be taken for granted.) One
Figure 12D4.-The three types of chain reactions.
neutron is lost in the first generation; the other two liberate three new neutrons to form second generation. One second-generation neutron is lost; the other two liberate two more to act as a third generation. By the end of the fifth generation, no neutrons remain to star new fissions. Therefore the reaction ends.
In a SUSTAINING (or STATIONARY) reaction the gains by new fissions exactly balance the various types of losses. Consequently, as shown in the central part of figure 12D4, the reaction continues at a constant strength.
The lower part of figure 12D4 shows a MULTIPLYING (or DIVERGENT) chain reaction. In each generation, the reaction products (in this instance, free neutrons) that are gained exceed those that are lost. If conditions were especially favorable, the ratio of gains to losses would be still higher.
The nature of any nuclear chain reaction depends, in part, on the purity of the fissionable material used. Impurities cause more neutrons to be lost through scatter or capture.
The nature of the reaction also depends, in part, on the mass and shape of the fissionable material. Even for highly refined fissionable substances, there are limits below which there are too few atoms to support a chain reaction. This brings us to the problem of CRITICALITY.
A mass (in a given shape) that is just great enough to support a sustaining chain reaction is called a CRITICAL mass. A SUBCRITICAL (smaller than critical) mass will support only a nonsustaining chain reaction. A SUPERCRITICAL mass will support a multiplying chain reaction.
FISSION BOMB POSSIBILITIES. As a very necessary safety precaution, the masses of fissionable material present in a nuclear bomb must be kept subcritical until time for the bomb to be detonated. Then a supercritical mass must be formed very rapidly. One way of producing a supercritical mass is by forcing two or more subcritical masses together. Another way is to squeeze a subcritical mass tightly into a new shape and/or a greater density that becomes supercritical without the addition of any more substance.
In a weapon, an efficient, rapidly multiplying chain reaction is essential. Ideally, no free neutron should be lost to the process. If the first fission produced two free neutrons, each of these neutrons should produce two more fissions, each of which fissions should liberate two neutrons, and so on. The fissions would then increase by geometric progression (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, and so on).
The eighty-first step of this process results in 2.5 x 1024 fissions-enough to transmute a kilogram of refined uranium. The time required
for the 81 steps is 1/108 second. These are the types of numbers that had to be considered in the design of the Hiroshima bomb.
UTILIZING SUSTAINING REACTIONS. When "atomic fuel" is used to produce power-as in some ships now in commission, other ships under construction, and certain experimental electric plants ashore-a sustaining type of chain reaction is required. Neutron production must not be allowed to get out of control; neither must it be allowed to die out.
The designers of nuclear reactors must face and solve many problems related to the production of an efficient, controllable chain reaction. These problems are beyond the scope of this chapter, but are discussed in texts on nuclear power engineering.
Other problems center about the safety factors-both for the equipment itself and for the people who operate it or live near it. Even the disposal of waste products must be carefully planned to avoid present and future dangers.
12D4. Nuclear fusion
As briefly mentioned before, fusion is the merging of two light nuclei to form a heavier one, with an accompanying conversion of mass to energy. For reasons that will be mentioned soon, fusion reactions are often called THERMONUCLEAR reactions.
SUITABLE SUBSTANCES. In order to fuse, two nuclei must come very close together with enough kinetic energy to break the binding force of one of the nuclei. Because protons repel one another, thus tending to keep the nuclei containing them apart, the single-proton hydrogen nuclei would seem to be the most promising materials for the fusion reaction.
Because they have no neutrons, two atoms of ordinary hydrogen cannot fuse to form a heavier element. Deuterium or heavy hydrogen, with one proton and one neutron per atom, is more promising. Experiments have shown that two atoms of deuterium can fuse, producing an atom of tritium (radioactive hydrogen) plus an atom of ordinary hydrogen. Alternatively, two atoms of deuterium can react to produce the helium isotope 2He3 plus a free neutron. Either reaction liberates nuclear energy.
Any tritium produced by fusion can react with deuterium to produce the helium isotope
2He4 plus a free neutron. This reaction again releases nuclear energy.
Deuterium, then, is an effective source of energy, provided it can be made to fuse at all. To achieve fusion, a very high value of kinetic energy must be expended in forcing any two deuterium nuclei to unite. In laboratory experiments, artificial acceleration of nuclear particles has produced enough energy to initiate small-scale fusion reactions. Mechanical acceleration of particles, however, is not feasible in nuclear weapons, nor is it adaptable to industrial energy production.
Thus far, heat has proved to be the only form of kinetic energy capable of initiating a fusion reaction large enough to have practical applications. Temperatures comparable to that of the sun-millions of degrees Centigrade-are required. A multiplying fission chain reaction produces these temperatures. Naturally the container is vaporized by the reaction; this is suitable for a weapon, but not for an industrial power plant.
Fusion weapons, then, are really fission-fusion weapons, in which fission occurs first and acts to trigger the still greater fusion reaction. Because heat provides the kinetic energy necessary for their functioning, fusion weapons are sometimes called THERMONUCLEAR weapons.
FUSION AND FISSION COMPARED. Both fusion and fission liberate nuclear energy. The two reactions differ in several respects.
One respect, THE MEANS OF INITIATION, has already been discussed. Fission results when a supercritical mass of suitable heavy material is rapidly formed. The reaction occurs automatically as soon as the free neutrons, already present in the fissionable material, are supplied with a large enough number of atoms to support the process. Fusion does not occur automatically; it must be initiated by the application of extremely high kinetic energy to suitable light substances-namely, deuterium and tritium.
There is a practical limit to the SIZE of a fission weapon and, therefore, to the amount of energy that can be released by this reaction. If a weapon contains more than a limited number of subcritical masses of fissionable
material, it becomes unsafe to handle and transport. No such limit is placed on the amount of heavy hydrogen a fusion weapon can contain; this weapon may be as large as the available launching devices permit. Much greater destruction, therefore, is possible with fusion weapons.
Fission produces a large number of RADIOACTIVE PRODUCTS-gamma rays, nuclear particles, and isotopes of various middleweight elements. Some isotopes decay in a short time. Others have a long half life and can remain dangerous for a comparatively long time. A few long-lived isotopes, including strontium90, tend, if they enter the body at all, to become lodged in the bones. There they can cause radiation damage over a period of years. Fusion, On the other hand, has tritium as its only radioactive product, and the tritium is itself fused with deuterium to produce stable helium.
12D5. Other nuclear reactions
Under laboratory conditions, particles other than neutrons have been (and still are) used to bombard atomic nuclei. Normally a very high concentration of energy is required to force one of these charged particles into a target nucleus.
Alpha particles, protons, deuterons, gamma rays, and beta particles can all produce transmutations.
ALPHA-PARTICLE bombardments produce stable isotopes plus protons or, alternatively, unstable isotopes plus neutrons.
PROTONS sometimes produce transmutations accompanied by the emission of neutrons, deuterons, alpha particles, and gamma rays.
DEUTERON-produced transmutations are accompanied by emissions of neutrons, protons, or gamma rays.
GAMMA RAYS are useful in releasing neutrons from light-element nuclides.
When captured by a nucleus, a BETA PARTICLE causes an X ray to be emitted.
Some, or perhaps all, of these reactions may take place during a complicated, high-energy process such as the explosion of a fission weapon.
CHAPTER 13 PRINCIPLES OF NUCLEAR WEAPONS
Nuclear weapons are sometimes called SPECIAL weapons, the name by which they were identified in early non-classified documents.
This chapter will deal with hypothetical fission and fusion weapons, and will make some comparisons between the two types. It should be clearly understood that the chapter makes no pretense of describing specific marks and mods of nuclear ordnance. The emphasis is on underlying principles, not on design details and operational sequences. The reader will find, however, that an understanding of the background information in this chapter is necessary to any of the more specific studies of nuclear ordnance he may make in the future.
Like the assembled weapons, the fuzes and other non-nuclear components will be
discussed without reference to specific service designs.
The latter portion of the chapter takes up some of the problems related to the use and handling of nuclear weapons.
The Navy has a wide variety of officer billets. Many of these billets are somewhat indirectly related to weaponry. All officer billets, however, are concerned with security, safety, defensive measures, and, when necessary, disaster relief. All these officer responsibilities are graver and more complex, now that nuclear warfare has become possible. Whether or not he expects ever to be directly in charge of any phase of the nuclear weapons program, every young officer needs such information as he will find in this chapter and in other non-classified summaries.
B. Fission Weapons
13B1. General requirements
FISSIONABLE MATERIAL. As mentioned in chapter 12, fissionable material for use in weapons consists of suitable isotopes of uranium and plutonium. When a mass of one of these isotopes-supercritical in size and shape-is very rapidly formed, a high-order multiplying chain reaction automatically begins. If all design features permit this reaction to continue in accordance with the laws of geometric progression, a powerful explosion occurs.
If the design features do NOT favor a rapidly multiplying chain reaction, the fission will produce only a low-order explosion; the weapon may even be a complete dud. In view of the high cost of fissionable materials and the military importance of the targets at which fission weapons are directed, it is important that these weapons perform reliably. This section will discuss some of the practical problems that have been met and solved by the designers of fission weapons.
SAFETY FEATURES. A fission weapon requires a device (or system of devices) to prevent a supercritical mass from forming prematurely or accidentally. Tragic results have followed the infrequent premature detonations of conventional weapons. Large-scale destruction would follow the premature detonation of a nuclear weapon.
It is, therefore, extremely important that every nuclear weapon be so designed and assembled that it will remain unarmed until the ordered conditions for arming have all been met. Section E, on fuzing techniques, will mention this topic again.
CONFINING THE REACTION. Obviously a nuclear explosion cannot be confined very long; the shock and heat of the reaction would soon overtax the resistance of even the strongest container. Yet the reaction MUST be confined until it has gained so much momentum that it cannot be stopped short of a full-scale explosion.
If the nuclear reaction were not confined during its early stages, the supercritical
mass would tend to fly apart, breaking into a number of subcritical masses.
One way to confine the reaction would be to make the outer case of the nuclear weapon abnormally thick and heavy. An equally effective arrangement, however, is to place the explosive elements within an inert but comparatively small inner container known as a TAMPER. The tamper need not have great tensile strength, but it must be a very dense substance. Information on the actual substances currently used is beyond the scope of this chapter.
In addition to its primary function of confining the nuclear reaction during its early stages, the tamper may also act as a REFLECTOR. If made of (or lined with) a reflecting substance, it will deflect stray neutrons back into the reaction, thus increasing the efficiency of the weapon.
ACHIEVING SUPERCRITICAL MASS. The subcritical mass or group of masses must be changed to a supercritical mass almost instantaneously. (Mathematical computations beyond the scope of this article set the maximum allowable time at less than 10-6 second.)
Too great a lapse of time in the formation of the supercritical mass might have one or the other of two undesirable results.
1. A continuing reaction, rather than a multiplying one, might be initiated. The heat generated by this reaction would be likely to melt enough of the fissionable material to return the mass to a subcritical state.
2. An inefficient multiplying reaction might begin and continue until it produced a low-order explosion.
Two means of achieving a supercritical mass will be discussed in the next two articles.
13B2. Gun principle
The hypothetical nuclear weapon sketched in cross section in figure 13B1 contains two sub-critical hemispheres of a suitable isotope of uranium, with their plane surfaces facing each other across an air gap.
If the intervening gap is very rapidly closed, the two hemispheres will unite to form a perfect sphere. This shape, by reason of its volume-to-surface ratio, is the configuration most favorable to a multiplying chain reaction, for it gives neutrons the least possible chance to escape. The masses of the
hemispheres are such that, when combined, they will constitute a supercritical mass.
Union of the two hemispheres is achieved by shooting one at the other, as a conventional gun shoots a projectile at a target. The hemisphere at the right is the projectile; the one at the left is the target. As in a gun, a firing mechanism and a primer initiate the propelling action.
Notice that the fissionable material occupies a comparatively small part of the interior of the weapon. The tamper is comparatively large.
13B3. Implosion principle
A single subcritical mass of fissionable material can be made supercritical by compressing it forcibly and rapidly. If the applied pressure is strong, and is equally distributed around the entire surface of the fissionable material, the space between atoms can be significantly reduced, with the result that fewer neutrons have a chance to become lost to the chain reaction. This is the principle behind the implosion type of fission weapon.
Uranium and plutonium are among the heaviest elements. To squeeze them effectively during the very brief time that is allowed, the compressive force must be an extremely powerful one. Conventional high explosives, if properly arranged and detonated, can furnish a shock wave that meets the requirements.
Figure 13B2 represents a cross section of the payload in a purely hypothetical implosion-type fission weapon. At the center is a sphere of fissionable material, either uranium or plutonium. Completely surrounding the sphere is a charge of conventional high explosive. A firing system provides for the SIMULTANEOUS detonation of the entire outer surface of the hollow sphere of conventional high explosive.
Figure 13B2.-Cross section of the explosive chamber in a hypothetical implosion weapon.
When the conventional explosive is detonated, a smooth shock wave moves inward against the sphere of fissionable material, striking it with equal force at all points. As a result of this suddenly applied squeezing force, the density of the plutonium is increased sufficiently to make the mass supercritical. A multiplying chain reaction automatically begins.
13B4. Neutron sources
A question may arise as to the origin of the first generation of free neutrons in a fission weapon.
One source is the air trapped within the weapon. Air always contains free neutrons, probably as a result of cosmic-ray reactions with its nitrogen and oxygen atoms.
Another neutron source is the uranium or plutonium as originally assembled in the weapon. Even in subcritical masses, these radioactive substances undergo a small amount of spontaneous fission that results in the production of stray neutrons.
The designer of a nuclear weapon, however, wishes to be absolutely certain that an abundance of neutrons will be available for use as soon as the supercritical mass is achieved. Therefore he places, in or near the fissionable material, a special capsule called the INITIATOR.
The initiator contains two substances. One substance is a reliable alpha-particle emitter such as radium or radon; the other is a light element such as beryllium. The light element, when subjected to alpha bombardment, will emit neutrons. The enclosing capsule is carefully designed to prevent any premature neutron development.
The same mechanical impulse that forms the supercritical mass shatters the initiator and makes emitted neutrons available to start the chain reaction.
C. Fusion Weapons
As chapter 12 explained, nuclear energy is released not only by the splitting of heavy nuclei (the fission reaction) but also by the joining of light nuclei to form heavier ones (the fusion reaction).
The light nuclei at the hydrogen end of the table of nuclides (fig. 12B5) have lower binding energies than uranium and plutonium. Upon hearing this statement for the first time, one might question quite justifiably why fusion weapons were not developed first. The reasons are several; but the main reason is the amount of initiating energy required.
ENERGY REQUIREMENTS. Neutrons, the tools used in producing large-scale fission reactions, possess the advantage of electrical neutrality. To cause fission, a free neutron has to pierce the binding energy of a heavy nucleus that is already in unstable equilibrium, but it does not have to overcome any electrostatic repulsion.
When two light nuclei fuse to form a new and heavier nucleus, however, at least two mutually repellant protons are involved. One proton is in the nucleus that (for the sake of simplicity) we can regard as the target; the other is in the nucleus that we can regard as the projectile. The projectile nucleus must be impelled with enough kinetic energy to pierce
the binding energy of the target, in spite of the repulsive forces that act between the two protons or sets of protons.
The energy requirements do not have to be guessed; for the various combinations of target and projectile nuclei, they can be computed by standard formulas of nuclear physics. When the United States began in earnest to develop a nuclear weapon, the fusion reaction was ruled out as being unlikely of achievement, on a practical scale, by any means available to man. After fission detonations had been achieved and their effects had been studied, however, the physicists began to suspect that the thermal energy released by a fission reaction might possibly be adequate to start a fusion detonation.
A fission explosion reproduces-briefly and in small space-intensities of light and heat comparable to those in the sun. A fusion explosion does still more; it duplicates a part of the actual process by which the sun and OTHER stars produce their light and heat. This process is not a chemical burning reaction; it is a nuclear fusion reaction in which four nuclei of simple hydrogen become one nucleus of stable helium, with a conversion of mass to radiant energy. The next article will summarize the solar cycle.
SOLAR FUSION CYCLE. Man has reproduced-on small scale under laboratory conditions-the six-stage process by which, according to the currently accepted theory, the sun "burns hydrogen as fuel." The process involves carbon, which undergoes a series of transmutations as it captures one proton (hydrogen nucleus) after another, then suddenly emits all the captured hydrogen (now fused into a single helium nucleus) and regains its original identity. Figure 13C1 shows the sun's continuously repeated cycle.
In the first stage simple carbon (C12) fuses with hydrogen (H1), with an accompanying release of radiant energy representing the mass lost in the fusion. A similar fusion and release of energy take place in the third and fourth stages. In the second and again in the fifth stage, the constantly growing nucleus emits a positive electron; this means, in each instance, that an excess negative charge remains on the nucleus and, in effect, converts a captured proton to a neutron by counterbalancing its positive charge.
In the sixth and final stage a fourth proton is captured. If the previous pattern were followed, the growing nucleus would emit energy and become simple oxygen (O16); but this doesn't happen. Instead, the nucleus becomes violently excited and emits all four of the captured particles, thus regaining its original identity as simple carbon.
By the time of emission, the four captured protons (two of which, as already noted, have been converted to neutrons) have achieved identity as an alpha particle (article 12B6), which is, of course, simply a helium nucleus. The carbon has merely acted as a catalyzing agent to bring four hydrogen nuclei together and hasten their fusion into helium. This is the cycle by which, for its millions of years of existence, the sun has been heating and lighting our solar system. The astrophysicists estimate that enough hydrogen remains to keep the cycle going for ten billion more years.
13C2. Fusionable materials
CHOICE. In the sun, then, hydrogen and carbon nuclei take part in a revolving, self-perpetuating process in which carbon becomes nitrogen and oxygen isotopes, and then reverts to carbon when all the captured hydrogen fuses into helium. In designing the first fusion weapon, the physicists decided against trying to reproduce the carbon-hydrogen cycle.
The reason was a practical one. Though not complex, the carbon, nitrogen, and oxygen nuclei involved in this cycle have higher binding energies than the two hydrogen isotopes deuterium (1H2) and tritium (1H3). Since the problem of supplying adequate initiating energy was known to be difficult at best, it seemed wise to choose the simplest reagents possible. The reagents must have both neutrons and protons; this requirement ruled out ordinary hydrogen, but not deuterium and tritium.
PRACTICAL FUSION REACTIONS. As mentioned briefly in chapter 12, two deuterium nuclei can fuse in either of two ways, as follows:
1H2 + 1H2 -> 1H1 + 1H3 + energy, and
1H2 + 1H2 -> 2He3 + neutron + energy.
A fusion weapon can, then, be based on deuterium. The tritium produced in one of the
Figure 13C1.-Fusion in the sun.
two possible deuterium fusion reactions is the radioactive hydrogen isotope identified as H3 in figure 12B5. It does not become an end product, but rather enters into the fusion reaction by combining with deuterium, as follows:
1H2 + 1H3 -> 2He4 + neutron + energy.
In the fusion weapon, then, as in the sun, hydrogen becomes helium, with a release of energy.
As a fusion reaction progresses and the heat intensifies, other nuclear reactions may (and sometimes do) take place. It is possible, for
example, for the neutrons liberated during fusion to cause various fissions that would be unlikely to occur at lower temperatures.
Because it is usable from the beginning of the fusion process, tritium as well as deuterium may be included in the payload of a fusion weapon.
13C3. Fusion weapon characteristics
As chapter 12 briefly mentioned, a fusion weapon (sometimes called a thermonuclear weapon and popularly known as an H-bomb) is a fission-fusion device.
SEQUENCE OF EVENTS. The first explosive action in a fusion weapon is the detonation of a conventional high explosive or the burning of a conventional propellant. As a result, the mass of uranium or plutonium becomes supercritical. Fission spontaneously begins.
When the heat of fission becomes high enough, fusion of the deuterium (or deuterium and tritium) nuclei begins. As fusion progresses, certain neutrons that first appeared to be byproducts may be drawn into the explosive process.
D. Weapon Comparisons
In the years since Hiroshima, means have been found to reduce the size and streamline the exteriors of fission weapons. There is, however, no nuclear equivalent of the hand grenade or the bazooka-nor is there likely to be, except in the comic strips.
A fission weapon must contain enough fissionable material to produce a supercritical mass, and enough ancillary material to keep the weapon from flying apart too soon. These requirements are likely to keep fission weapons permanently outside the minor-caliber classification.
According to present knowledge of FUSION, it will not occur at all, on a practical scale, unless it is triggered by a fission reaction. A fusion weapon, therefore, meets all the size and weight requirements of at least a minimum fission weapon. In addition, it must have space for a fusionable payload and strength to confine the fusionable material until an extremely high temperature is reached.
FISSION. There are practical limits to the amount of subcritical material that can be placed in a single fission weapon.
If an implosion weapon contains a mass that is very nearly critical, or if a gun-type weapon contains subcritical masses in excess of certain size and weight combinations, that weapon becomes unsafe to store and handle. There is always the consideration that some accident or incident may occur, during normal shipping and handling, to produce criticality and start a premature detonation.
The premature reaction might be a flash rather than a full-scale detonation. Even this, however, would be a catastrophe. If it failed to kill nearby personnel immediately, it would nevertheless subject them to a fatal dose of
ionizing radiation. It might contaminate the area with fission products. News of the accident would tend to lower the military and civilian morale throughout the nation.
Therefore fission weapons, by their nature and for reasons of safety, are limited as to the amount of fissionable material they contain. Consequently they are likewise limited as to the size of the explosion each can produce.
FUSION. Since a multiplying fission reaction is used as the fusion trigger, even a "small" fusion explosion is large in comparison with any other type. Furthermore, in theory at least, there is no upper limit to the size of a fusion reaction. A large amount of deuterium or deuterium-tritium is no easier to detonate, and no more susceptible to accidental actuation, than a small one. Actually, then, the size and weight of a fusion weapon need not be limited by the peculiar properties of its fusionable payload.
Its size and weight are likely, however, to be restricted by other factors-such, for example, as the facilities for handling and launching. Another limiting factor might be called military judgment. Using a bomb that "over-destroys" a target has no merit in itself.
The fact that nuclear weapons, and particularly fusion weapons, may (and under some conditions probably will) be used in combat has caused the military planners to revise many earlier concepts of strategy and tactics.
FISSION. All fission reactions practicable in ordnance require radioactive materials and scatter large amounts and varieties of radioactive products. As chapter 12 explained, the various radioactive byproducts vary in half life from a few seconds to many years. All these byproducts are hazardous until they have decayed to negligible amounts.
FUSION. On the other hand, the fusion reaction, as such, produces only one radioactive isotope; namely, the tritium that is consumed in the reaction. Of course the fission that triggers the fusion produces radioactive isotopes, as always. Since the area covered by a fusion explosion is comparatively large, however, the radioactive contamination is spread out widely and thereby diluted, unless additional and unusual fissions occur.
One of the current problems of scientists and ordnance engineers is the development of
"cleaner" nuclear weapons that will accomplish military objectives without widespread radiation hazards. In event of nuclear warfare, it is probable that fission weapons will be used over our own territory. An example would be the use of nuclear war heads in missiles to destroy attacking enemy aircraft. It is also certain that windborne radioactive products will be carried across national boundaries. It is, therefore, important that the incidental contamination be reduced as much as possible.
E. Fuzing Techniques
In the hypothetical nuclear weapons described in this chapter, a conventional explosive reaction always acts as the trigger. This is followed by the fission and finally (if arranged for) the fusion reactions. Actually, then, the problem of fuzing a nuclear weapon is similar to the problem of fuzing conventional bomb-type or gun-type ammunition. The techniques may be somewhat more sophisticated, but the essential fuze components-arming system, detonator, safety arrangements, and so on-are all present.
A nuclear weapon may be designed to explode in the air, at the surface of the ground or water, or below one or the other of these surfaces. These considerations affect the choice of a fuze.
13E2. Fuzing for air burst
Heavy blast damage to a target area is frequently the most effective means of gaining a military objective. As chapter 14 will explain, a nuclear detonation in the air above a target produces widespread blast effects. Several types of fuzes can be used to initiate a detonation at a selected altitude.
RADAR TYPE. A fuze may be constructed to operate much like a small radar. It may transmit, receive, and compare electromagnetic pulses, and may fire when the signal (or combination of signals) meets specific built-in requirements.
The forerunner of current radar-type fuzes was the proximity (or VT) projectile fuze of World War II. Because recent years have seen
many improvements in electronic equipment, radar-type fuzes are likely to be used extensively in the future.
BAROMETRIC TYPE. As the reader will recall from earlier studies in basic sciences, the atmosphere exerts pressure. Like water pressure, atmospheric pressure increases with depth. The barometric switch (familiarly called the BARO) is a fuzing device that responds to a predetermined value of atmospheric pressure.
The barometric initiating device has no exact counterpart in older bomb-type weapons. It corresponds roughly, however, to the conventional hydrostats that are used in underwater ordnance.
TIMER. Mechanical timing devices, representing various applications of the clock principle, are used in many conventional projectiles, bombs, and mines. Not all of these older clock mechanisms are used to cause firing at a preselected instant. Some are used, instead, to prevent firing from occurring BEFORE a given instant; these timers are associated with impact or influence devices that initiate the actual firing.
In an aircraft-launched nuclear weapon, of course, safety delays are vitally important to protect the launching craft and personnel. A timer can be designed to provide these delays, and also to close the firing circuit when the bomb has fallen a selected distance below the launching altitude. The dropping speeds of the various types of nuclear bombs are known; therefore the dropping distance is easily converted to an equivalent time interval.
13E3. Fuzing for surface burst
A nuclear weapon, like a conventional one, may be designed to burst at the surface of the ground or water. An impact fuze, in which the shock of landing causes one operating component (or group of components) to move with respect to another, is used for surface bursts.
If a delayed explosion is desired, an impact fuze may contain a timing device that prevents the detonation from occurring immediately upon impact. A heavy weapon, falling from a high altitude, will penetrate some types of soil and may even bury itself. A delayed-action fuze permits this burial to take place.
13E4. Fuzing for underwater burst
Nuclear weapons, like conventional ones, are capable of underwater detonation by hydrostatic fuzes. Timing devices can also be used to produce underwater detonations.
13E5. Safety devices
Because of the high destructive capacity of all nuclear weapons, the fuzes used in them must have positive protection against accidental or premature arming. Certain types of safety arrangements and accessories have performed reliably in conventional weapons. With adaptations and refinements, similar safety devices are usable in nuclear weapons.
ARMING WIRES. The arming wire has long been familiar to naval aviators. This wire is threaded through a fuze to keep a movable component from taking its armed position. During launching, the fuzes (or equivalent devices) are freed from the arming wires.
In conventional aircraft bombs a delay period, provided by a windmill-type vane and a gear train, keeps the fuze from becoming armed immediately after the arming wire has been removed. In the safe interval, the planting craft escapes from the danger zone.
OTHER SAFETY FEATURES. Inertia, as the reader will recall from basic physics, is the natural tendency of a material object (if stationary) to resist being set in motion and (if moving) to resist any change in the direction or speed of motion. In weapons, the force of inertia can be utilized to produce a safety delay by retarding the relative motion between two components.
During the acceleration of a projectile in a gun barrel, inertia tends to force all parts of the fuze mechanism toward the rear. This manifestation of inertia-called SETBACK in gunnery-can be used to delay arming. Similarly, in several types of mine accessories and components, inertia is used either to prevent an undesired action or to cause a desired one.
Some fuzing arrangements for nuclear weapons use inertia as a means of achieving a safety delay.
Electrical arrangements constitute another means of assuring safety before and during launching, and for a selected period thereafter. Nearly all naval mines, for example, are assembled and planted with several breaks in the battery-to-detonator circuit. Each break consists of a normally open switch that will not close until specific requirements have been met. Similar but not identical electrical interrupters are used in nuclear weapons.
F. Practicable Weapon Types
Probably because the two fission weapons used in World War II were bombs, the popular press tends to group all nuclear ammunition under that classification. Frequently the only distinction it makes is between the A-bomb and the H-bomb. Though not strictly accurate, this popular manner of writing reflects the fact that nuclear weapons have been (and still are) adaptable to the
delivery techniques used with conventional bombs.
TYPES OF BURST. As the foregoing section on fuzing techniques has already implied, nuclear bombs may be arranged to burst in the air, at the surface of the ground or water, or below either of these surfaces.
The effects on military objectives vary, to some extent, with the type of burst. The next chapter will summarize and compare those effects.
13F2. Missile heads
Almost as soon as fission weapons had been proved practicable, the thought of a guided missile with a nuclear explosive charge began to interest the ordnance designers. Since that time, all the armed services have developed reliable missiles capable of delivering nuclear payloads.
A quick check of unclassified sources indicates the following missiles having a nuclear capability: The Navy' s Regulus I and II, Talos, and Polaris; The Army' s Matador, Honest John, Corporal, Sergeant, Nike-Hercules, Redstone, and Jupiter; and the Air Force's Snark, Thor, Bomarc, Atlas and Titan.
13F3. Other applications
PROJECTILE. The Army has designed at least one nuclear projectile that can be
fired successfully from a long-range field gun.
UNDERWATER WEAPONS. A conventional torpedo has two interchangeable heads-an inert head for exercise runs and an explosive-loaded war head. A third head bearing a nuclear charge is entirely practicable.
Nuclear equivalents of conventional mines and depth charges are also feasible.
13F4. Practical limitations
Whenever nuclear explosives are substituted for conventional ones, sever al practical problems must be faced and solved. One problem is to avoid trapping the planting craft and personnel within the explosion area. Another is to make reasonably certain that the military advantage to be gained is great enough to justify the expenditure of nuclear ordnance.
G. Delivery Systems and Techniques
The usual advantages and disadvantages of conventional aircraft bombing missions apply to missions involving nuclear weapons. The airplane is swift and maneuverable; it can penetrate far into enemy territory. It is, on the other hand, susceptible to radar detection and to countermeasures.
The airplane that drops a nuclear bomb must assure safety for itself. High-altitude launching can, of course, use distance as a safety factor. Pinpoint accuracy, however, is hard to achieve from a great height.
To assure greater accuracy of aim, a nuclear weapon may be launched from a comparatively low altitude. An adaptation of the loft, toss, or over-the-shoulder technique may be used. Accurate maneuvering and high escape velocity now have to be utilized as the safety factors.
As previously mentioned, the gun that fires a nuclear projectile is a long-range artillery type. Either a proximity fuze or a timer, as ordered, is usable with this projectile.
13G3. Guided missiles
Any guided missile, whether or not it bears a nuclear payload, requires a dependable launching system manned by a skilled team. Chapter 11 of this volume deals with representative shipboard missile-launching systems. Like the waterborne systems, the various land-based missile systems differ among themselves.
In the Polaris missile system, a submerged submarine is, in effect, the launching platform for a heavy-duty airborne weapon. When used for this purpose, the submarine has advantages that may revolutionize a number of military concepts. Except at close ranges, the submarine is harder to detect than a bombing plane. Nuclear propulsion and improved design have given the submarine a ranging power and a degree of maneuverability that would have seemed fantastic as recently as World War II.
13G4. Underwater ordnance
Torpedo tubes and other launching devices for conventional underwater weapons can be adapted to take nuclear weapons of comparable weight and shape, for use against submerged targets.
H. Safety and Security
13H1. Safety precautions
In a nuclear weapon, three main classes of components are subject to specific safety precautions. One class is the conventional high explosive or propelling charge that must be used to achieve the supercritical mass of fissionable material. A second class is the complement of mechanical and electrical devices that provide a safe period, an arming cycle, and an initiating impulse. The third and final class, of course, is the nuclear material.
CONVENTIONAL EXPLOSIVES. General safety precautions for conventional explosives are summarized in NavOrd Instruction 5100.1. These basic rules apply in equal measure to nuclear ordnance, and to any separately stowed non-nuclear components that contain explosives. These rules should be familiar to the reader from his previous studies.
ELECTRICAL AND MECHANICAL COMPONENTS. As section E of this chapter has implied, the safety delays, arming arrangements, and fuzes used in nuclear ordnance are not radically different from their counterparts in other bomb-type and gun-type ammunition. Certain refinements have been made, but the underlying principles have not been superseded. For these components, as for conventional explosives, the Navy's standard rules must be known and enforced.
Special Weapons Ordnance Publications (SWOP's) on specific nuclear weapons supplement the standard Navy safety precautions wherever necessary.
NUCLEAR COMPONENTS. All personnel assigned to work with fissionable or fusionable materials must receive special training in the handling, stowage, and accounting methods peculiar to these materials.
Fissionable substances are, of course, radioactive. Therefore, as chapter 12 has explained, they constitute a radiation hazard that must be recognized and guarded against.
Whether assembled in weapons or stored separately, radioactive substances are very costly and potentially very hazardous. Also, the details regarding the design and operation of specific nuclear weapons are extremely important to national defense. A breach of security regarding these matters might have grave consequences.
It is essential, therefore, that only personnel of unquestionable integrity, judgment, and patriotism have access to classified information on nuclear weapons. Officers who work in any capacity with these weapons must be careful at all times-on and off duty alike-to observe the spirit as well as the letter of the security regulations.
I. Elements of Organization
The organization for nuclear weapons is too large a topic to receive more than brief mention in this chapter. The general informational manuals of the several interested agencies, including the Bureau of Ordnance, give additional background information.
13I2. Atomic Energy Commission
In 1946 the Atomic Energy Commission (AEC) was established by act of Congress for the control of nuclear source material and the products derived from it.
Until 1953 the AEC developed and produced all United States nuclear ordnance. The three
services participated jointly in formulating nuclear ordnance requests for the Department of Defense. These requests were forwarded to the AEC by way of a liaison committee.
Since 1953 many of the original functions of the AEC have been distributed among the three armed services. Each service develops its specific weapons systems, but parts of all these systems remain the responsibility of the AEC. Such parts must be procured from the AEC as explained in the next article.
13I3. Armed Forces Special Weapons Project
A tri-service agency called the Armed Forces Special Weapons Project (AFSWP) acts as the Department of Defense planning,
programming, and procurement agency for all AEC -developed ordnance items. Navy requirements for such items must be approved by the Joint Chiefs of Staff. They are then submitted to the AEC by way of the Bureau of Ordnance and AFSWP.
AFSWP reports directly to the Secretary of Defense regarding the status of all United States nuclear ordnance, including those items produced by the services.
TRAINING. AFSWP furnishes training and technical services to the three military departments. At first ALL training in nuclear weapons was under AFSWP control. Although the services have now developed their own training programs, AFSWP continues to furnish supplementary training, both to individuals and to groups. In particular, it orients and indoctrinates key officers.
TECHNICAL SERVICES. AFSWP determines the safety standards and physical procedures involved in the handling and transportation of nuclear weapons. It prepares plans and budget estimates for the military phases of nuclear weapon tests. It continuously reviews and evaluates the results of these tests, as a guide to the development of improved weapons and more effective defense measures.
13I4. Storage sites
Within the continental United States, three types of storage sites, discussed below in the order of their relative size and complexity, are set aside for nuclear weapons and their components. Outside the continental United States, both land-based and shipboard storage facilities have been developed.
NATIONAL STOCKPILE SITES. The largest and most completely equipped land-based storage site for nuclear weapons is called a national stockpile site (NSS). It has facilities for storage, inspection, monitoring, maintenance, and calibration of a rather large number of weapons.
The weapons stored at an NSS are in AEC custody. The site itself is controlled and operated by AFSWP.
OPERATIONAL STORAGE SITES. For the non-nuclear components of nuclear weapons,
an operational storage site (OSS) performs all the services that an NSS performs for the complete weapons. It may have similar facilities for the nuclear components.
Weapons at an OSS are in AEC custody or, sometimes, in Department of Defense custody. The site is controlled by one of the armed services.
LIMITED STORAGE SITES. Handling, temporary storage, and partial monitoring of nonnuclear components (and SOMETIMES of nuclear components) may be performed at a limited storage site (LSS). Complete storage and monitoring facilities may, under some conditions, be available at an LSS.
Weapons at an LSS are in Department of Defense custody. The site is controlled by one of the armed services.
A limited storage site located OVERSEAS is designated an OLSS.
VESSELS AS SITES. Two classes of naval vessels-the NVA and the NVB-serve as floating storage sites for nuclear weapons under Department of Defense custody. The NVA is roughly similar to the NSS, and the NVB to the LSS, in equipment and functions, though not necessarily with respect to size or capacity.
The selection of officers and men for assignment to nuclear weapons duties is under cognizance of the Chief of Naval Personnel. As has been mentioned, the standards for selection are necessarily very high.
Service schools and A FSWP both offer basic technical training. Individuals who have completed the content courses are organized into teams and trained to function as unified groups. Ordinarily a team specializes in one of the following duties: assembly and storage, loading, or disposal.
Naval special weapons teams are under the administrative control of the Gunnery Officer. Aboard ship each team is in the department most closely related to its functions. On an aircraft carrier, for example, the storage and assembly teams are in the gunnery department, while the loading team is part of the air department.
CHAPTER 14 EFFECTS OF NUCLEAR WEAPONS
Whenever a new weapon is proposed, two questions arise. First, what can this weapon do for us in combat? Second, if the enemy uses the weapon against us, what defensive action can we take? The answers to these questions are seldom simple, even when the weapon is a "conventional" type. For nuclear weapons, the answers are complicated by two major factors. 1. The explosion is a very large one; 2. the explosion is accompanied, and often followed as well, by ionizing radiation.
When these two facts first became public knowledge, a certain amount of hysteria was inevitable. Hysteria still characterizes much of the popular thinking about nuclear weapons. Unbiased information, honestly faced and analyzed is an antidote to hysteria. A great deal of information on the effects of nuclear weapons has now been made available in unclassified Government publications. The data for
these publications have come from two main sources-the World War II detonations over Japan and the postwar testing program.
Out of the wealth of available information, this chapter endeavors to summarize the details that are most likely to be useful to a junior officer. Regardless of specialty, every officer has cause to be familiar with the effects of nuclear explosions.
The brief second section of this chapter will review-and, where necessary, amplify-the major comparisons and contracts between conventional and nuclear weapons. The body of the chapter will analyze the effects of several possible types of nuclear explosions. Concluding sections will analyze the types of damage that can be expected, and will mention defensive measures.
14B1. Conventional reaction
A conventional explosion is a chemical reaction. An initiating impulse-usually heat or shock-is applied to a substance whose molecules contain oxygen, carbon, and hydrogen in abundance. When initiated, explosive substances oxidize (burn) much more rapidly than ordinary combustible materials. HIGH explosives (the substances used as the burster charge in conventional bomb-type ammunition) .are said to detonate rather than to burn in the usual sense. The detonation propagates itself as an intense shock wave, followed immediately by a release of energy in the form of intense heat.
During this almost instantaneous process, the original molecules break up and their atoms recombine to form more stable compounds. Most of the energy of heat is converted to energy of motion that bursts the container and sends a blast wave through the air, or a shock wave through the earth (or
water). It is primarily this blast or shock wave that causes damage.
However, the amount and type of damage can be modified by a number of considerations. These include (but are not necessarily limited to) the type and amount of the explosive substance, the strength of the target, and the distance between the target and the point of detonation. Frequently the target is shattered; sometimes it is ignited immediately. More frequently, fire damage to the target occurs (if at all) as a secondary result of shock damage to fuel systems, stowed explosives, or power lines. If the target is a ship, it may sink because it has been damaged beyond its capacity for rapid repair; or it may remain waterborne but be unfit for combat.
A successful conventional detonation is likely to kill or injure at least a few personnel. Some fatalities or casualties occur immediately and are unavoidable. Still others occur as a result of secondary effects. Their causes may be falling or flying objects, short circuits, fire,
flooding, or other resulting manifestations of explosive violence. A taut ship or station endeavors to keep secondary casualties to a minimum. This can be accomplished with enlightened foresight, training, and discipline.
14B2. Nuclear reaction
A nuclear explosive reaction, like a conventional one, is characterized by intense heat and a heavy wave of blast or shock. The heat is many times higher than in a conventional explosion; the shock wave, in addition to being stronger, moves more slowly and covers a much greater area. If all or even part of a nuclear explosion takes place in the air, winds of a high velocity are generated.
Secondary effects -falling and flying objects, damaged pipelines and wiring systems, and fires-are more numerous and extreme than after a conventional explosion. Unavoidable
casualties may be numerous. Unhappily, other casualties (some of which could be avoided by using elementary knowledge and taking simple precautions) are liable to be very numerous.
In these respects a nuclear explosion differs from a conventional one in degree more than in kind. In another respect-the certainty of concomitant nuclear radiation and the possibility or probability of secondary radioactive contamination-the nuclear explosion is in a class by itself. Because nuclear radiation cannot ordinarily be discerned by any of the five senses, and because the average person has a vague and partially erroneous idea of the phenomenon, this aspect of a nuclear explosion-even more than the heavy blast and shock damage-is a possible source for panic.
The next section will describe the several major classes of nuclear explosions, and will summarize the effects of each on a target area.
C. Nuclear Explosions
14C1. Distribution of energy
Figure 14C1 shows how energy is distributed in a representative nuclear explosion. About 85 percent of the total energy appears first as intense heat. Almost immediately a considerable part of this heat is converted to blast or shock; the remaining thermal energy moves radially outward as heat and visible light.
Some 5 percent of the total energy appears immediately as invisible but extremely powerful nuclear radiation-alpha particles, beta particles, gamma rays, and neutrons. The residual nuclear radiation occurs over a long time; it is produced by the decay of the numerous radioactive isotopes that are formed by fission reaction.
Thus far we have been thinking of nuclear explosions in very general terms. The next few articles will take up weapons for which the yield (power) and the conditions accompanying the detonation are specifically mentioned. The first of these will be a nuclear weapon detonated in the air.
14C2. Representative air burst
As chapter 13 mentioned, an air burst over a target is frequently the most efficient means of accomplishing a military objective. A 1-megaton detonation (equivalent in destructive
Figure 14C1.-A typical energy-distribution graph.
power to a million tons of TNT) has been selected for study in this article. For a weapon of lower yield, the distances and the time intervals would be shorter; for a more powerful weapon, they would be longer.
The three parts of figure 14C2 show what happens during the first 11 seconds after detonation.
Figure 14C2.-Three stages in the development of a 1-megaton air burst.
Very soon after the nuclear weapon is triggered, a rapidly multiplying nuclear reaction vaporizes all parts of the weapon and its container. The reacting matter appears as an extremely hot and brilliant fireball resembling a small sun. The fireball radiates heat, light, and nuclear emissions.
The reaction causes a blast wave (the primary shock front) to move outward from the fireball. The air immediately behind this front acts as a terribly violent wind. In the first portion of figure 14C2 the blast wave has not yet reached GROUND ZERO (the point directly below the detonation point). The light rays and the equally swift gamma rays, however, have done so.
When the primary blast wave strikes ground zero with an impact like that of a tremendous hammer, a second or REFLECTED blast wave begins to move upward and outward from ground zero. The second part of figure 14C2 shows the reflected wave. At points on the surface, the impact of the two waves is felt simultaneously. This is true also, for practical purposes, of points ABOVE the surface in the vicinity of ground zero.
At points somewhat farther out, such as Pi and Pii in figure 14C3, however, an object above the surface, such as the top of a tall smokestack or a television tower, would receive
two distinct blows. It would be struck first by the incident wave moving radially outward from the fireball and, shortly thereafter, by the reflected wave moving radially outward from ground zero.
As one goes farther out from ground zero, however, the angular distance between the incident wave and the reflected wave decreases. In other words, the two waves are moving more nearly in the same direction. Also, the reflected wave tends to move faster, since the incident wave has compressed the air through which it will move. At some point between Pii and Piii in figure 14C3, the two waves begin to be felt as a single strong shock, not only at the surface (as before) but above it as well. This point marks the beginning of the MACH FRONT. For the explosion shown in figure 14C2, the overpressure (excess over normal atmospheric pressure) of the Mach front at its point of origin is 16 pounds per square inch.
As the combined waves move further from ground zero, the Mach front elongates itself, forming the Mach STEM shown extending almost vertically from points Piii and Pii in figure 14C3. An airplane or a tall object located ABOVE the triple point at the upper end of the Mach stem will feel two separate blast waves. An object BELOW the triple point will feel the combined blast waves as a single powerful
Figure 14C3.-Formation of the Mach front.
blow. The Mach effect is one reason for the long-range shattering power of a nuclear air burst.
Behind the primary shock wave and, after its formation, behind the Mach stem, a strong, swift wind blows almost horizontally outward from ground zero. In its destructive power, this wind is like a concentrated, short-lived hurricane.
While the Mach front is being formed, the fireball is still radiating large amounts of heat, light, and nuclear emissions. By the end of 11 seconds, for a 1-megaton explosion, the Mach stem has moved outward about 3 miles from ground zero. The overpressure is about 6 pounds per square inch, and the wind is blowing at 180 miles per hour. This is the situation shown in the third part of figure 14C2.
By the end of 37 seconds, however, significant changes have taken place, as shown in figure 14C4. The overpressure has dropped to a single pound per square inch, and the velocity of the wind behind the Mach stem is merely 40 miles per hour. The fireball has ceased to radiate much heat, but it is still emitting gamma rays given off by the decay of various
short-lived radioactive isotopes formed during the fission reaction. This is an example of SECONDARY radiation, as distinguished from the primary radiation given off as an immediate result of the explosion.
Though it no longer glows, the fireball is still very hot. It rises swiftly, like a hot-gas balloon, sucking air inward and upward after it. This suction phase of the burst creates strong winds, opposite in direction to the Mach wind. Near ground zero these AFTERWINDS pull upward a large amount of surface dirt, plus much of the lighter debris from buildings shattered by the blast. This windborne material forms the stem of the mushroom cloud that is characteristic of a nuclear air burst. In figure 14C4 the cloud has begun to form.
Within the second minute after a 1-megaton detonation, the top of the mushroom cloud is about 7 miles in the air. The afterwinds are blowing inward toward ground zero at about 200 miles per hour.
The mushroom cloud consists mainly of vaporized fission products and other bomb residues, plus some of the lighter material carried up through the stem.
Figure 14C4.-Formation of the mushroom cloud after a 1-megaton air burst.
The fission products, of course, are highly radioactive.
After 10 minutes the mushroom cloud is about 15 miles in the air and has spread out considerably. In time, normal winds disperse the cloud, thus spreading its contents over a wide area and diluting them.
Because some of the radioactive fission products have very short half lives, the total radiation hazard is constantly decreasing by decay as well as by dispersal. It does not completely vanish, however. Fission products with long half lives, and diminishing quantities of those with short half lives, remain. Some of these may, in time, be borne earthward on raindrops, fog droplets, or dust particles; or they may descend by their own weight. This returning radioactive material constitutes the FALLOUT that is a peculiar hazard of nuclear explosions. (It may be mentioned in passing that the fallout from a high air burst is unlikely to be one of its serious threats. Fallout from some of the other types of bursts, however, is a major hazard).
The student should clearly understand that a non-fissioned water droplet or dust particle does not itself become radioactive by acting as a vehicle for a radioactive isotope. All it does is to convey this product of the original explosion from the upper atmosphere to some place where it may possibly be picked up by a living organism.
In considering a bomb of greater or lesser yield than a megaton, the order and nature of the events in an air burst will be as outlined in this article, but the statistical values will be different.
An air burst, then, produces intense heat radiation, primary nuclear radiation from the fireball, secondary nuclear radiation from the fission products in the mushroom cloud, great changes in atmospheric pressure, and strong, high-velocity winds, first away from ground zero and later toward it. At and near ground zero, any or all of the primary effects are fatal to personnel. The combination of pressure and wind destroys all light buildings, and possibly all buildings whatsoever.
Beyond the area of total immediate destruction, blast and wind damage are still heavy. Fires-resulting either from the initial heat radiation or from various secondary causes-soon reach dangerous proportions. Unprotected personnel may be killed or injured by
radiation, by falling buildings, by blows or lacerations from falling or windborne objects, or by secondary fires. Many in underground shelters, and many above ground who have learned and applied elementary defense procedures, can save themselves or, if injured, can be saved by well-drilled rescue teams. The human body is much more tolerant of short-term overpressure than even the strongest buildings are. It is the secondary effects of overpressure-crumbling walls and flying glass, for example-that cause most injuries.
Because it is particularly destructive of structures and equipment (and because of minimized radioactive after effects), an air burst above a target area is likely to be a preferred method of nuclear attack. Other classes of bursts are possible, however. It is therefore necessary to notice how each compares with an air burst.
14C3. Surface burst
If an air detonation takes place at a very low altitude, part of the fireball, in its rapidly growing early stages, touches the surface of ground or water. This type of nuclear explosion is defined as a surface burst.
The intense heat of the fireball vaporizes a large amount of soil or water. This vaporized (but ordinarily not fissioned) extraneous material remains in the fireball as it rises. In addition, the suction phase of the explosion carries much more debris into the mushroom stem than would be expected in an air burst.
As the fireball cools, the vaporized foreign material condenses into minute particles in the mushroom cloud. The heavier debris falls back fairly near the point of burst; the lighter particles may remain airborne for along time. Radioactive fission products may cling to any or all of the non-fissioned particles. The surface burst, therefore, carries a much greater threat of hazardous radioactive fallout than an air burst does. Though the danger from the fallout of heavy particles is greatest near the target (where damage from other causes is also severe) the airborne lighter particles may seriously contaminate wide areas.
It is estimated, for example, that a 1-megaton bomb, exploded on the surface of the ocean, would convert about 100,000 tons of water to vapor. At a high altitude, this water vapor would condense into droplets like those
in an ordinary cloud, with the serious difference that many droplets would be vehicles for radioactive fission products (plus an amount of induced radioactivity).1 By the time these contaminated droplets fell as rain, they might be hundreds of miles from the point of detonation.
If any significant portion of the fireball touches land, a crater remains to mark the site of the explosion. The crater is formed partly by vaporization of the soil and partly by updraft into the stem during the suction phase. An observer at a distance can recognize a surface burst over land by the dirty color of the mushroom stem and cloud.
A varying portion of the kinetic energy of a surface burst goes into ground shock similar to that produced by a penetrating high-explosive bomb. This shock aids the atmospheric overpressure in demolishing buildings near the point of burst.
14C4. Underwater burst
A nuclear underwater burst is defined as one whose origin is beneath the surface of a body of water. Most of the energy of the underwater burst appears as underwater shock, but, a certain proportion (dependent on the depth) may escape and produce air blast.
A "true" underwater burst is one in which the detonation and the formation of the complete fireball both occur below the surface of the water. Because it is subject to hydrostatic pressure, the fireball is believed to be smaller than for a bomb of comparable yield detonated in the air. As the rising fireball touches the surface, its glow disappears, because the gases expand and cool when they meet the lesser resistance of the air.
While it is still under the surface, the fireball (or gas bubble) generates a shock wave, much as a fireball in the air generates a blast wave. A later paragraph will mention some of the peculiarities and military uses of this shock wave.
Two phenomena give advance warning that the fireball from an underwater detonation is approaching the surface. First a rapidly
expanding white circle, called the SLICK, appears on the surface. The slick is composed of countless droplets of surface water that have been tossed up by the advancing shock wave. At the center of the slick, a dome of water and spray rises, directly over the detonation point.
Neither the slick nor the spray dome contains any radioactive matter. Neither of them has any interest except as a forerunner of the true explosion phenomena. (A very deep detonation may fail to produce a spray dome.)
When the radioactive fireball (or gas bubble) touches the surface, the hot gases are violently expelled into the atmosphere, drawing up with them a hollow column (sometimes described as a PLUME, or a CHIMNEY) of water. The complex pressure relationships cause water droplets to form a "Wilson" condensation cloud about the hollow column. The cloud formation reproduces, on a large scale, the conditions in the laboratory cloud chamber mentioned in chapter 12. The Wilson cloud remains only for a second or two, and is not radioactive.
Figure 14C5 shows three characteristic steps in a typical underwater burst. (Baker test at Bikini atoll in 1946-a 100-kiloton weapon was used in comparatively shallow water.) The upper part of the illustration shows conditions 2 seconds after detonation. Notice that the shock wave that surrounded the fireball in the water has become a blast wave in the air, surrounding the Wilson cloud.
Twelve seconds after detonation, as shown in the second part of figure 14C5, the water column has reached a height of about 3,300 feet. The fission products venting through the center of the column have begun to condense into an atomic cloud resembling a giant cauliflower.
The cauliflower cloud is strongly radioactive, but is too high to be a serious threat to shipborne personnel at this time. A much greater immediate threat is the BASE SURGE that has begun to form around the lower end of the hollow column. The base surge consists of radioactive mist from the contaminated water in the hollow column, which is now dropping
1 Another source of radioactivity is the activity induced by neutrons when they are captured in the various elements present in the earth, sea, or other substance in the explosion environment. It may be mentioned in passing, that radioactivity induced by gamma rays from a nuclear explosion is either insignificant or completely absent.
Figure 14C5.-Three stages in the development of a 100-kiloton shallow underwater burst.
backward due to gravity. The base surge spreads radially outward, giving the appearance of a doughnut-shaped cloud on the surface of the water.
By this time, too, large water waves have begun to form and move outward from the base of the hollow column.
By the twentieth second after detonation, conditions are as shown in the third part of figure 14C 5. The base surge is growing higher as it moves outward. Large quantities of contaminated water, the MASSIVE WATER FALLOUT, begin to pour down from the mushroom cloud. The hollow column is continuously shrinking.
A minute after detonation, the hollow column is much lower and the ring of outward-rushing base surge much higher. Contaminated
water and spray from the cauliflower cloud encircle the hollow column. Water waves continue to form and move outward. The first wave has traveled almost a mile from the column.
Two and a half minutes after detonation, as shown in figure 14C6, the central column has been completely replaced by a radioactive mist or cloud that extends downward to the surface of the water. The base surge still forming an outward-moving ring around the central cloud, has lifted slightly. It appears, therefore, as a low-hanging cloud from which radioactively contaminated rain is pouring. This rain is hazardous to surface vessels in its path. The reader is to assume that the two portions of the base surge shown in figure 14C6 are cross sections of the ring-shaped cloud.
Figure 14C6.-Conditions 2-1/2 minutes after a 100-kiloton shallow underwater detonation.
Though diffusion, and the natural decay of isotopes with short half lives, have reduced the intensity of the nuclear radiation given forth by the central cloud, the level of radiation is still dangerously high.
Eventually, the central cloud and the base surge mingle and are carried off in the downwind direction.
An underwater detonation at greater depth may fail to produce any of the phenomena shown in figures 14C5 and 14C6. Instead, the hot gas bubble may break into a large number of smaller bubbles as it rises through the water. When the small bubbles reach the
surface, they may break into radioactive froth, perhaps with a thin layer of contaminated mist above it. The mist is not likely to create a large fallout problem, but dangerous amounts of the radioactive foam maybe washed against surface vessels or even against the shore.
During any type of underwater nuclear explosion, all or a great percentage of the radiant heat is absorbed by the water. Many of the first neutrons and gamma rays are also absorbed. When and if the fireball reaches the surface and bursts, however, the various fission products are still emitting gamma rays and beta particles.
The hollow column, the cauliflower cloud, and the base surge all contain large numbers of radioactive particles. The fallout (or rain-out) of these particles is liable to be the most serious danger to surface ships and shore installations BEYOND the region of heavy shock (and blast). It is important, therefore, that naval officers in general should have knowledge of decontamination procedures (as well as other damage control and first aid procedures).
14C5. Underground burst
When the fireball is formed below the surface of the soil, the hot, pressurized gas within it is mingled with bomb residues and vaporized earth. Upon breaking through the surface, the expanding gases throw up a hollow, outward-flaring column consisting of earth debris mingled with fission products.
As in an underwater burst, a hemispherical blast front surrounds the hollow column in its early stages. The upper part of figure 14C7 shows conditions two seconds after a 100-kiloton shallow underground burst. In addition to the phenomena shown in the drawing, this type of detonation produces a ground shock resembling a small earthquake, except that it occurs nearer the surface.
In rising the hollow column produces a THROWOUT of contaminated debris. The lighter products of the explosion form a radioactive cloud about the upper part of the column.
The CRATER is deeper and wider2 than the one produced by a surface burst of equivalent yield.
By the end of 9 seconds, as shown in the second part of figure 14C7, the expanding cloud is still giving off hazardous amounts of radiation. Some of the heavier fragments in the throwout are falling back to the earth.
Forty-five seconds after detonation, the throwout is rapidly falling to the ground. It can be expected that finer dust particles from the hollow column will form a ring of base surge, much like the mist surge that characterizes a shallow underwater burst. The dust particles in the base surge are heavily contaminated with nuclear byproducts.
After a few minutes, as shown in the final part of figure 14C7, the central column loses its separate identity. The lightest particles from the column have now become part of the radioactive cloud. This cloud spreads out, especially in the downwind direction. If a base surge has formed, it rises toward the cloud and moves ahead of it in the downward direction. Thus, radioactive particles can be carried downwind for considerable distances, seriously contaminating a large area.
It is estimated that a 1-megaton shallow underground burst would blow into the air some ten million tons of soil and rock. The area around the crater would be heavily contaminated, and the fallout of lighter particles might be hazardous over a great distance.
D. Effects of Nuclear Explosions
14D1. Damage criteria
BASIC GRAPH. In assessing the damage caused by any explosion-whether "conventional" or nuclear-it is convenient to represent the various intensities of damage, and the areas subjected to each intensity, as a series of concentric circles about the detonation point. See figure 14D1.
Of course figure 14D1 is a simplified and generalized graph. To show the data gathered from the study of any particular explosion, this graph will have to be modified in one or
several ways. For a nuclear explosion the several kinds of damage, and their separate or combined effects on equipment and personnel, are so varied that a series of graphs often becomes necessary to tell the story.
It may be desirable, also, to show a larger number of damage intensities than the four indicated in figure 14D1.
In actual practice there are no lines of demarcation between one damage area and another. Furthermore, the damage areas will seldom be perfect circles; sometimes they will vary greatly from the circular form.
2Comparison of CRATERING in dry soil for 100 KT weapon: contact surface burst-dia. 580' X depth 80'; underground burst at 50'-dia. 720' X depth 120'.
Nevertheless, for preliminary considerations, figure 14D1 is a useful tool.
DAMAGE AREAS. In any effective explosion of bomb-type ammunition, there is a large or small area about the point of burst where total destruction of equipment and personnel must be taken for granted. In a nuclear explosion at or below the surface, nothing
within or near the fireball will be salvageable. With an air burst (except a very high one), ground zero and a greater or lesser area surrounding it can be considered completely demolished. For a nuclear weapon of any type, the area of TOTAL destruction is many times larger than for a conventional weapon of comparable size.
Figure 14C7.-Development of a 100-kiloton shallow underground burst.
Figure 14D1.-A basic damage graph.
Ashore, in the HEAVY damage area, many buildings, much equipment, and many persons will be lost, either within a few seconds after detonation or as a result of secondary phenomena. Some buildings and equipment, and some people as well, may suffer only minor primary damage. The final number of casualties in the heavy damage area will depend, in part, on the speed, level-headedness, ingenuity, and cooperation displayed by disaster-relief personnel.
This chapter will not go into details about "atomic defense". As a junior officer you will receive further indoctrination in fundamental procedures and will be assigned a definite responsibility for some part of the total program of your ship or station.
In the zone of MODERATE damage, there will, of course, be some heavy damage to light equipment and structures. There will also be some fatalities and severe casualties to personnel. Some persons, however, will be unharmed, and many will be able to do useful work after receiving simple first aid. The great problems will be (1) to prevent panic and (2) to utilize all able-bodied (and mentally or emotionally competent) personnel in damage control and disaster relief. At a shore station, fire fighting (possibly with severely damaged equipment) will be vitally necessary.
Within the zone of SLIGHT damage, the main problems will be to prevent panic, to
ascertain that previously trained teams and groups are functioning properly, and to make such adaptations as are ordered by higher authority. One duty, even in this area, will be to watch for fires and get them under control.
The next few articles will explain the several kinds of damage that can be expected from nuclear explosions.
14D2. Air blast
This chapter has already mentioned that an air burst of a nuclear weapon above a target has tremendous destructive capability and, therefore, great military usefulness. For a short distance from the fireball, the blast damage from a surface burst may be even greater, but the effective range tends to be shorter. Blast damage can also stem from shallower subsurface bursts.
Blast damage from a nuclear explosion really has two distinct causes. One cause is the OVERPRESSURE that has already been defined and described. The other cause is the DRAG exerted by the nuclear windstorm.
OVERPRESSURE DAMAGE. A given point in space is subjected to peak overpressure when the primary blast wave (or, in the Mach region, the Mach wave) strikes it. This is the time when a structure or vehicle is most liable to collapse, as though from a hard blow. After the peak, the atmospheric pressure at the given point gradually drops back to normal. Shortly afterward, the pressure is reduced below normal by the suction phase of the explosion. The drop below normal is never as great as the previous rise above it; but it, too, can cause damage.
Massive, comparatively low buildings of reinforced concrete, and low masonry buildings strengthened by heavy steel skeletons, are the only structures likely to withstand 15 or more psi (pounds per square inch) of peak overpressure without severe damage. Light wood or masonry buildings-typical living accommodations-receive moderate damage from 2 to 3 psi.
Naval vessels are constructed to withstand battle shock and constant pounding from the waves. Peak overpressures of 5 psi cause light damage to most types of surface ships, while overpressures required for severe damage vary from 25 psi for destroyers to 40 psi for heavy cruisers. A ship's boilers, uptakes,
and ventilation system are especially vulnerable to overpressure.
Some tanks and other heavy-duty shore equipment have withstood 20 to 30 psi.
Strangely enough, the human body has been known to stand short-term overpressures up to 100 psi without severe or permanent damage. Of course, resistance to overpressure, like other resistances, varies with the individual. Nevertheless it is safe to assume that personnel injuries and fatalities traceable to overpressure will be caused indirectly-usually by the collapse of heavy equipment and structures.
DRAG DAMAGE. Drag (sometimes called DYNAMIC PRESSURE) refers to the effects of the WINDS, as distinguished from the pressure changes that cause and immediately precede these winds.
Article 14C2 mentioned that an air burst is characterized by violent winds blowing radially outward from ground zero and, a short time later, by afterwinds blowing inward. The drag of these winds is particularly destructive to lightweight walls, and to tall objects such as antennas and flagpoles. Power lines, bridge spans, and parked vehicles are also vulnerable to drag.
Drag, rather than overpressure, is the blast phenomenon that seriously threatens the many personnel who might otherwise suffer only slight injuries. The winds of a nuclear explosion can impel heavy or sharp objects with tremendous force, thus converting everyday materials into deadly weapons. A man who has survived peak overpressure intact may leave cover too soon, only to be killed by a brickbat hurled against his temple, or a glass splinter driven into or through his body.
Table 14D1 and figure 14D2 give some indication of the relationships between overpressure, wind velocity and dynamic pressure (drag force). Dynamic pressure is a function of the wind velocity and the density of air behind the shock front. Like the peak shock overpressure, the peak dynamic pressure decreases with increasing distance from the explosion center, although at a different rate as can be seen in the illustrations. (The dynamic pressure decreases more rapidly than does the shock overpressure.)
For the purpose of this orientation, let it be said that certain structures are more susceptible to damage by the drag forces inherent
Peak over-pressure (psi)
Peak dynamic pressure (psi)
Maximum wind velocity (mph)
with air blast, while others are more sensitive to shock overpressure.
Figure 14D3 indicates the damage-distance relationships for a sampling of structures more sensitive to overpressure. Other damage-distance relationship data is available in the bibliography.
NUCLEAR-HIGH EXPLOSIVE COMPARISON. Although the blast effects of nuclear and conventional explosives were compared in the beginning of this chapter, an additional difference between the two should be pointed out here. The combination of very high peak overpressures, together with the much longer duration of the positive phase of the blast wave from nuclear explosions, results in "mass distortion" of buildings and structures-similar to that caused by earthquakes. An ordinary explosion will usually damage only part of a large building, but the nuclear blast can surround and destroy it entirely.
Figure 14D2.-Variation of overpressure and dynamic pressure with time at a fixed location.
Figure 14D3.-Damage-distance relationships for structures (diffraction type).
When all or part of the fireball strikes or is formed below the surface, a shock front in the earth (or water) corresponds to the blast front in the air.
GROUND SHOCK. As has already been mentioned, ground shock resembles a small earthquake, except that it originates much nearer the surface.
Ground shock is a threat to land-based personnel, because it can demolish or damage underground shelters. In the bomb crater, of course, these would be totally destroyed. For a short distance beyond the actual crater, the zone of tot al destruction would continue. Beyond that would be a zone of heavy damage consisting of severe distortion and partial collapse.
The effects of underground shock tend to fall off rapidly, however, Too, when shock ceases to be severe, effects from it become almost negligible. In a subsurface burst, if any part of the fireball breaks through the surface, the blast damage above ground is likely to be more extensive than the shock damage below it.
Buried utility pipelines would be destroyed within the crater and would be damaged at distances up to three times the radius of the crater. Near the crater, the pipes themselves would rupture. Farther out, the joints, especially between horizontal pipes and risers, would tend to rupture.
Well constructed tunnels and subways, particularly in granite bedrock, are resistant to underground shock. Complete demolition would be likely to occur only within or near the bomb crater.
UNDERWATER SHOCK. A shock wave formed under the surface of the water behaves much like a similar wave in air. Since water is a denser fluid than air, the values of normal pressure and overpressure are correspondingly higher. The reduction after peak overpressure is more gradual than in air.
The shock wave in water may produce a reflected wave by striking the bottom or any rigid submerged object. If conditions are favorable, the primary wave and the reflected wave may fuse to produce a phenomenon comparable to the Mach front in air.
When the shock wave touches the upper (or air) surface of the water, a peculiar
phenomenon occurs. Because air is lighter and less resistant than water, the wave reflected back from the contact point is a RAREFACTION (or suction) wave. When the rarefaction wave reaches any given point below the surface, a sharp pressure reduction, called CUTOFF, occurs.
From the military point of view, cutoff is important because at points near the surface it follows so closely after peak overpressure that one phenomenon tends to neutralize the other, thus reducing the damaging power of the explosion. For shallowly submerged targets, therefore, nuclear weapons may not always be fully effective.
The primary shock wave of an underwater nuclear explosion strikes the target ship or other object with a sudden violent blow. In this action a nuclear weapon resembles a conventional one-with one significant difference. The conventional weapon delivers its blow at a single point or over a comparatively small area; while the nuclear explosion acts simultaneously over a large area with all encompassing force.
Underwater shock damages a vessel in one (or both) of two ways. First, it may rupture or at least weaken the hull. Second, it may distort, rupture, or break loose any of the various ship's components or installations. Piping, shafting, air vents, and boiler brickwork are susceptible to damage. Platforms supporting heavy equipment may be weakened or thrown out of proper alignment. Light objects may be thrown about so violently that they become a serious threat to personnel (missile hazards).
In the effects just mentioned, an underwater nuclear burst is similar to a conventional mine or depth charge. The major difference lies in the extended damage radius of the nuclear weapon.
14D4. Thermal radiation
Within a few milliseconds after the detonation of a nuclear weapon, intensely hot gases, at tremendously high pressures, rapidly form a highly luminous mass known as the "fireball" or "ball of fire." At about seven-tenths of a millisecond, the fireball from a 1-megaton nuclear weapon would appear to be more than 30 times as bright as the sun at noon to an observer 60 miles away. Although the size of
the fireball will vary with the bomb energy, the luminosity does not vary greatly. However, the larger the yield of the weapon, the longer will be the PERIOD of luminosity. Within this seven-tenths of a millisecond from time of detonation, the fireball of a 1-megaton weapon will have reached a diameter of 440 feet. The fireball increases to maximum diameter of about 7200 feet3 at plus 10 seconds. It is then rising at the rate of approximately 200 mph. After a minute, the ball of fire has cooled to an extent that it is no longer visible.
The nuclear explosion has often been compared to the conventional high explosive detonation in that, except for the yield and nuclear radiation involved, they can be considered similar. When referring to thermal effects, this can be a poor comparison because of the very large proportion of energy released as thermal radiation by a typical nuclear explosion. As was illustrated in figure 14C1, over one-third of the energy of a typical nuclear explosion manifests itself in the form of thermal radiation. Too, the temperatures involved in a nuclear explosion are much higher than with conventional explosives.
Thermal radiation travels with the speed of light, so that the time elapsing between its emission from the ball of fire and its arrival at a target a few miles away, is quite insignificant.
Much like the sun, the fireball radiates ultraviolet (short wave length) as well as visible and infrared (long wave length) rays. Due to certain phenomena associated with the absorption of thermal radiation by the air in front of the expanding fireball, the SURFACE temperature undergoes a curious change. While the interior temperature of the fireball falls steadily, the surface temperature decreases more rapidly for a small fraction of a second, then it increases again for a somewhat longer time, after which it falls continuously. In other words, there are effectively TWO SURFACE-TEMPERATURE PULSES-the first of very short duration, the second lasting for a relatively long period of time (see fig. 14D4). These surface-temperature pulses correspond to the pulses of thermal energy radiated from the fireball. In a
1-megaton nuclear explosion, the first pulse lasts for about a tenth of a second. The temperatures are very high, and much of the radiation is in the ultraviolet region. Moderately large doses of ultraviolet radiation can produce painful blisters. Even small doses can cause reddening of the skin. However, in most circumstances, the first pulse of thermal radiation is not a significant hazard. The situation with regard to the second pulse is quite different. This pulse may last for several seconds, and it carries about 99% of the total thermal radiation energy of the nuclear explosion.
Figure 14D4.-Emission of thermal radiation in two pulses.
The large amount of thermal radiation characteristic of the nuclear explosion has important consequences. For although most of the destruction from a nuclear air burst is the result of blast-thermal radiation will make a significant contribution to the overall damage through the ignition of combustible materials. Additionally, thermal radiation is capable of causing skin burns on exposed personnel at distances where the effects of blast and initial nuclear radiation are insignificant. This difference between the injury ranges of thermal radiation and the other effects mentioned becomes more marked with increasing nuclear weapon yield.
The most important physical effects of the high temperatures resulting from the
3Through the use of a scaling law, together with the results of various nuclear test explosions, it is possible to compute the fireball radius "R" for a nuclear weapon of "W" kilotons equivalent. R (in feet) = 230 W to the 2/5 power.
absorption of thermal radiation are: burning of the skin and scorching, charring, and possible ignition of combustible organic substances such as wood, fabrics, and paper.
Thin or porous materials, such as lightweight fabrics, newspaper, dried grass, and dried rotted wood, may flame when exposed to sufficient thermal radiation.
Table 14D2 supplies a comparison of approximate thermal energies required to produce a variety of physical effects.
Approx. cal/cm2 required:
Second-degree bare skin burn
White pine charring
Army khaki summer uniform destruction
Navy white uniform destruction
4Thermal energies are expressed in calories per unit area--square centimeter. Note that the amount of energy required for burning, charring, etc., varies inversely with the yield of the nuclear weapon. This is because of the rate at which the energy is delivered. For a given total amount of thermal energy received by each unit area of exposed material, the damage will be greater if the energy is delivered rapidly than if it is delivered slowly. This means that in order to produce the same thermal effect in a given material, the total amount of thermal energy (per unit area) received must be larger for a nuclear explosion of high yield than for one of lower yield, because the energy is delivered over a longer period of time, i.e. more slowly, in the former case.
Figure 14D5 indicates how thermal energy varies with distance for a selected yield nuclear weapon. The graph assumes a reasonably clear state of atmosphere.
Thick organic materials, such as plastics, heavy fabrics, and wood more than 1/2 inch thick, char but do not burn. Dense smoke, even jets of flame may be emitted, but the material does not sustain ignition. This type of behavior is illustrated in the photographs taken of a white-painted wood frame house during one of
Figure 14D5.-Thermal energy received at various slant ranges.
the nuclear tests in Nevada. Thus, figure 14D6a indicates that at virtually the instant of the explosion, the house became covered with thick black smoke, and no sign of flame. Very shortly thereafter, but before the arrival of the blast wave, the smoke ceased as indicated in figure 14D6b. Thin combustible material would probably burst into flame at the same location.
But perhaps the most serious consequence of thermal radiation is its ability to produce serious burn injury to personnel at long ranges. Figure 14D7 is included to show the ranges for moderate first-, second-, and third-degree burns from nuclear explosions. The graph is computed assuming a typical air burst with clear atmospheric conditions prevailing. For a typical surface burst, the distances would need to be scaled down to about 60% of those stated.
Figure 14D6.-a. Thermal effects on wood frame house almost immediately after explosion (about 25 cal/sq cm); b. thermal effects on wood frame house 2 seconds later.
Reading the graph, it can be seen that personnel exposed to a typical air burst (1 MT explosion) at 9 miles might be expected to receive moderate second-degree burns.
Conventionally, burns are classified according to their severity, in terms of degree (or depth) of injury. In first-degree burns there is only redness of the skin. A moderate sunburn is an example of a first-degree burn. Healing should occur without special treatment and there will be no scar formation.
Second degree burns are deeper, more severe, and are characterized by the formation of blisters. A severe sunburn is an example of a second-degree burn.
In third-degree burns, the full thickness of the skin is destroyed. Unless skin grafting techniques are employed, there will be scar formation at the site of the injury.
Figure 14D7.-Distances at which burns occur on bare skin.
The extent of the area of skin which has been burned is also important. Thus, a first degree burn over the entire body may be more severe than a third degree burn to one spot. The larger the area burned, the more likely is the appearance of symptoms involving the whole body. Further, there are certain critical, local regions, such as the hands, where almost any degree of burn will incapacitate the individual.
Thermal radiation can be the cause of flash burns or flame burns. Flash burns are directly caused by the radiant energy of the fireball. Flame burns are distinguished from flash burns in that they are caused by fire, no matter what the origin. Flame burns occur as a secondary result of thermal radiation, for example, those resulting from the fires started by thermal radiation.
A highly significant effect of the nuclear explosion is the very large number of flash burns. This was one of the most striking facts about the nuclear bombing of Japan in World War II. It has been estimated that 20 to 30 percent of the fatal casualties at Hiroshima and Nagasaki were due to flash burns, as distinct from flame burns. Though significant, it should be realized that these illustrated results were magnified due to the fact that the atmosphere was very clear and that the summer clothing worn was light and scanty.
Another danger of a nuclear explosion is its possible effect on the eyes. Thermal radiation can cause both retinal burns and flash blindness.
Because of the focusing action of the lens of the eye, enough energy can be collected to produce a burn on the retina at such distance from a nuclear explosion that the thermal radiation intensity is too small to produce a skin burn. As a result of accidental exposures during nuclear tests, a few retinal burns have been experienced at a distance of 10 miles from the explosion of a 20-KT weapon. It is believed that under suitable conditions, such burns might have resulted at even greater distance. Retinal burns occur so soon after the explosion that reflex actions, such as blinking and contraction of the eye pupil, give only limited protection. In all instances, there will be at least a temporary loss of visual acuity, but the ultimate effect will depend on the severity of the burn and on its location on the retina.
Because of the more or less remote chance that an individual will be looking directly at the ball of fire, the chance of temporary "flash blindness" or "dazzle," due to the flooding of eye with brilliant light is much more prevalent than retinal burns. Flash blindness is of a temporary nature and vision is regained within a comparatively short time. However, flash blindness is of military significance, since it may extend to 2 or 3 hours.
When thermal radiation falls upon any material or object, part may be reflected, part will be absorbed, and the remainder, if any, will pass through and ultimately fall upon other material. It is the amount of radiation ABSORBED by a particular material that produces heat and so determines the damage suffered by that material. Highly reflecting and transparent substances do not absorb much of the thermal radiation and so are relatively resistant to its effects. A thin material will
often transmit a large proportion of the radiation falling upon it, and thus escape serious damage. A dark fabric will absorb a much larger proportion of thermal radiation than will the same kind of fabric when it is white. However, a light-colored material which blackens (or chars) readily in the early stages of exposure to thermal radiation will behave essentially as a dark material regardless of its original color.
Unless scattered, thermal radiation travels in straight lines like ordinary light. For this reason any solid, opaque material, such as a bulwark, gun shield, hill, or tree, between a given object and the fireball will act as a SHIELD and thus provide protection from direct thermal radiation.
ATMOSPHERIC CONDITIONS also play a part in the amount of thermal radiation received by a particular object. However, they do not play as important a part in attenuating thermal radiation as was once suspected. When visibility is in excess of 2 miles (light haze or clearer), the total amount of thermal radiation received will be essentially the same as that on an "exceptionally clear" day (visibility more than 30 miles). This is because any decrease in direct radiation is largely compensated for by an increase in scattered radiation.
When visibility is less than 2 miles because of rain, fog, or dense industrial smoke, there will be a definite decrease in radiant energy received at any specified distance.
CLOUDS can also affect the amount of radiant energy received. For example, if an explosion occurs above a cloud layer, there will be considerable attenuation at ground level. Conversely, should an explosion occur beneath a cloud layer, some of the radiation which would normally have been lost to space will be scattered back to earth.
Artificial white (chemical) SMOKE can be used to attenuate thermal radiation, for it acts like fog in this respect. A dense smoke screen between the point of burst and a given target can reduce thermal radiation to as little as one-tenth of the amount which would otherwise have been received at the target.
14D5. Nuclear radiation
It has been previously pointed out that 15% of the total energy yield of a typical nuclear
weapon is distributed in the form of nuclear radiations (fig. 14C1). Let us explore further what this radiation consists of, how it occurs, and what its dangers are.
In any nuclear explosion there is an initial flux of radiations consisting mainly of gamma rays and neutrons. Both of these (especially gamma radiation) travel great distances through the air, and can penetrate great thicknesses of material. Remaining within the fireball are fission products and unfissioned bomb5 material. These fission products and unfissioned bomb material are also radioactive, and emit gamma rays and beta particles6. This emission of beta particles and gamma rays from the radioactive substance is a gradual process, and its hazard therefore remains over a significant period of time.
INITIAL NUCLEAR RADIATION is arbitrarily7 defined as that radiation emitted within (approximately) the first minute after the explosion. Initial nuclear radiation includes those neutrons and gamma rays given off almost instantaneously, as well as the gamma rays given off by the radioactive fission products in the rising cloud8.
It follows that RESIDUAL RADIATION is that emitted after approximately one minute from the instant of a nuclear explosion. This radiation originates mainly from the bomb residues; that is, from the fission products and, to a lesser extent, from the uranium and/or plutonium which has escaped fission. Additionally, the residues will usually contain some radionuclides as a result of "neutron capture" by other weapon materials. Still another source of residual nuclear radiation is the activity induced by neutrons9 captured in
various elements present in the explosion environment.
All of the nuclear radiation discussed thus far in this section is the result of fission reactions. As the student will recall, there are no fission products associated with the fusion reaction. Rather, neutrons are the only significant nuclear radiations produced in pure fusion reactions. Thus, it can be seen that for explosions in which both fission and fusion (thermonuclear) processes occur, the proportions of specific radiations will differ from those of typical fission explosions. However, for present purposes, the difference may be disregarded.
As the height of burst of a nuclear explosion occurs nearer the surface of the earth (or sea) larger and larger proportions of the earth (or water) enter the fireball and are fuzed or vaporized. When sufficient cooling has occurred, the fission products become incorporated with the earth particles as a result of the condensation of the vaporized products into fuzed particles of earth, etc. As the violent disturbance due to the explosion of the nuclear weapon subsides, these contaminated particles fall gradually back to the earth. This effect is referred to as the FALLOUT. The extent and nature of the fallout can range between wide extremes-dependent on the energy yield and design of the bomb, the height of the explosion, the nature of the surface beneath the point of burst, and the meteorological conditions. In the case of an AIR BURST occurring at an appreciable distance above the earth's surface, so that no large amounts of dirt (or water) are sucked into the cloud, the inherent radiation will be widely dispersed. On the other hand, a nuclear explosion occurring at or near the earth's
5Bomb, as it is used here, refers specially to the fissionable material in the nuclear weapon.
6In an analogous manner, ALPHA PARTICLES are also expelled as the result of the natural decay of the uranium (or plutonium) which has escaped fission in the explosion.
7This arbitrary amount has its origin with the classic 20-KT air burst, where because of the height of burst and the range of nuclear radiation, there are no significant radiological effects after one Minute of time. For nuclear weapons of higher energy, the range over which the gamma rays are effective will be larger than that for the 20-KT air burst, but the rate at which the atomic cloud rises is higher. The converse is true for bombs of lower energy, so that the period over which the initial nuclear radiation extends may be taken as essentially the same--approximately one minute, irrespective of the energy release of the bomb.
8It should be noted that, although alpha and beta particles are present in the initial radiation, they are not considered significant for they are so easily absorbed. They will not reach more than a few yards, at the most, from the atomic cloud.
9Radioactivity induced by gamma rays from a nuclear explosion is either insignificant or completely absent.
surface can result in SEVERE contamination by the radioactive fallout.
It should be understood that fallout is a gradual phenomenon extending over a period of time. There can be considerable fallout-many hours after the surface detonation of a nuclear weapon, and many miles away. Additionally, there is a phenomenon called WORLD WIDE FALLOUT which occurs for years after a nuclear explosion. Fallout that occurs within 24 hours of a nuclear explosion is referred to as LOCAL FALLOUT
Fission products, which make up the greatest hazard in residual radiation, are initially very radioactive. However, this activity falls off at a fairly rapid rate as the result of decay. Figure 14D8 shows the exponential rate of decay of fission products after a nuclear explosion.
Figure 14D8.-Rate of decay of fission products after a nuclear explosion.
The RADIOLOGICAL EFFECTS from a typical AIR BURST are completely overshadowed by the effects of blast and thermal radiation. An exception to this would be a "low" air burst of a high yield weapon where there would be extensive induced radioactivity in the vicinity of ground zero. Radiological effects might also be of some consequence to those persons shielded from the primary causes of casualties.
A SURFACE BURST nuclear explosion presents an entirely different picture. With a surface burst, even though the induced activity will be considerable, the activity of the
FALLOUT will be of so much greater consequence that the former may be neglected in comparison
The surface burst causes large amounts of earth (water), dust, and debris to be taken up into the fireball in its early stages. Here they are fuzed or vaporized and become intimately mixed with the fission products and other bomb residues. As a result there is formed, upon cooling, a tremendous number of small particles contaminated to some distance below their surfaces with radioactive matter. In addition, there are considerable quantities of pieces and particles, covering a range of sizes from large lumps to fine dust, to the surfaces of which fission products are more or less firmly attached.
The larger (heavier) pieces, which will include a great deal of contaminated material scoured and thrown out of the crater, will not be carried up into the mushroom cloud, but will descend from the column. Provided the wind is not excessive, these large particles, as they fall, will form a roughly circular pattern around ground zero (though the circle will be somewhat eccentric as the result of any wind). Most of this heavier material referred to above will descend within an hour or so.
The smaller particles present in the atomic cloud will be carried up to a height of several miles, and may spread out some distance in the mushroom cloud before they begin to descend. The actual time taken to return to the earth, and the horizontal distance traveled, will depend upon the original height attained, the size of the particles, and upon the wind in the upper atmosphere.
The fraction of the total radioactivity of the bomb residues that appears in the fallout depends upon the extent to which the fireball touches the surface. Thus, the proportion of available activity increases as the height of the burst decreases and more of the fireball comes in contact with the earth (or water). In the case of a "contact burst," some 50% of the total residual radioactivity will be deposited on the ground within a few hundred miles of the explosion. The remainder of the activity will remain suspended for a long period of time as with an air burst.
As a general rule, the pattern of contamination will be as illustrated in figure 14D9. Of course this pattern will vary with the wind velocities and directions at all altitudes between the ground and the height of the atomic cloud.
Figure 14D9.-a. Dose rate contours from fallout at 1, 6, and 18 hours after a surface burst of a nuclear weapon in the megaton range (115 mph effective wind); b. total accumulated dose contours from fallout at 1, 6, and 18 hours after a surface blast with fission yield in the megaton range (115 mph effective wind).
Note that the areas downwind are not immediately contaminated. Rather, most of the downwind area will not be seriously contaminated until hours after the explosion. For an example (fig. 14D9a), a location 32 miles downwind will have a DOSE RATE of about 30 roentgens/hour one hour after the detonation. At 6 hours, the dose rate has increased to 800 r/hr. Finally at 18 hours, it is down to roughly 200 r/hr. The increase in dose rate from 1 to 6 hours means that fallout was not complete at 1 hour after the explosion. With respect to the ACCUMULATED DOSE received, figure 14D9b shows that at one hour, the stipulated point will not have received any appreciable radiation because the fallout has only started to arrive. While at the end of 6 hours, the total dose has reached over 3000 roentgens. In general then, at any given location, at a distance from a surface burst, some time elapses before the fallout arrives.
Although the example given above is for the surface burst of a high fission yield nuclear weapon, the fallout phenomena associated with a low fission yield weapon are essentially the same except for differences in degree. Thus, a high energy fission yield explosion will mean a larger area contaminated to a more serious extent than would a low fission yield weapon.
The extent of residual radiation accompanying an UNDERGROUND BURST will depend primarily on the depth of burst and the weapon yield. With regards to initial radiation, it is either non-apparent or inconsequential by comparison to the residual radiation.
If the explosion occurs at sufficient depth below the surface, essentially none of the bomb residues and neutron-induced radioactive materials will escape to the atmosphere. There will be no appreciable fallout.
On the other hand, if the burst is near the surface so that the ball of fire actually breaks through, the consequences as regards fallout will not vary greatly from those of a surface burst. Other circumstances being more or less equal, the contamination in the crater area following an underground burst will be about the same as for a surface explosion of equal fission yield. However, the total contaminated area for a shallow underground burst will be greater because of the larger amount of fission products present in the fallout.
Radiological effects of UNDERWATER BURSTS closely parallel those of underground origin. The base surge, consisting of a contaminated cloud or mist of small water droplets also has a parallel in the underground phenomena. It is interesting to note that experts are placing lesser significance on the base surge as a source of contamination. It is now felt that though the base surge will materially contribute to the overall contamination, "rain-out" from the atomic cloud is of more consequence.
An important difference between an underwater burst and one occurring under the ground, is that the radioactivity remaining in the water is gradually dispersed, whereas that in the ground is not. Therefore, as a result of diffusion of the various bomb residues, mixing with large volumes of water outside the contaminated area, and the natural decay, the radiation intensity of the water in which a nuclear explosion has occurred will decrease fairly rapidly. Additionally, fission products will settle to the bottom of the body of water, thus greatly attenuating the radiological hazards.
Of specific naval interest is the fact that, after being distilled, contaminated sea water is perfectly safe for drinking. This is because the radioactive material remains behind in the residual scale and brine of the distillation process. It should be emphasized, however, that the mere boiling of water is of no value as regards the removal of radioactivity.
As the student will recall from chapter 12, the injurious effects of nuclear radiation represents a phenomenon completely absent in conventional explosions. For this reason, the subject of RADIATION INJURY will be discussed here in more detail.
The harmful effects of radiation appear to be due to the ionization (and excitation) produced in the cells that make up living tissue. As a result of ionization, some of the constituents that are essential to normal functioning are damaged or destroyed. Some of the products formed may act as cell poisons. Additionally, the living cells are frequently unable to undergo mitosis, so that normal cell replacement is inhibited.
The effects of nuclear radiations on living organisms depend not only on the total dose, that is, on the amount absorbed, but also on
the rate10 of absorption, i.e. on whether it is ACUTE or CHRONIC. In an acute exposure, the whole radiation dose is received in a relatively short period of time. It has somewhat arbitrarily been defined as that dose received during a 24-hour period. Delayed radiations, like those which may be received from fission products, persist over a longer period of time and this type of exposure is of the chronic type.
The distinction between acute and chronic exposure lies in the fact that, if the dose rate is not too high, the body can achieve partial recovery from some of the consequences of the nuclear radiations while still exposed. In addition to the above, the percent of body exposure has significance. It follows then, that whereas a person would most probably die as the result of acute exposure of 700 roentgens whole-body radiation, he would probably suffer no critical effects if the dose were spread over a year-or if it were localized to a hand or foot.
The data provided in tables 14D3 and 14D4 are also plotted in figure 14D10. Each shows the effects of acute, whole-body radiation.
It can be noted in both the tables and the illustration that a particular effect is associated with a range of exposure doses. The reason for this uncertainty is that there are many factors, some known and some unknown, which determine the effect on the body of a specified
Figure 14D10.-Incidence of sickness and death due to
acute exposure to various doses of nuclear radiation.
Table 14D3.-Expected Effects of Acute Whole-Body Radiation Doses
Acute dose (roentgens)
0 to 50
No obvious effect, except possibly minor blood changes.
80 to 120
Vomiting and nausea for about 1 day in 5 to 10 percent of exposed personnel. Fatigue but no serious disability.
130 to 170
Vomiting and nausea for about 1 day, followed by other symptoms of radiation sickness in about 25 per cent of personnel. No deaths anticipated.
180 to 220
Vomiting and nausea for about 1 day, followed by other symptoms of radiation sickness in about 50 percent of personnel. No deaths anticipated.
270 to 330
Vomiting and nausea in nearly all personnel on first day, followed by other symptoms of radiation sickness. About 20 percent deaths within 2 to 6 weeks after exposure; survivors convalescent for about 3 months.
400 to 500
Vomiting and nausea in all personnel on first day, followed by other symptoms of radiation sickness. About 50 percent deaths within 1 month; survivors convalescent for about 6 months.
550 to 750
Vomiting and nausea in all personnel within 4 hours from exposure followed by other symptoms of radiation sickness. Up to 100 percent deaths; few survivors convalescent for about 6 months
Vomiting and nausea in all personnel within 1 to 2 hours. Probably no survivors from radiation sickness.
Incapacitation almost immediately. All personnel will be fatalities within 1 week.
radiation exposure dose. For in addition to the biological variations among individuals, there are such considerations as the ages of exposed personnel and their state of health, depth of penetration into the body and the organs absorbing the radiation, and the orientation of the body with reference to the source of the
10A few radiation phenomena, such as the genetic effects, apparently depend on only the total dose received and are independent of the rate of delivery.
Table 14D4.--Summary of Clinical Symptoms of Radiation Sickness
radiation (possible shielding of one part of the body with another).
A further matter of note is that the sooner the symptoms of radiation sickness appear
after exposure, the more serious the consequences will be. Additionally, there is a latent period between the first symptoms of radiation exposure and a further condition of sickness.
E. Atomic Warfare Defense
Foresightedness and an understanding of the effects of nuclear weapons will have great bearing on survival in event of nuclear warfare. In general, there are two broad
categories of protection that can be used to avoid the stunning effects of nuclear weapons. They are DISTANCE and SHIELDING. In other words, it is necessary to get beyond the reach of the effects, or to provide protection against them within their radii of damage.
The naval forces afloat have a distinct advantage in that they are readily dispersible. In addition to this, the ships of the Navy are so designed that they are comparatively resistant to the blast (and/or shock) and the thermal effects of nuclear weapons. Too, the features built into Navy ships for protection against gas attacks provide some measure of protection against radiation hazards.
Both long- and short-range preparation goes into proper readiness of Navy ships. Strict specifications are set for the designers and builders in order that the ships are as resistant as feasible to the effects of nuclear weapons. Command action is taken to keep fire and missile hazards minimized. Tactics include such things as greater than normal dispersal, the placing of all possible personnel under cover, establishing the highest state of material readiness, and the activation of WATER WASHDOWN systems.
After a nuclear explosion, the primary problem is to keep and/or return the particular ship to a maximum state of readiness by preventing the avalanching casualties resulting from secondary effects, by restoring normal services or rigging alternate (or emergency) services, attending to the wounded, conducting radiological surveys and decontaminating or localizing as applicable, and by assessing the extent and nature of damage and restoring the unit as nearly as possible to its original condition.
Tactics included after a nuclear explosion would include maneuvering to avoid, or minimizing the transit time through, any base surge, fallout areas, or radioactive waters.
Ashore, the military must also plan to disperse and/or provide suitable protection for personnel and material. The civil defense authorities should plan accordingly for the civilian population. Much can be done towards reducing blast and fire hazards in existing structures. Shelters and shelter areas can be provided. Disaster teams can be organized and trained to keep losses to a minimum.
14E2. Protective measures-individual action
For an individual, in the event of a surprise attack, proper and immediate action can mean the difference between life and death.
From experience gained in both nuclear and conventional explosions, there is little doubt
that as a general rule it is more hazardous in the open than inside a structure. In an emergency, therefore, the best available shelter should be taken.
Aboard ship, TAKE COVER should be directed at the appropriate time for those in exposed stations. Once properly shielded, and other operations permitting, personnel should take a position with knees flexed, and with a firm grip on a substantial piece of the ship's structure. This position should be held until passage of the blast and/or the shock wave.
Ashore, civil defense authorities (or military, where they have jurisdiction) should have designated shelter areas and/or shelters. Subways would provide a good emergency shelter; however, these are found in only a limited number of cities. As an alternative, the basement of a building should be chosen. In this connection, a fire-resistant, reinforced-concrete or steel frame structure is to be preferred, since there is less likelihood of a large debris load on the floor above the basement. Even basements of good buildings are not, however, an adequate substitute for a well designed shelter.
Should there not be any opportunity to take the best shelter, alternate immediate action will be necessary. The first indication of an unexpected nuclear explosion (other than a subsurface explosion) would be a sudden increase in the general illumination. It would be imperative to avoid the instinctive tendency to look at the source of light, but rather to do everything possible to cover all exposed portions of the body (another reason for proper and suitable battle dress). A person inside a building should immediately fall prone and crawl behind a table or desk. This will provide a partial shield against splintered glass and other flying missiles. No attempt should be made to get up until the blast wave has passed, as indicated by the breaking of glass, cracking of plaster, and other signs of destruction. The sound of the explosion also signifies the arrival of the blast wave.
A person caught in the open by the sudden brightness due to a nuclear explosion, should drop to the ground, while curling up to shade the arms, hands, neck, and face with the clothed body. Although this action will have little effect against the initial nuclear radiation, it may help in reducing flash burns due to the thermal radiation. Of course, the degree
of protection from thermal effects will vary with the energy yield of the explosion. For as you will recall, low yield weapons expel all their thermal radiation in a short interval of time, while the higher the yield, the longer the thermal pulses of energy will last. Nevertheless, there is nothing to be lost, and perhaps much to gain through such action. The curled-up position should be held until after the blast wave has passed.
If a shelter of some kind, no matter how minor, e.g. in a doorway, behind a tree, or in a ditch or trench, can be reached within a second, it may be possible to avoid a significant part of the initial nuclear radiation, as well as the thermal radiation. But shielding from nuclear radiation requires considerable thickness of material and this may not be available in the open. By dropping to the ground, some little advantage may be provided by the ground and surrounding objects.
14E3. Protection from fallout
Protection against the residual radioactivity present in LOCAL FALLOUT presents a number of difficult and involved problems. This is because the radioactive products are not normally visible11 and require radiac equipment for detection and measurement, and
because of the widespread and persistent character of the fallout, and too, because fallout prediction is a function of complicated meteorological processes.
Ships will have to depend on proper maneuvers, the GAS TIGHT ENVELOPE, water wash-down systems and decontamination procedures for protection. Ashore, the civil defense and/or military authorities must be depended on for any evacuation of contaminated areas. Radiological surveys must be made to ascertain the extent and nature of the contamination. Once this is known, it is possible to take other corrective actions, such as orderly evacuation of sheltered survivors and decontamination of essential areas or equipments. Shifting winds and other unknown variables complicate any prediction of safe evacuation routes. A person may leave a comparatively safe location and end up the loser for his effort.
Of the passive protective measures that can be taken, shelter is the foremost. Where approved shelters are not available, even the basement of a frame house can attenuate nuclear radiation by a factor of about 10. Greater reduction is possible in large buildings or in shelters covered with several feet of earth. Three feet of earth will provide a radiation attenuation factor in the neighborhood of 1000.
F. Employment of Nuclear Weapons Effect
Many factors enter into the selection of the burst height (or depth) and yield of a particular weapon. Among these are fusing limitations, available delivery systems, and the degree of damage desired.
From an EFFECTS standpoint, the basic criteria which govern weapon selection are peak blast wave overpressure, peak dynamic pressure, duration of the positive wave (of blast wave), crater extent, thermal radiation, initial nuclear radiation, residual fission product fallout, and induced ground contamination.
The actual mechanics of weapon selection is a very complex operation. This operation is the function of relatively high echelons of command. The student should be aware that there also are great moral and political issues involved in the use of nuclear weapons. For these reasons, the actual committing of nuclear weapons to use by our country is the responsibility of the President of the United States. Notwithstanding, some generalized statements concerning the relative importance of various effects for different burst conditions is considered essential to a complete orientation in the nuclear weapons subject area.
11Although there are cases on record, where the fallout was visible as a white powder or dust, different circumstances would probably have made it impossible to see the fallout.
14F2. Surface burst
A SURFACE BURST will increase the range at which peak overpressures greater than about 12 psi occur. It will reduce thermal radiation received by ground targets compared to that received from an air burst at the same slant range and it will produce significant cratering and ground shock. A peak overpressure of 12 psi will cause severe damage to all structures except those of reinforced-concrete, blast-resistant construction. It will also cause moderate to severe damage to most military equipments. Most naval ships operating today will receive moderate damage (immobilization) when subjected to 20 psi peak air overpressure. Five psi will cause light damage to all naval and mercantile shipping. Light damage to naval ships consists of damage to electronic, electrical, and mechanical equipments-however the ships may still be able to operate effectively.
The surface burst will overdestroy some area. It is therefore not as economical (in its damage capabilities) as an air burst. Conceivably, therefore, the surface burst would be used against resistant targets or where assured destruction is desirable.
The LOCAL FALLOUT associated with a surface burst is a very significant factor in nuclear weapons selection.
14F3. Air burst
An AIR BURST will increase the ground range at which overpressures of about 10 psi or less are obtained; maximize areas at which significant thermal radiation is received on the ground; and eliminate local fallout contamination. Windowpane breakage is associated with 0.5 psi overpressure, while severe damage to wood frame houses occurs with 3 psi, and to reinforced-concrete buildings with approximately 10 psi12.
Figure 14F1 accumulates certain cardinal damage criteria for air burst explosions.
Figure 14F1.-Limiting distances from ground zero at which various effects are produced in an air burst.
14F4. Subsurface burst
With a SUBSURFACE BURST, peak air overpressure, thermal radiation, and initial nuclear radiation decrease as the depth of the burst is increased. Cratering, ground (or water) shock, and fallout contamination will increase with the depth of burst up to a maximum (the optimum depth depends on the effect being considered) and then decrease. Maximum water waves will be produced at a certain critical depth of burst.
12Because the degree of damage depends upon the duration of blast, the same structure would require 10.5, 9.5, and 9 psi overpressure with yields of 1 KT, 100 KT, and 10 MT, respectively.
Bibliography for Nuclear Weapons Orientation
Basic Nuclear Physics, NavPers 10786
Atomic Weapons General Information, OP 2508 (classified)
The Effects of Nuclear Weapons, Gov't Printing office (1957)
Atomic Warfare Defense, NavPers 10097
Capabilities of Atomic Weapons, OpNav Instruction 03400.1B (classified)