The mechanism shown raises and lowers the control rods. The control rods block the movement of neutrons and thereby reduce the reactivity and heat generated. They are slowly raised to start the nuclear reaction. When needed they can be reinserted in seconds. This is called a scram (from scramble) to very quickly shut down the nuclear reaction.
A special high pressure system is used to seal around the control rods and prevent leaking of the high pressure water in the reactor. The pumps and controls for this system are located under the fin stabilizers shown later in the tour.
The plastic sheeting shown was part of the recent cleaning process.
Looking down at the fuel rod assemblies separated by the + shaped control rods.