Every 12 to 24 months (also called a cycle), the pressurized water reactor must be refueled. During a refueling, fuel assemblies that are the oldest and have typically been in the reactor for 3 to 6 years are removed. Fuel assemblies that have been in the reactor for 1 or 2 cycles are typically moved to new locations in the reactor. These changes are made to ensure that the power distribution across, and along, the reactor is optimum (not too high or too low) throughout the future cycle. Typically newer fuel assemblies are loaded towards the outer sections of the reactor.
The photo on the left shows a VVER reactor after completion of a refueling. The reactor vessel head cover has been installed. The rods seen sticking out of the top are the control rods. Cables will then be connected for operating the control rods and for providing indication of position of the control rods to the operators in the control room. The reactor will subsequently be heated and pressurized to the normal operating conditions (about 550F and 2250 pounds per square inch).
The photo on the right shows a reactor that has had the power cables attached for operating the control rods and for the rod position indication.
Copyright © 1996-2005. Joseph Gonyeau, P.E.. The Virtual Nuclear Tourist. All rights reserved. Revised: March 15, 2001.