Reactor Theory (Reactor Operations)
About 7 percent of the 200 MeV produced by an average fission is released at some time after
the instant of fission. This energy comes from the decay of the fission products. When a
reactor is shut down, fission essentially ceases, but decay energy is still being produced. The
energy produced after shutdown is referred to as decay heat. The amount of decay heat
production after shutdown is directly influenced by the power history of the reactor prior to
shutdown. A reactor operated at full power for 3 to 4 days prior to shutdown has much higher
decay heat generation than a reactor operated at low power for the same period. The decay heat
produced by a reactor shutdown from full power is initially equivalent to about 5 to 6% of the
thermal rating of the reactor. This decay heat generation rate diminishes to less than 1%
approximately one hour after shutdown. However, even at these low levels, the amount of heat
generated requires the continued removal of heat for an appreciable time after shutdown. Decay
heat is a long-term consideration and impacts spent fuel handling, reprocessing, waste
management, and reactor safety.
The important information in this chapter is summarized below.
Reactor Operation Summary
An installed neutron source, together with the subcritical multiplication process,
may be needed to increase the neutron population to a level where it can be
monitored throughout the startup procedure.
Reactivity balances, such as Estimated Critical Position calculations, typically
consider the basic reactivity of the core and the reactivity effects of temperature,
direct xenon, and indirect xenon.
A reactivity balance called an Estimated Critical Position is used to predict the
position of the control rods at which criticality will be achieved during a startup.
To arrive at an ECP of the control rods, the basic reactivity, direct and indirect
xenon reactivity, and temperature reactivity are added together to determine the
amount of positive reactivity that must be added by withdrawing control rods to
attain criticality. A graph of control rod worth versus rod position is used to
determine the estimated critical position.