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.

Summarv

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.

Three methods are used to shape or flatten the core power distribution. Use of reflectors

Installation of neutron poisons

Axial or radial variation of fuel enrichment

Power tilt is a non-symmetrical variation of core power in one quadrant of the core relative to the other quadrants.

Shutdown margin is the instantaneous amount of reactivity by which a reactor is subcritical or would be subcritical from its present condition assuming all control rods are fully inserted except for the single rod with the highest integral worth, which is assumed to be fully withdrawn.

The stuck rod criterion is applied to the shutdown margin to ensure that the failure of a single control rod will not prevent the control rod system from shutting down the reactor.

Several factors may change during and after the shutdown of the reactor that affect the reactivity of the core.

Control rod position

Soluble neutron poison concentration Temperature of the fuel and coolant Xenon

Samarium

Decay heat is always present following reactor operation due to energy resulting from the decay of fission products.

The amount of decay heat present in the reactor is dependent on three factors. The pre-shutdown power level

How long the reactor operated

The amount of time since reactor shutdown

Decay heat immediately after shutdown is approximately 5-6% of the preshutdown power level. Decay heat will decrease to approximately 1% of the pre-shutdown power level within one hour of reactor shutdown.