REACTOR OPERATIONDOE-HDBK-1019/2-93Reactor Theory (Reactor Operations)Reactor Operation Summary (Cont.)Three methods are used to shape or flatten the core power distribution.Use of reflectorsInstallation of neutron poisonsAxial or radial variation of fuel enrichment Power tilt is a non-symmetrical variation of core power in one quadrant of thecore relative to the other quadrants.Shutdown margin is the instantaneous amount of reactivity by which a reactoris subcritical or would be subcritical from its present condition assuming allcontrol rods are fully inserted except for the single rod with the highest integralworth, which is assumed to be fully withdrawn.The stuck rod criterion is applied to the shutdown margin to ensure that thefailure of a single control rod will not prevent the control rod system fromshutting down the reactor.Several factors may change during and after the shutdown of the reactor thataffect the reactivity of the core.Control rod positionSoluble neutron poison concentrationTemperature of the fuel and coolantXenonSamariumDecay heat is always present following reactor operation due to energy resultingfrom the decay of fission products.The amount of decay heat present in the reactor is dependent on three factors.The pre-shutdown power levelHow long the reactor operatedThe amount of time since reactor shutdownDecay heat immediately after shutdown is approximately 5-6% of the pre-shutdown power level. Decay heat will decrease to approximately 1% of thepre-shutdown power level within one hour of reactor shutdown.NP-04Rev. 0Page 34
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