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Shutdown - h1019v2_124
Reactor  Operation  Summary (Cont.)

Nuclear Physics and Reactor Theory Volume 2 of 2
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Reactor Theory (Reactor Operations) DOE-HDBK-1019/2-93 REACTOR OPERATION Decay  Heat 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. Summary 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. Rev. 0 NP-04 Page 33







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