DOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)SAMARIUM AND OTHER FISSION PRODUCT POISONSsufficient amount of tritium may decay to helium-3 to add a significant amount of negativereactivity. Any helium-3 produced in the reactor during a shutdown period will be removedduring subsequent operation by a neutron-proton reaction.SummaryThe important information in this chapter is summarized below.Samarium and Other Fission Product Poisons SummarySamarium-149 is produced directly from fission and from the decay ofpromethium-149 during reactor operation. Samarium-149 is removed from the core by neutron absorption.The equation for equilibrium samarium-149 concentration is stated below.NSm(eq) g Pm SfuelfsSmaThe equilibrium samarium-149 concentration is independent of power level.Following a reactor shutdown, the samarium-149 concentration increases due to thedecay of the promethium-149 inventory of the core and the loss of the burnupfactor.If the reactor is restarted following a shutdown, the samarium-149 concentrationdecreases as samarium is burned up and returns to its equilibrium operating value.Helium-3 will become a significant neutron poison if significant amounts of tritiumare left in a reactor during a shutdown period that lasts longer than several months.Rev. 0NP-03Page 47
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