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