DOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)SAMARIUM AND OTHER FISSION PRODUCT POISONSThe fission yield of samarium-149, however, is nearly zero; therefore, the equation becomes thefollowing.dNSmdtlPm NPm NSm sSmafSolving this equation for the equilibrium concentration of samarium-149 and substituting gPmSffuelf/ lPmfor NPm(eq) yields the following.NSm(eq) g Pm SfuelfsSmaThis expression for equilibrium samarium-149 concentration during reactor operation illustratesthat equilibrium samarium-149 concentration is independent of neutron flux and power level. Thesamarium concentration will undergo a transient following a power level change, but it will returnto its original value.Samarium-149ResponsetoReactorShutdownSince the neutron flux drops to essentially zero after reactor shutdown, the rate of samarium-149production becomes the following.dNSmdtlPm NPmBecause samarium-149 is not radioactive and is not removed by decay, it presents problemssomewhat different from those encountered with xenon-135, as illustrated in Figure 7. Theequilibrium concentration and the poisoning effect build to an equilibrium value during reactoroperation. This equilibrium is reached in approximately 20 days (500 hours), and sincesamarium-149 is stable, the concentration remains essentially constant during reactor operation.When the reactor is shutdown, the samarium-149 concentration builds up as a result of the decayof the accumulated promethium-149. The buildup of samarium-149 after shutdown dependsupon the power level before shutdown. Samarium-149 does not peak as xenon-135 does, butincreases slowly to a maximum value as shown in Figure 7. After shutdown, if the reactor isthen operated at power, samarium-149 is burned up and its concentration returns to theequilibrium value. Samarium poisoning is minor when compared to xenon poisoning. Althoughsamarium-149 has a constant poisoning effect during long-term sustained operation, its behaviorduring initial startup and during post-shutdown and restart periods requires special considerationsin reactor design.Rev. 0NP-03Page 45
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