DOE-HDBK-1019/2-93SAMARIUM AND OTHER FISSION PRODUCT POISONSReactor Theory (Nuclear Parameters)For the purpose of examining the behavior of samarium-149, the 1.73 hour half-life ofneodymium-149 is sufficiently shorter than the 53.1 hour value for promethium-149 that thepromethium-149 may be considered as if it were formed directly from fission. This assumption,and neglecting the small amount of promethium burnup, allows the situation to be described asfollows.Rate of change of 149Pm = yield from fission - decay 149Pm concentrationtherefore:dNPmdtg Pm Sfuelff lPm NPmwhere:NPm=149Pm concentrationgPm=149Pm fission yieldlPm=decay constant for 149PmSolving for the equilibrium value of promethium-149 gives the following.NPm(eq) g Pm SfuelfflPmThe rate of samarium-149 formation is described as follows.Rate of change of 149Sm = yield from fission + 149Pm decay - 149Sm burnuptherefore:dNSmdtg Sm Sfuelff lPm NPm NSm sSmafwhere:NSm=149Sm concentrationgSm=149Sm fission yieldsaSm=microscopic absorption cross section of 149SmNP-03Rev. 0Page 44
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