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 ^{149}Pm = yield from fission - decay ^{149}Pm concentrationtherefore:dN_{Pm}dtg Pm Sfuelff lPm NPmwhere:N_{Pm}=^{149}Pm concentrationgPm=^{149}Pm fission yieldlPm=decay constant for ^{149}PmSolving for the equilibrium value of promethium-149 gives the following.N_{Pm}(eq) g Pm SfuelfflPmThe rate of samarium-149 formation is described as follows.Rate of change of ^{149}Sm = yield from fission + ^{149}Pm decay - ^{149}Sm burnuptherefore:dN_{Sm}dtg Sm Sfuelff lPm NPm NSm sSmafwhere:N_{Sm}=^{149}Sm concentrationgSm=^{149}Sm fission yieldsaSm=microscopic absorption cross section of ^{149}SmNP-03Rev. 0Page 44