The isotope Sm-149 is the second most important fission-product poison because of its high thermal absorption cross section of 41,500 barns. Sm-149 stems from Pm-149 in a manner analogous to the formation of Xe-135 from I-135. The complete decay chain is represented thus:

(Nd-149 is a fission product which occurs in about 1.4 percent of uranium-235 fissions by slow neutrons.) Since the half-life of Nd-149 is short compared to that for Pm-149 (promethium) it may be assumed that Pm-149 is a direct fission product with a fission yield of 1.4 percent.

Because Sm-149 is not radioactive, it presents problems somewhat different from those encountered with Xe-135. The equilibrium concentration and the poisoning during reactor operation are independent of the neutron flux and reach a negative reactivity maximum value of 0.01 (see Figure 9). The maximum change in relative reactivity due to samarium in an operating reactor is thus -0.01, irrespective of the thermal neutron flux. Because Sm-149 is a stable nuclide it can be eliminated in one way, and that is by absorbing a neutron and changing to Sm-150, which has a low absorption cross section and is of no importance, thus:

The build-up of Sm-149 after shutdown depends on the power level before shutdown. Sm-149 does not peak as Xe-135 does but rather increases slowly to a maximum asymptotic value. Thus, for example, a steady flux of 2 x 10¹⁴ neutrons/cm²s (which appears to be a practical limit for a thermal reactor) the samarium poisoning after shutdown increases to ~0.027. It is, therefore, evident that the addition of about 0.03 to the reactivity will handle any samarium problems which may arise. Notice that samarium poisoning is minor when compared to that of xenon.