Samarium-149

149Sm is a stable isotope of samarium, and a fission product (yield 1.0888%) which is a neutron-absorbing nuclear poison with significant effect on nuclear reactor operation. Its neutron cross section is 40140 barns.

The equilibrium concentration and (thus the poisoning effect) builds to an equilibrium value during reactor operation in about 500 hours (about 20 days), and since Sm-149 is stable, the concentration remains essentially constant during reactor operation.