Analysis of 149Sm time evolution and the reactivity contribution in nuclear reactors

In this work the theory to describe atomic density time evolutions of 149Sm and precursor 149Pm is developed. The isotope 149Sm is one of neutron poisons in nuclear reactors; this is the reason for studying its concentration time evolution and the associated reactivity. Here, different from other theories already existent, the 149Pm concentration time evolution equation is corrected to neutron absorption and the production of 149Sm due to 235U thermal fission (fission products) is also considered. Albeit the influence of 149Sm is lesser than that of main neutron absorber 135Xe (mainly because of absorption cross section values) it is important to determine the 149Sm perturbation in neutron balance equations and reactivity calculations in nuclear reactors design. Here 149Pm and 149Sm time evolution concentrations were calculated for different power cycles, using values for NPP Atucha U-II. The reactivity associated to 149Sm concentration was estimated using results from two-groups energy diffusion equations.