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

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