149Sm evolution behavior in a small modular molten salt reactor

Compared with the well-studied neutron poison 135Xe, 149Sm received much less attention in the conventional reactors because of the comparatively smaller neutron absorption cross section. But in Molten Salt Reactor (MSR), gaseous fission products such as xenon and krypton are largely removed by the gas removal system, the behavior of 149Sm, which exists in molten salt, therefore becomes important. During MSR's operation, the liquid fuel continuously circulates through the core (with neutron flux functioning) and outer-loop (the part of primary loop at external core, with negligible neutron flux), the nuclide evolution law is much different from that in the solid fueled reactors, thereby accurately evaluating the time behavior of 149Sm is much needed. In this work, the control equations precisely describing the nuclide evolution of 149Sm as well as its precursor 149Pm entrained in the flowing salt are deduced, and a time behavior during different power cycles is then investigated for a small modular molten salt reactor based on the deduced equations. To fundamentally understand the flow effect on 149Sm time behavior, the corresponding results are compared with those calculated by using the existent method, in which the flowing fuel is treated as the static fuel. Because the disappeared 149Pm regardless at the core and outer-loop is all converted to 149Sm, the flow state has a same equilibrium atomic density with that at the static state. But the time for achieving the equilibrium at the flow state is prolonged due to the time for 149Pm decayed to 149Sm being extended at a same 149Pm production rate. This effect also leads to a lower “samarium peaking (overshoot)” for power up/down, as well as a lower 149Sm concentration after scram which subsequently decreases the neutron poison (around 210 pcm) as the core start-up again. This work provides a precise description for the evolution of 149Sm for circulating fuel, which should be included in the core design and source term analysis of safety study of a MSR.