149Sm evolution behavior in a small modular molten salt reactor

Highlights: • ¹⁴⁹Sm and ¹⁴⁹Pm evolution behavior in a small modular molten salt reactor is analytically deduced. • ¹⁴⁹Sm equilibrium concentration is independent with the mass flow rate and core to outer-loop volume ratio. • The circulation of fuel salt contributes a lower "samarium peak" and a longer time for ¹⁴⁹Sm attaining equilibrium. • A lower neutron poison (around 210 pcm) at the flow state is obtained while the core restarts from the SCRAM. - Abstract: 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.