Ensuring subcriticality of space lithium-cooled reactors during submerged accidents: A study on ex-core reactivity control

Space lithium-cooled fast reactors have a high power density and are gradually becoming an ideal power source for deep space probes. However, in the case of launch failure, the reactor may fall into the water, leading to a supercritical accident with increased core reactivity. In this paper, a control ring design scheme and an axial gasket design scheme are proposed, and a spectral shift absorber design is added to the core to improve the submerged criticality safety of the core. The results show that both the control ring scheme and the axial gasket scheme can effectively absorb the excess thermal neutrons generated due to the dropout accident. When B4C is used as the neutron poison, the reactivity of the control ring scheme in a flooded accident is reduced by 9637 pcm compared to the traditional design. The proposed axial gasket design reduces the number of sliding reflector blocks to be slid out by 16.7 % in a submerged accident, which effectively improves the reactivity control efficiency. The simulation results of this paper provide a basis for the safety design and research of small space lithium-cooled fast reactors.