Submersion criticality safety analysis of an in-core reactivity control structure for space gas-cooled reactor

•Design concept of a fuel assembly for a space gas-cooled reactor is presented.•Two neutron absorber materials are evaluated in the concept design.•Neutronic analysis in different cases of this design have been performed. For space gas-cooled fast reactors, the neutron spectrum will be thermalized and increase reactivity in the submerged accidents, having an impact on the safety of the reactor. In this paper, the submersion criticality safety analysis of an in-core reactivity control structure for space gas-cooled reactor is presented. Two different neutron poisons are considered to be doped into the control ring, the variations of the effective multiplication factor with the fuel enrichment, the poison atom density and the thickness of the control ring are analyzed. The burnups with different poison atom densities and thicknesses of the control ring are also calculated. The result shows the increasing safety with the reduction of the control ring thickness and increase of the poison atom density. The thickness of the control ring and the poison atom density only influence the initial effective multiplication factor, but have no influence on the decrease rate of the effective multiplication factor with the increase of burnup. The result also indicates the higher safety of 10B doped into the control ring than 155Gd doped, but also the higher negative reactivity of 10B than 155Gd. These results can provide a reference for the design of space gas-cooled nuclear reactors.