Study on in-vessel shielding design concepts for small-size pool-type sodium-cooled fast reactors

We study the characteristics of in-vessel shielding design for small-size pool-type burner and breeder sodium-cooled fast reactors (SFRs). The breeder SFR shows a better performance as regards axial neutron shielding due to the existence of the axial blanket. The diameter of the reactor vessel with respect to the no-shield case is increased by 17% for the breeder SFR, while the diameter of the reactor vessel is increased by 23% for the burner SFR because of the shielding assemblies. However, this disadvantage of the shielding characteristics of the burner SFR is noticeably mitigated upon adopting the cylindrical shielding design concept. Although there are several other parameters that influence the size of the reactor vessel, the configuration of the in-vessel shielding structure is one of the most important parameters that determine the final size of the reactor vessel for a pool-type SFR. Therefore, the cylindrical fixed shield design concept is more efficient than the design concept utilizing replaceable shield assemblies to prevent secondary sodium activation at the intermediate heat exchangers (IHXs) for small-size pool-type burner SFRs.