Sodium boiling stabilisation in a fast breeder subassembly during an unprotected loss of flow accident

Abstract

Extended sodium boiling in the fissile zone of a fast breeder reactor (FBR) implies a risk of transient over-power (TOP) and subsequent core melting. Extended sodium boiling may be a consequence of an unprotected loss of flow accident (ULOF). The paper proposes a mitigation strategy, based on boiling stabilisation. This stabilisation consists in limiting boiling in the outlet zone of the subassembly (in the upper axial blanket and outlet tube) linked to stable two-phase natural circulation flow. The paper describes: • the pressure drop measurements obtained for sodium boiling in subassemblies, • the flow stability criterion, • the methodology derived to analyse two-phase flow stabilisation in a subassembly, • the application to optimise the geometry of an outlet zone of an FBR subassembly, • simulations of transients leading to flow stabilisation and analysis of the effect of pump coast down inertia.