Evaluation of the fuel melting character of FBR core caused by seismic reactivity insertion

A seismic probabilistic risk assessment (PRA) for fast breeder reactors (FBRs) has been carried out to confirm that the seismic safety is equivalent to that of light-water reactors (LWRs). FBR plants consist of components with thin walls, so the seismic responses tend to be large in comparison with LWRs. The seismic response of the reactor structure of FBRs is caused by seismic reactivity. The group motion of fuel subassemblies is one typical seismic response. Thus, much attention has been paid to the reactivity insertion mechanism and its consequences during earthquakes beyond the design basis ground motion (DBGM) condition. Continued seismic reactivity insertion may cause fuel melting. To prevent a core disruptive accident (CDA), it is necessary to terminate the expansion of the fuel melting zone by the insertion of control rods. The behaviors of both the fuel melting and the scram delay depend on the seismic exiting acceleration, so the limit to prevent a CDA is defined as the critical acceleration for which the scram time is equivalent to the limit time to prevent fuel pin failure from the viewpoint of the fuel melting behavior. To clarify the critical acceleration and to clarify the uncertainty in the evaluation of the fragility, the evaluation method is studied in terms of the seismic response of the reactor structure, the neutronic characteristics and thermal hydraulic characteristics. (author)