Evaluation of a group motion for FBR core subassemblies under seismic condition of design basis earthquake ground motion

To re-evaluate the design basis ground motions (DBGM) at nuclear power plants (NPPs) in Japan, the 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). The seismic response on the reactor structure of FBRs causes seismic reactivity. The group motion of fuel assemblies is one of the typical seismic responses. Therefore, much attention has been paid to the mechanism of reactivity insertion due to the group motion of fuel assemblies and its consequence during an earthquake of the DBGM condition. The group motion of subassemblies is the phenomenon that almost all subassemblies move and collide simultaneously because the gaps between neighboring subassemblies are narrow. When each gap is reduced coherently and core compaction in the lateral direction occurs, positive reactivity insertion is introduced. We evaluate the gap reducing characteristic at the midplane of the core by using a correlation coefficient. As a result, we found that subassembly vibration modes depend on the excitation acceleration and frequency. The gap reduction exceeds that of the total deflection condition at the acceleration of 40 m/s² and the frequency of 5 Hz. It is important to confirm that the seismic motion of such a condition does not cause a CDA, from the point of view of the reactor structure. (author)