A new design concept and seismic margin assessment for a spent fuel storage system

Abstract

The design and structural performance of a spent fuel storage system, which plays a role in storing irradiated and damaged fuel assemblies generated during the operation of a reactor, have been regarded as very important issues in relation to nuclear safety. The recent accident at the Fukushima Daiichi Nuclear Power Plant once again highlights the importance of the safe design of such a system. Thus, this paper proposes a new design concept of a spent fuel storage system composed of spent fuel storage racks (SFSR) and a support frame. The design of each SFSR is enhanced by increasing the natural cooling capacity of the pool water. The support frame is newly devised such that it physically prevents the overturning and sliding of racks and collisions among them during an earthquake event.In addition, to verify the structural integrity of the proposed design and evaluate its seismic margin, static analyses, response spectrum analyses and nonlinear time history analyses are conducted. The response spectrum analyses provide the maximum response of the structures during and after seismic events. The nonlinear time history analyses were carried out to predict contact sliding, rocking, twisting, and turning between the floor of the pool bottom and the support frame. The numerical results were analyzed based on the allowable code limits to assess the structural integrity. The possibility of a collision between the support frame and the adjacent pool wall is investigated. The seismic margin of the proposed design is studied within the seismic fragility analysis framework.The analysis results show that the maximum stress values of these SFSRs and their support frame under seismic loads are within the specified code limits. An impact between the support frame and the adjacent pool wall will not occur because the sliding distance calculated from the nonlinear time history analysis is less than the gap between the two elements. These results confirm that they cannot be overturned but will instead slide under an SSE event. Finally, the seismic fragility analysis results demonstrate that the designed spent fuel storage system has a sufficient seismic margin which exceeds the targeted seismic design level.