PROPOSAL OF PERFORMANCE-BASED SEISMIC FRAGILITY CONCEPT RELATED TO FAILURE MODES FOR A PIPING SYSTEM

After the Great East Japan earthquake, the countermeasures for beyond design basis events (BDBEs) have become more important. For a component, mitigation of a consequence of a failure in terms of impact on safety performance is required in addition to prevention of the failure. One of the key issues regarding BDBE is a structural failure of a piping system subjected to excessive earthquakes. The piping systems have various kinds of failure modes such as collapse, break, ratchet deformation and fatigue. Collapse and break can lead to significance degradation of core cooling performance. On the other hand, small deformation or fatigue clack have small impact on the cooling performance. However, the existing seismic fragility does not take into account the failure consequence and its dependency on failure modes. In this paper, we propose a new concept regarding performance-based seismic fragility for BDBEs by including failure consequence. In the concept, the core cooling performance is taken as the most important safety performance. The performance degradation is evaluated as failure consequence considering the dependency on the failure modes by the following approaches. As for collapse, the approach differs by the existence of a crack in the piping system. In case a crack is not included, the large deformation is estimated to occur in the piping. Based on the deformation, change in geometry of a flow channel of coolant is estimated. Considering the flow channel, change in a flow rate is estimated. Finally, the performance degradation by the flow rate change is evaluated. On the other hand, in case a crack is included, a break of the piping and a leak from the break point can occur. Based on the break size, a leak rate and change in flow rate is estimated. Finally, the performance degradation based on the flow rate change is evaluated. As for ratchet, deformation of the piping due to an earthquake is estimated. Based on the deformation, the performance degradation is evaluated by the same approach as the collapse of piping with no cracks. As for fatigue, the size of a through-wall crack initiated by an earthquake is estimated. Based on the crack size, the performance degradation is evaluated by the same approach as the collapse of piping with a crack.