Ultimate Failure Criteria Evaluation of Elbow Pipe Components in Seismically Isolated NPPs

Seismic isolation system can be an effective alternative to protect the nuclear power plants (NPPs) against to the strong seismic events. Therefore, some research activities to adopt the seismic isolation concept to the design of the next generation NPPs have been progressed for last few years in Korea. If seismic isolation devices are installed in nuclear power plant for seismic stability, safety against seismic load of power plant may be improved. But in some equipment, the seismic fragility capacity may decrease because the relative displacements may become larger compared to the non-isolated case. It is well known that the interface pipes between isolated & non-isolated structures will become the most critical component when the seismic isolation system will be introduced. Therefore, seismic performance of such interface pipes should be evaluated comprehensively especially in terms of the seismic fragility capacity. To evaluate the seismic capacity of interface pipes in the isolated NPP, firstly, we should define the failure mode and failure criteria of critical pipe components. In many previous studies, the failure mode and failure criteria of pipes under severe seismic loading condition were defined in terms of the low-cycle fatigue and/or ratcheting failure. However, for the interface pipes in the seismically isolated NPPs, the failure mode may become different from the conventional failure modes of ordinary pipes because of the extremely large relative displacement between the support anchors which are located in the isolated and non-isolated structures. Hence, in this study, we performed the dynamic tests of elbow components which were installed in a seismically isolated NPP, and evaluated the ultimate failure mode and failure criteria by using the test results. To do this, we manufactured 24 critical elbow component specimens and performed cyclic loading tests under the internal pressure condition. The failure mode and failure criteria of a pipe component will be varied by the design parameters such as the internal pressure, pipe diameter, loading type, and loading amplitude. From the tests, we assessed the effects of the variation parameters onto the failure criteria. For the tests, we generated the seismic input protocol of relative displacement between the ends of elbow component. The results of ultimate failure mode and failure criteria are presented in terms of the number of cyclic loading counts, damage indices which are the functions of dissipated energy and inelastic deformation. From the results, we found that the increase of the internal pressure will slightly increase the failure criteria. Tested elbow components had a very good sustainability against to the earthquake loading since that more than 34 times of 0.5g earthquakes (40 mm relative displacement) were required to make a penetration crack at the most critical point.