Study on Evaluation Method for Safety Margin of Degraded Piping Against Seismic Loading with Multi-Fracture Mode

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The piping system is one of the major equipment in the nuclear power plant. Because it is assumed that fracture of the piping cause serious damage to whole system of the nuclear power plant, sufficient safety must be ensured against the defect and caused degradation of the piping. In addition, large margin of seismic safety has been taken in Japan because of a lot of earthquakes. However, effect of aged piping on seismic safety has not been clarified.The aged piping of a nuclear power plants has possibility to have some defects such as SCC (Stress Corrosion Cracking) cracks and local thinning. In order to evaluate tolerance limit of the degraded piping, a series of large scale four-point bending tests on straight pipe and shaking table tests using 3-D piping system models, low cycle fatigue tests for the degraded pipes were carried out as a part of the "Aged Piping Committee" project sponsored by National Research Institute for Earth Science and Disaster Prevention. And local thinning bent pipes bending tests were done in National Research Institute for Earth Science and Disaster Prevention in 2002.The purpose of this study in this paper is to establish analytical approaches by which the low cycle fatigue strength and failure behavior of degraded piping can be simulated against seismic loading with multi-fracture mode. A series of finite element analyses, dynamic analysis and static analysis, have been performed. A series of elasto-plastic analyses have been carried out for the locally thin-walled straight piping loaded cyclic four-point bending and locally thin-walled elbow piping loaded cyclic in-plane or out-of-plane bending and then 3D piping system loaded dynamic loading. It has been found that the ratcheting or ovaling behavior observed in the experiment can be simulated well by finite element analysis.