Study on initial crack assumed in the flaw tolerance approach applied to piping of nuclear power plants

It is known that there is a large safety margin in the current fatigue damage assessment method for piping of nuclear power plants. One of the evaluation methods in order to rationalize it, there is a flaw tolerance method that estimates the continuable operation period by crack growth calculation assuming initiation of fatigue cracks even when the fatigue cumulative usage factor exceeds 1. It is already standardized by ASME Boiler and Pressure Vessel Code Section XI Appendix L in the United States, but the assumed initial crack shape was based on fatigue damage data and evaluation conditions in the United States. Therefore, in this paper, investigation of domestic fatigue data and evaluation conditions of Appendix L were performed, and the depth and length of the assumed initial crack for the flaw tolerance method were studied for the standardization in Japan. Multiple cracks due to fatigue tend to initiate on the inner surface of piping based on the operating experience. Therefore, assumed initial crack was modeled as an equivalent single crack with priority given to simplicity of the evaluation procedure, where the aspect ratio of the equivalent single crack was determined so that the failure probability of the equivalent single crack was equivalent to that of the assumed multiple cracks. The depth of the assumed initial crack was set as the allowable flaw depth of the JSME Codes 'Rules on Fitness for Service for Nuclear Power Plants' to assure the detectability of the crack. Moreover, in order to be able to apply this method over a wide range, the aspect ratio of the assumed initial crack have been developed for many kinds of piping shapes and materials. Finally, residual lives of typical pipes with the assumed initial crack were evaluated assuming virtual loadings that roughly envelope realistic loadings and the continuable operation periods were surveyed. (author)