Determination of equivalent single crack based on coalescence criterion ofcollinear axial cracks

In a nuclear power plant the steam generator tubes cover a major portion of the primary pressure-retaining boundary. Thus, very conservative approache s have been taken in the light of steam generator tube integrity. According to the present criteria, tubes wall-thinned in excess of 40% should be plu gged whatever the cause. However, many analytical and experimental results have shown that no safety problems exist even with thickness reductions gre ater than 40%. The present criterion was developed about 20 years ago when wear and pitting were dominant causes for steam generator tube degradation, and it is based on tubes with single cracks regardless of the fact that th e appearance of multiple cracks is more common in general. The objective of this study is to review the conservatism of the present plugging criteria of steam generator tubes and to propose a new coalescence model for two adj acent through-wall cracks existing in steam generator tubes. Using the exis ting failure models and experimental results, we reviewed the conservatism of the present plugging criteria. In order to verify the usefulness of the proposed new coalescence model, we performed finite element analysis and so me parametric studies. Then, we developed a coalescence evaluation diagram. (C) 2001 Elsevier Science B.V. All rights reserved.