COMPARATIVE REVIEW OF DEFECT ASSESSMENT REQUIREMENTS (FRANCE, GERMANY, US AND UK)

A review of French, German, US and UK defect assessment codes; practic es and requirements has been undertaken in the context of future licen sing of advanced PWR (APWR) systems. The main issues addressed have be en fracture mechanics methodology, fracture toughness and leak-before- break (LBB). The review has been conducted mainly in relation to the i ntegrity of Class 1 components, principally the reactor pressure vesse l and primary pipework. The review has highlighted the fact that all c ountries use an ASME III Appendix G type of approach for a basic asses sment of the avoidance of the risk of fast fracture in ferritic pressu re vessels. The method is based on a comparison of linear elastic frac ture mechanics (LEFM) and stress intensity factors, calculated using a reference defect size, with a reference value of fracture toughness, Although over most of the thickness range of interest the reference de fect size is the same for all cases, there are differences in the appr oaches adopted in the determination of stress intensity factors, permi tted loading cases, permitted areas of application and factors of safe ty. Details of these differences are presented. More detailed LEFM and /or simplified elastic plastic fracture mechanics (EPFM) methods are a lso specified in all the codes. These are highlighted, as are the diff erences identified in relation to specified fracture toughness values, the onset of upper-shelf definition and the allowance of enhanced tou ghness, warm prestressing and crack arrest. The main difference in app roach from the licensing aspect on LBB has been identified as being th e acceptance of such arguments as part of the case for the elimination of pipe-whip restraints for PWR primary and auxiliary pipework in the US and Germany, but not in the UK and France. Explanations are given of the principal differences in the various LBB procedures which arise from the different treatment of crack growth.