Comparative review of defect assessment requirements (France, Germany, US and UK)

A review of French, German, US and UK defect assessment codes, practices and requirements has been undertaken in the context of future licensing of advanced PWR (APWR) systems. The main issues addressed have been fracture mechanics methodology, fracture toughness and leak-before-break (LBB). The review has been conducted mainly in relation to the integrity of Class 1 components, principally the reactor pressure vessel and primary pipework. The review has highlighted the fact that all countries use an ASME III Appendix G type of approach for a basic assessment of the avoidance of the risk of fast fracture in ferritic pressure vessels. The method is based on a comparison of linear elastic fracture mechanics (LEFM) and stress intensity factors, calculated using a reference defect size, with a reference value of fracture toughness. More detailed LEFM and/or simplified elastic plastic fracture mechanics (EPFM) methods are also specified in all the codes. These are highlighted, as are the differences identified in relation to specified fracture toughness values, the onset of upper-shelf definition and the allowance of enhanced toughness, warm prestressing and crack arrest. The main difference in approach from the licensing aspect on LBB has been identified as being the 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. (author)