Protection against manufacturing defects for the safety case of an IoF nuclear component

The paper addresses two questions raised in relation to a TAGSI multi-legged safety case for Incredibility of Failure components in nuclear power plant, specifically in relation to calculation of tolerance to manufacturing defects, which are typically most significant at welds. Calculation is referred to as the Failure Analysis leg of the safety case. There are locations where the start of life target safety margin of 2, expressed as the Validation Factor (VF), can no longer be claimed after a period of service for a postulated defect with a through wall extent equal to that of the inspection validation defect used to independently qualify the manufacturing inspections. If justified, changes to the size and aspect ratio of postulated defects could allow achievement of a VF of 2 in support of life extension, as follows: (i) by reducing the size of the postulated defect from the validation defect size to a smaller size that has demonstrated high capability of detection, and (ii) excluding postulated defects with higher aspect ratios from the Failure Analysis, which although often presenting the limiting position, can be difficult to conceive from the welding processes used.This article is part of the theme issue 'Future challenges for structural integrity of high-integrity components'.