Development and observations on the multi-legged structural integrity safety case.

The principles for the assessment of the structural integrity of nuclear components were published by TAGSI in 2001 with the adoption of a multi-legged approach. This was based on work in the 1990s, to aid the development of UK 'conceptual defence in depth' safety cases relating to 'incredibility of failure' components. Since 2001, multi-legged approaches have been used for assessments of the design, the construction and operation of nuclear power plants in the UK, with the approach also adopted for proposed UK-based nuclear power generation design safety cases. Over time, nuclear design and construction codes have improved strengthening arguments relating to design and manufacture. For example, the value of the proof test has reduced as modern pressure retaining components are expected to be less sensitive to defects, and less likely to have such defects present, than components manufactured decades ago. In considering the recent generic design assessments, several applications have rearranged the scope and number of legs in the 'conceptual defence in depth' safety cases. Noting application and developments since the original proposal, it is observed that a generalized three-legged model is more appropriate for a new plant, while remaining consistent with the original TAGSI principles and multi-legged proposal.This article is part of the theme issue 'Future challenges for structural integrity of high-integrity components'.