Assessment of Defence in Depth for Nuclear Power Plants

This publication describes the updated version of the method for the assessment of comprehensiveness of defence in depth and demonstrates the overall improvement in assessment results when using it. For assessment of comprehensiveness, five levels of defence in depth are considered. To ensure that safety objectives are met at each level of defence in depth, the integrity of relevant fission product barriers is maintained by the safety functions. A set of challenges to the performance of safety functions and the mechanisms leading to the challenges are specified by the method. Finally, a comprehensive list of safety provisions, which contribute to preventing these mechanisms from occurring, is specified. These provisions encompass the inherent safety features, equipment, procedures, personnel availability, personnel training and safety culture aspects. The challenges, mechanisms and provisions for all levels of defence in depth are presented in the assessment method in the form of objective trees.