LEVEL 3 PSA use and development in the United Kingdom

The nuclear power plant most recently commissioned in the United Kingdom was Sizewell B in 1995, a pressurized water reactor. As part of the planning phase for Sizewell B, Level 3 PSA codes were used to determine the impact of a range of nuclear accident sequences from design basis accidents to accidents involving major core damage with containment bypass. With renewed interest in potential new nuclear build in the UK, the current state of readiness for a potential new series of Level 3 PSA assessments required evaluation. Existing Level 3 codes were reviewed to identify the underlying modeling approaches and data. The results were compared to identify differences and the applicability of the codes for use within the existing Level 3 PSA framework within the UK was considered. The review also considered the advances in scientific knowledge since major PSA system development last occurred and hence highlighted areas that may require further development for future Level 3 assessments. Finally, the implications of developments in Level 2 PSA accident source terms and ongoing ANS and IAEA Level 2 Standards development were considered. The paper summarizes the findings of the review, showing the main differences between the codes and the areas of scientific knowledge that have developed since the last major round of Level 3 PSA development and summarizes an initial programme of further development to update PSA capability for use in the UK. The updates in the initial programme relate to dosimetry, health effects and population/agricultural production spatial distribution data.