Primary to secondary leaks in WWER nuclear power plants. A publication of the extra budgetary programme on the safety of WWER and RBMK nuclear power plants

The IAEA initiated in 1990 a programme to assist the countries of Central and eastern Europe and the former Soviet Union in evaluating the safety of their first WWER-440/230 NPPs. The main objectives were: to identify major design and operational safety issues; to establish international consensus on priorities for safety improvements; and to provide assistance in the review of the completeness and adequacy of safety improvement programs. The scope of the program was extended in 1992 to include RBMK, WWER-440/213 and WWER-1000 plants in operation and under construction. As steam generators play a central role in residual heat removal for WWERs, the loss of integrity of steam generators leading to interfacing LOCA was found to be of high significance for ensuring the NPP safety. Cracks that were revealed in the primary steam generator collector of several WWERs have potential to propagate into larger cracks and to develop collector integrity failure. This might cause large primary to secondary leaks (PRISE) accidents which are unique to the WWER design. The main concerns identified by IAEA are related to loss of core cooling and radioactive release due to long term los of coolant bypassing the containment. Adequate automatic means to cope with large PRISE events do not exist because safety systems are typically designated to mitigate consequences of loss of primary or secondary coolant into the containment. That is why such events should be further studied. The development of national approaches on treating PRISE events as design basis accidents varies considerably from country to country. In this context, cooperation between the various international groups is very desirable. This guidance report is intended to provide both a comprehensive understanding of PRISE events for plant operators and regulators with practical guidance for the treatment of PRISE events, particularly large PRISE accidents, as design basis accidents for both WWER-440 and WWER-1000 NPPs. This document comprises rules and practices related to PRISE events applied in Member States and the strategy for dealing with these events. This deterministic approach should be complemented by well qualified PSA studies to evaluate risks involved, to select scenarios which need to be considered as design basis accidents and to set priorities for improving defence in depth