A compendium of practices on safety improvements in low-power and shutdown operating modes - Principal Working Group No. 5 - Risk Assessment

Traditionally, the focus of safety assessment of Nuclear Power Plants has been on severe accidents during full power operation. It has been assumed that the risk originating from events during other modes of operation would be small in comparison with full power events. This perception has been based on the fact that the potential impact of an accident during low power and shutdown (LPS) conditions are lower due to decreased decay heat and that more time is available for recovering actions, should adverse situations occur. During the last years low power and shutdown analysis has come into the focus of nuclear safety considerations. Operational experience and performance of Low-Power and Shutdown Probabilistic Safety Analysis (abbreviated LPS PSA or SPSA) have highlighted that the risk from those operational modes is significant and could contribute up to 50% to a total core damage frequency. Based on this considerations the OECD/NEA Principal Working Group 5 for Risk Assessment decided to form a Task Force (Task 13) to cope with this issue. It was decided that the Task Force should compile a report on plans, methods and experiences in the area from the different OECD countries. The Task Force prepared and published its first report in 1993. That report was appreciated by many SPSA practitioners. As the activities in this area are developing rather fast, a review of appropriateness of the 1993 Task Force 13 report was undertaken. This review determined that many programs had advanced in the mean time and that an update of the 1993 report was needed. The review of the 1993 document also pointed out that the advances had occurred in the area of implementation of findings of the SPSAs and other safety studies. An update of the 1993 Task Force 13 report focuses on both the techniques and in particular the measures taken to improve the safety in shutdown and low power operation. This update of the 1993 Task Force 13 report was initiated in 1996, and was completed in 1997. As the update actually meant a complete rewrite of the report, the Task Force was renamed Task 97-1 'Risk Based Control of NPP systems during shutdown - Safety in shutdown operations-Compendium of practices on safety improvements'. The Task Force leader of both the Task Force 13 and the new Task Force 97-1 was Mr. Bo Liwang, Head of Reactor Safety Analysis Division of SKI of Sweden. Other members of the Task Force included Dr. Lanore of IPSN, Mr. Versteeg of GNKK, Mr. Viroleinen of STUK, Mr. Szikszai of Paks NPP and Mr. Murphy of NRC. Additionally, the Task Group wishes to provide acknowledgement of the specific service of Dr. Bojan Tomic and the personnel at Enconet Consulting who provided much of the in-depth technical analyses as well as overall coordination in editing and compiling the report. This new report provides a compendium of practices on safety improvements/enhancements in Shutdown and Low-Power operation modes in OECD Member Countries. Content: 1. Introduction (Background to the Project, Project Objective, Project Scope, Structure of this report); 2. Approaches to SLP PSA analysis (Background, Full Scope SLP PSA Models, Barrier Analysis Approach, ORAM (Streamlined Configuration Control type tool); 3. Modelling issues related to SLP PSA (Introduction, Plant Operating Modes and Plant Operational States, Initiating Events, Screening Analyses, Modelling of Accident Sequences, Human Interaction, Dependency Analysis, Plant Damage States, Quantification of Accident Sequences, Application of SLP PSA); 4. International experience with SLP PSA (Introduction, Findings, Summary of Countries Activities); 5. Safety enhancement in shutdown and low power operation (Background, Administrative Improvements, Hardware Improvements, Operational Improvements); 6. Conclusions; 7. References and other sources; Appendix A - Comparison of improvements by categories; Appendix B - List of enhancements for shutdown operation of NPPS