The accident that occurred in Japan at the Fukushima Daiichi nuclear power plant (NPP) on 11 March 2011 highlighted the need to examine more thoroughly the impact of extreme events in extended design basis conditions on the level of protection provided at nuclear facilities, and to identify possible vulnerabilities that the protection systems may have with respect to extreme events. Systematic assessment of an NPP’s response to extreme events, with a focus on long term accident progression and the identification of potential cliff edge effects affecting the provision of important safety functions and their associated support functions (alternating and direct current power supply, essential service water, etc.), is usually beyond the scope of the licensing basis. Plant systems have usually been assessed mainly against design basis accidents, including certain postulated external and internal hazards. However, for certain design extension conditions, success paths to perform safe plant shutdown and maintain the reactor in a safe state may exist due to margins embedded in the design of safety-related systems, structures and components. Investigations are performed by many utility organizations to demonstrate the existence of safe shutdown paths using deterministic and probabilistic methods.
In order to promote further research and development, an extra budgetary project (EBP) entitled "Further Enhancement of Nuclear Power Plant (NPP) Safety by Developing and Applying PSA-Based Methods for Complementary Assessment of NPPs’ Robustness against the Impact of Extreme Events”, financially supported by Russia, was launched in 2014 (the project reference number is NSAT EBP 2.6). The project was originally designed as a series of three consultants’ meetings and a technical meeting to exchange and collect relevant information (available in Russia and internationally), process it, and produce a technical report. The latter contributes to the discussion on the items needing further attention in safety assessment of nuclear installations, taking into account lessons learned from the Fukushima Daiichi accident, and provide information on recent developments in the area of PSA-based methods and tools for complementary safety assessment of plant protection against the impact of extreme events.
Generally, it was concluded in this Russian project under the Action Plan that though the stress-test-type safety analyses performed in Member States were an adequate response to the Fukushima Daiichi accident, it is useful to continue developing a range of comprehensive systematic approaches for complementary analysis of nuclear power plant protection against the impact of extreme events to cover the wide range of possible hazards and their credible combinations, as well as long duration accident sequences. Several examples of new methods that may assist in the analysis of combined hazards and long duration accident sequences were noted (e.g. the Fault Sequence Analysis method being developed by IAEA that combines minimal cutsets from PSA and operability limits of systems, structures and components for plant-specific external hazards). Further needs were identified, e.g. the need for useful, practical advice for performing analyses and activities, e.g. conducting walkdowns for the existing plants aimed at identifying vulnerabilities related to extreme events (seismic walkdowns, flood walkdowns), and taking into consideration portable equipment in PSA and human reliability analysis for extreme events conditions.
The IAEA Scientific Secretaries responsible for carrying out the activities under NSAT EBP 2.6 and producing this report are Irina Kuzmina and Artur Lyubarskiy of the Safety Assessment Section/NSNI/NS.