Safety assessments of nuclear installations and the definition of the engineering design basis for nuclear power plants (NPPs) require the assessment of extreme natural events with low or extremely low occurrence probabilities, the definition of maximum credible/maximum possible events, or both. Definitions of the design basis of NPPs, for example, require data on events with occurrence probabilities not higher than 10-4 per year. Today, even lower probabilities, down to 10-8, are expected and typically used for probabilistic safety analyses (PSA) of NPPs and the examination of so-called design extension conditions.
NPPs are exposed to all types of natural hazards. Hazard assessments are consequently required for a large variety of individual hazards, including seismotectonic, hydrological, meteorological and biological hazards. For these hazards the establishment of the maximum credible/maximum possible events by deterministic methods, and the extrapolation of hazard curves to occurrence probabilities of 10-4 and below by probabilistic approaches is highly challenging for natural sciences. This is mainly due to the restriction of data and records of natural events to decades or, at best, centuries, and the limited understanding of the physical processes underlying the hazards. The session therefore searches to compare the needs of nuclear safety assessment with the methodological possibilities available for natural hazard assessment, and the limitations of these methods. We encourage contributions on methodologies and case studies assessing extremely rare natural events, approaches to extend the time/area coverage of datasets of natural hazards (e.g., by using geological records of paleo-earthquakes, storm, flood, and tsunami), the use of simulation tools to obtain information on extreme events, and assessments of hazards at the background of global change (e.g., climate change, biological infestation by invasive species). Case studies for the assessment of natural hazards for nuclear installations are particularly welcome.
This session is associated to the European project ASAMPSA_E (www.asampsa.eu) which gathers more than 30 organizations (industry, research, safety control) from Europe, US and Japan and which aims at identifying some meaningful practices to extend the scope and the quality of the existing probabilistic safety analysis developed for nuclear power plants.