External event risk assessment: Methodology and application

The 2011 Fukushima accident revealed various gaps related to probabilistic approach, used for the plant risk assessment against external events. In consideration of that and analysing the evolution of the accident scenario, some issues need to be re-considered and/or improved for a reliable NPP risk evaluation in Fukushima conditions. This study is therefore aimed at the evaluation of the NPP safety performance when subjected to external events, such as the earthquake and/or tsunami (inundation phase), by applying the IDPSA (Integrated Deterministic Probabilistic Safety Analysis) approach, which combines the Probabilistic and Deterministic Safety Assessment ones. To the purpose "stress tests" suggestions, presently foreseen by the European and International Associations, have been taken into account. The main issues as emerging from the Fukushima accident, specifically the LOOP/SBO (Loss Of Offsite Power/Station Black Out) caused by a flooding or tsunami, were analysed. The results indicated that although the containment building is suffering local damaging, no loss of structural integrity occurs.