As a consequence of the Fukushima nuclear power plant accident, the safety of all EU nuclear plants is being reviewed with a focus on evaluating the response of nuclear power plants to beyond design basis events. In addition, previous evaluations of the safety margins are being reassessed. Lloyd's Register Consulting was involved in the reassessment of Swedish and Finnish nuclear power plants.
The project was performed by Westinghouse and ourselves in support of the Nordic Owners Group (NOG). It covered potential effects of external flooding and other extreme natural events on Swedish and Finnish boiling water reactor (BWR) plants. A review of existing analyses was conducted with the aim of identifying and re-evaluating assumptions and simplifications in previous analyses which may have hidden important aspects of events. Additional analyses were performed with different approaches and can be used as benchmarks for existing analyses.
We carried out two sub-projects within the framework of the larger project. These focused on:
- Extreme water levels in the Baltic Sea and Skagerrak Strait;
- Proposing a methodology for the analysis of multiple external events.
The objective of the sub-project on extreme water levels was to evaluate the relevance of the existing analyses, which are based on extreme value analysis of sea level measurement series for a period of 30 to 110 years. In this project, historically observed extreme high sea level events from around the year 1000 until today were also analysed. Several events with sea levels of 3 m or more above average had occurred but were not included in the previously analysed measurement series. The project also included an improvement of the description of the development and duration of extreme water levels, as well as a survey of potential medium-term impacts of climate change.
In the second sub-project, a methodology for identification of multiple external events, i.e. combinations of external events, previously developed by Lloyd's Register Consulting for use in Probabilistic Safety Assessment (PSA) was used as a basis for proposing a screening methodology applicable for deterministic analyses.