Shutdown probabilistic safety assessment-method and results

Shutdown probabilistic safety assessment is a method for assessment and improvement of nuclear power plant safety for the periods of their shutdown. It is developed similarly as the power probabilistic safety assessment with several similarities and some differences representing the realistic plant behavior within the safety models. The method is developed and starts with definition of representative plant operating states. For each of representative plant operating states, which have different durations, separate model is prepared. The model preparation includes: definition and analysis of initiating events, accident sequence development analysis, success criteria analysis, system analysis, human reliability analysis, parameter estimation analysis, quantification and results interpretation. Each model for each plant operating state can consider either one or more sets of initiating event sets such as internal events, fires, floods, earthquakes and other external events. The results for every plant operating state are obtained by evaluation of the respective model. The results have been obtained for all plant operating states and they show the risks related with respective plant operating states. Their comparison with the results of plant full power operation show reduced and identifies the plant operating states with larger and smaller risks in terms of core damage frequency, which is selected as the main risk measure.