Risk-informed assessment of defence in depth, LOCA example. Phase 1: Mapping of conditions and definition of quantitative measures for the defence in depth levels. Rev 0

The concept of defence-in-depth (DID) is fundamental to the safety of nuclear power plants. It calls for multiple successive methods or barriers against radioactive release to the environment. DID principle is partly reflected in a probabilistic safety assessment (PSA), but not all of the DID levels are included in the model. In addition, events included in PSA are not typically labelled with DID information. PSA could however be a powerful tool to assess the status of various DID levels in an NPP. This work is a start of a development of the PSA-methodology towards an assessment of DID levels. It includes: 1) mapping of conditions that should be considered for the defence in depth levels, and 2) definition of quantitative measures that should be considered for the defence in depth levels. The work has been limited to loss-of-coolant accidents (LOCA) and DID levels 1 and 2, i.e., prevention of abnormal operation and failures and control of abnormal operation and detection of failures. Examples are chosen both from power operation LOCAs and LOCAs during cold shutdown. The methods that are used today in PSA are applicable for evaluating defence-in-depth levels 1 and 2. In the framework of these methodologies there are many different conditions and measures used. Failure data can be determined through: human reliability analysis (HRA), risk-informed in-service-inspection (RI-ISI) methodology, system reliability analysis and directly from plant specific failure data for the components. Many DID activities against LOCA are not explicitly modelled in typical PSA-studies. The risk importance of in-service-inspection is analysed and quantified in RI-ISI applications but so far results from RI-ISI have not been incorporated into PSA. Very few leakage detection systems are modelled in PSA-studies. Normally leakage detection systems that is part of the automatic actuation system are modelled while leakage detection systems in DID levels 1 and 2 typically are omitted. DID activities and systems identified in this study can play a role in several DID levels, and the evaluation of the DID level must therefore be judged by the initiating event. The next step is to implement the ideas in a real PSA to demonstrate how the DID levels 1 and 2 can be incorporated more explicitly in PSA than in today's PSA. Another task is to develop a method for the presentation of results. By these developments PSA can then become a tool for identifying relative and absolute weaknesses in activities for preventing and controlling abnormal events