To promote the practical fire Probability Risk Assessment (PRA) among Japanese industries based on the domestic fire experience, it is important to estimate generic fire ignition frequency distributions for Japanese nuclear power plants (NPPs), which are applicable to the fire PRA during full power operation.

Firstly, the Central Research Institute of Electric Power Industry (CRIEPI) proposed the estimation process of the distributions, which consists of the four steps including sub steps as follows:

Step 1: Fire event data collection including development of criteria

Step 2: Fire event data analyses including development of criteria:

Step 2.1: Screening of fire events to identify events to be classified

Step 2.2: Classification of fire events from the viewpoint of fire severity of each event.

Step 2.3: Categorization of fire events from the viewpoint of fire ignition source types

Step 3: Estimation of generic fire ignition frequency distributions

Step 4: Comparison of the results of Step 3 with fire ignition frequency estimated in other countries

In this paper, CRIEPI introduced the detail of Step 1 and 2 above and discussed the study results for them, which are essential to estimate generic fire ignition frequency distributions properly. Through Step 1, CRIEPI established a preliminary fire event database based on fire records at Japanese NPPs provided by domestic utilities. In step 2.1, CRIEPI identified the 180 fire events for future steps. Finally, CRIEPI classified the fire events and defined the fire ignition source categories in Step 2.2 and 2.3. As a result, CRIEPI found the major three risk contributors among fire ignition source categories: transient related fires, electrical cabinet fires and pump fires.

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