Plant incident progress scenario analysis for Mihama nuclear emergency exercise of Kansai Electric Power Co., Inc. 2021

A scenario analysis that assumed a severe accident phenomena progression scenario was carried out in FY2021 for the Mihama nuclear emergency exercise of Kansai Electric Power Co., Inc. The analysis assumed event occurrence and subsequent multiple failures of engineering safety equipment, emergency safety measures, etc. From the obtained results, progression of events, such as information of the specific event occurrence time and the corresponding plant response, was identified based on the Nuclear Emergency Response Special Measures Act Evaluation. The following points were seen. (1) It was 50 minutes from the start of the event as defined by Article 10 of the Nuclear Disaster Act, and 1 hour and 20 minutes from the start of the event to the time of declaration of a nuclear emergency situation under the provisions of Article 15 of the Act on Special Measures Concerning Nuclear Emergency Preparedness. (2) It was confirmed that direct cavity water injection and alternative core injection and alternative containment spray, which were the selected accident management (AM) measures, were carried out after core damage, and the rises in core temperature, containment pressure and temperature were suppressed and the events converged without causing damage to the reactor vessel and containment vessel. (3) As a result of performing MAAP analysis when AM measures could not be implemented after GE21 dispatch, the time from event occurrence to core damage was 2 hours and 34 minutes, and the time to reach double the maximum working pressure was 32 hours and 35 minutes. (author)