Study on loss-of-cooling and loss-of-coolant accidents in spent fuel pool. (8) Safety margin of spent fuel in large LOCA event by the simple assessment method

After the Fukushima Dai-ichi accident, regulations for nuclear power plant have been strengthened in Japan and the following assessment is required in case of the decommissioning of Spent Fuel Pool (SFP); that is "Prevention and mitigation measures for an accident of SFP should be maintained until spent fuel assemblies are removed from the SFP, or it should be demonstrated that these measures are not necessary". From this background, the simple (but conservative) assessment of spent fuel integrity in large LOCA event has been carried out in Japan. Although the safety margin of the simple assessment is considered to be sufficient, it is important to capture it quantitatively. In this study, CFD analysis has been conducted for the assessment of spent fuel integrity in large LOCA event and the maximum temperature of spent fuel assemblies has been evaluated. Then, it has been compared with the result of the simple assessment method. As a case study, additional CFD analysis has been conducted, where water level in SFP decreases to the Bottom of Active Fuel (BAF) due to boil-off. Since this scenario might be more severe than large LOCA scenario, the number of spent fuel assemblies, their decay heat and loading pattern to maintain spent fuel integrity are investigated. A summary of the result and the insight from the study are provided in this paper. (author)