Features of Fukushima Daiichi Nuclear Power Plant accident and information on fuel debris obtained from PCV internal investigations

TEPCO has been estimating the situations of reactor pressure vessels (RPVs) and primary containment vessels (PCVs) and the distribution of the fuel debris by multiple approaches, in order to safely and efficiently carry out decommissioning work and to contribute to enhancing NPP safety by revealing the complete picture of the accident. As a result of analyses of the data obtained at the time of the accident and after the accident, and evaluation using analysis codes on the accident progression of Units 1-3, the timing of core damage and core melt is estimated to be differ in each unit due to the differences of the period without water injection and the heat generated by water-Zirconium (Zr) reaction, etc. As a result of PCV internal investigations, the states of the inside of the pedestal and the states and the distributions of fuel debris were confirmed to be differ in each unit. This may be due to the differences of the accident progression. The contaminated material samples collected from each unit were analysed to evaluate the Uranium (U) isotope ratios, which are important in examining the criticality of fuel debris, etc. As a result, the U isotope ratios in many of samples were confirmed to be close to the average value by the analysis code. This may indicate the state of fuel debris resulted in molten mixing of fuels. Based on this information obtained by multiple approaches, the extent of damage inside the RPV was estimated to be in the order of Unit 1, Unit 3, and Unit 2. (author)