Knowledges and Insights Obtained from the Fukushima Daiichi Accident

It was very hard to estimate how the accident had progressed at each unit of Fukushima Daiichi NPS at the early stage of post-accident analytical activities, because the knowledge relevant to BWR severe accident phenomena and progression was insufficient and the collected data was very limited since complete station blackout (loss of AC and DC power) was occurred at Fukushima Daiichi NPS. However, because of the progression of the measured data analysis, system behavior or response of reactors before the core melt has already been clarified. System behaviors after the core melt have been clarified, as well. Measured data and circumstantial evidence clearly indicated that the fuel temperature went up to melting point and the RPV was damaged by the molten core, and the Fukushima accident is known as a more serious accident than TMI-2 because of including ex-vessel phenomena. But, the information regarding detailed accident progression (for example, amount of melted fuel and timing of RPV breach, etc.) is still unclear and we are struggling to solve such a detailed accident progression. At present, there are two challenges in front of us. One of them is to improve nuclear reactor safety by applying the insights from the Fukushima accident and the other is to solve unclear issues before conducting the first RPV internal investigation. Tokyo Electric Power Company is willing to stand up against these two challenges as the company concerned and appreciate supports of the world nuclear community. (authors)