The Accident Analysis for Unit 1-3 at Fukushima Dai-ichi Nuclear Power Station

摘要

Due to the Great East Japan Earthquake which occurred on March 11, 2011, Units 1 to 3 of Fukushima Daiichi Nuclear Power Station experienced severe accidents. After the accidents, data from interview with operators and on-site surveys has been continuously developed. Based on the data, plant-state analysis by severe accident analysis code "MAAP" (Modular Accident Analysis Program) has been conducted. In parallel with MAAP analysis, plant reaction to site operation such as water injection, is analyzed and core condition is comprehensively evaluated. According to the evaluation, in Unit 1, it is presumed that almost no fuel was left at the original position, and moved downward after it melted. The moved fuel likely damaged RPV and assumed that most of it had dropped to the PCV pedestal. In Unit 2 and 3, it is presumed that some part of fuel was left at the original position, and the other dropped to the bottom of RPV or to the PCV pedestal. In MAAP analysis, behavior of the plants before core melt is reproduced. On the other hand, RPV damage of Unit 2 and 3 does not occur in MAAP analysis, which is contrary to the observed facts. This shows the limitation of analysis capability of the current MAAP code. Therefore, by developing severe accident analysis codes to achieve higher analysis accuracy levels and by evaluating the plant reaction to the site operation, we will continue the effort to obtain a clear picture of the states inside the reactor in order to remove fuel debris.