Benchmark study of the accident at the fukushima daiichi NPS best estimate case comparison

The Great East Japan earthquake, occurred on March 11th 2011 at 14:46, and the subsequent tsunami led the TEPCO's Fukushima Daiichi Nuclear Power Station (NPS) to endure beyond design basis accident. After the accident, the Japanese government and TEPCO compiled the roadmap towards an early resolution to the accident including, among the main activities, the employment and development of severe accident (SA) computer codes. In the member countries of the Organization for Economic Cooperation and Development, Nuclear Energy Agency (OECD/NEA), SA codes have been developed after the accident at the Three Mile Island Unit 2 and widely employed to asses NPS status in the postulated SA conditions. Therefore, the working plans have been set up to conduct a benchmark study of the accident for the Fukushima Daiichi NPS units 1-3 with the country members of the OECD/NEA, using SA codes, constituting an international program named Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station (BSAF). The objectives of the BSAF project are: To analyze the accident progression of Fukushima Daiichi NPS, to raise the understanding of SA phenomena, to contribute to the improvements of methods and models of the SA codes and to define the status of debris distribution in the reactor pressure vessels and primary containment vessels for decommissioning. The present technical paper summarizes the achievements obtained through the results' comparison, emphasizing the portions of the accident where all the participants reached a common consensus and identifying still open questions where future work should be directed. Consensus exists on the current condition of the Unit 1, where a large fraction of the fuel is assumed to have relocated ex-vessel. On the other hand larger uncertainties exist for Units 2 and 3 where in-vessel and ex-vessel scenarios produce a reasonable prediction of the accident progression.