Overview and outcomes of the OECD/NEA benchmark study of the accident at the Fukushima Daiichi NPS (BSAF) Phase 2-Results of severe accident analyses for Unit 1

Phase 2 of the OECD/NEA Project "Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF)" was established in mid-2015. The objectives have been similar to Phase 1 of the project but with an extended analysis period of 3 weeks from the occurrence of the earthquake, a major focus on fission product (FP) behaviour and releases to the environment and the comparison to various data including radiological data and results of backwards calculations of the source term. Nine organizations of six countries (Ciemat Spain; IAE, JAEA and NRA Japan; CEA, IRSN France; IBRAE Russia; KAERI Korea; NRC/DOE/SNL U.S.A.; VTT Finland) submitted results of their calculated severe accident scenarios for Unit 1 at the Fukushima Daiichi site using different severe accident codes (ASTEC, MAAP, MELCOR, SAMPSON, SOCRAT, THALES-KICHE). This paper describes the findings of the comparison of the participants results for Unit 1 against each other and against plant data, the evaluation of the accident progression and the final status inside the reactors. Special focus is on RPV status, melt release and FP behaviour and release. Unit specific aspects will be highlighted and points of consensus as well as remaining uncertainties and data needs will be summarised. The results for Units 2 and 3 are presented next in separated papers.