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 3

This is the third part of the three part paper describing the accidents at the Fukushima Daiichi nuclear power station as analyzed in the Phase 2 of the OECD/NEA project “Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant” (BSAF). In this paper, we describe the accident progression in unit 3. Units 1 and 2 are discussed in parts 1 and 2 of this series of papers. In the BSAF project, eight organizations from five countries (CRIEPI, IAE, JAEA and NRA, Japan; IRSN France; PSI, Switzerland; SNL, USA; VTT, Finland) analyzed severe accident scenarios for Unit 3 at the Fukushima Daiichi site using different severe accident codes (ASTEC, MAAP, MELCOR, SAMPSON, THALES). The present paper for Unit 3 describes the findings of the comparison of the participants’ results against each other and against plant data, the evaluation of the accident progression and the final status inside the reactors. Special focus is on the status of the reactor pressure vessel, melt release and fission product release and transport. Unit 3 specific aspects, e.g., the complicated accident progression following repeated containment venting actuations and attempts at coolant injection at the time of the major core degradation, are highlighted and points of consensus as well as remaining uncertainties and data needs will be summarized. Fission product transport is analyzed, and the calculation results are compared with dose rate measurements in the containment. The release of I-131 and Cs-137 to the environment is compared with analysis conducted using WSPEEDI code.