Main Findings, Remaining Uncertainties and Lessons Learned from the OECD/NEA BSAF Project

The OECD/NEA Benchmark Study at the Accident of Fukushima project (BSAF) has started in 2012 until 2018 as one of the earliest responses to the accident at Fukushima Daiichi with 15 participating organizations out of 10 countries (see list of authors). The project divided into two phases addressed the investigation of the accident at Unit 1, 2 and 3 by Severe Accident (SA) codes until 500 h focusing on thermal-hydraulics, core relocation, Molten Corium Concrete Interaction (MCCI) and Fission Product (FP) release and transport. The objectives of OECD/NEA BSAF were to make up plausible scenarios based primarily on SA forensic analysis, support the decommissioning and inform SA codes modeling. The analysis and comparison among the institutes have brought up vital insights regarding the accident progression identifying periods of core meltdown and relocation, and Reactor Pressure Vessel (RPV) and Primary Containment Vessel (PCV) leakage/failure through the comparison with various plant data (presented in the related papers in the same session). The combination of code and inspections results (muon radiography, PCV inspection) has provided a picture of the current status of the debris distribution and plant status. All units present a large relocation of core materials and all of them present ex-vessel debris with Unit 1 and Unit 3 showing evidences of undergoing timely limited MCCI. Uncertainties have been identified in particular on the time and magnitude of events such as corium relocation inside and from the RPV onto cavity floor, RPV and PCV failure events. Main uncertainties resulting from the project are the large and continuous MCCI progression predicted by basically all the SA codes and the leak pathways from RPV to PCV and PCV to reactor building and environment. The OECD/NEA BSAF project represents a pioneering exercise which has set the basis and provided lessons learned not only for code improvement but also for the development of new related projects to investigate in details further aspects of the Fukushima Daiichi accident.