Estimation of the in-depth debris status of Fukushima Unit-2 and Unit-3 with Multi-physics modeling (Contract research). FY2021 Nuclear Energy Science & Technology and Human Resource Development Project

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), had been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to 'the Project') in FY2021. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company Holdings, Inc. (hereafter referred to '1F'). For this purpose, intelligence was collected from all over the world, and basic research and human resource development were promoted by closely integrating/collaborating knowledge and experiences in various fields beyond the barrier of conventional organizations and research fields. The sponsor of the Project was moved from the Ministry of Education, Culture, Sports, Science and Technology to JAEA since the newly adopted proposals in FY2018. On this occasion, JAEA constructed a new research system where JAEA-academia collaboration is reinforced and medium-to-long term research/development and human resource development contributing to the decommissioning are stably and consecutively implemented. Among the adopted proposals in FY2019, this report summarizes the research results of the 'Estimation of the in-depth debris status of Fukushima Unit-2 and Unit-3 with multi-physics modeling' conducted from FY2019 to FY2021. Since the final year of this proposal was FY2021, the results for three fiscal years were summarized. Continuous update on understanding of the damaged 1F reactors is important for safe and efficient decommissioning of the reactors. This study aimed to estimate the in-depth debris status of the damaged 1F Unit-2 and Unit-3 through multi-physics modeling, which comprises of MPS method, simulated molten debris relocation experiment and high-temperature melt property data acquisition in the three-year project from FY2019. As the results, a new insight has been gained, indicating that the preceding release from the reactor pressure vessel (RPV) is unlikely to contain large amount of UO₂ for both Unit-2 and Unit-3. Then, for Unit-3, the following UO₂ rich release from the RPV relocated to the pedestal region without significantly melting the pedestal structures. Any changes to the assumed distribution of the relocated debris may not have significant influences on the estimated criticality of the debris, and overall, the debris in Unit-3 pedestal region is expected to be well below criticality. (author)