Investigation for damage condition and radiation dose in Unit 3 at Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company Holdings (December 2019). Photo: Japan Nuclear Regulation Authority

The Analysis of Information from Reactor Buildings and Containment Vessels of Fukushima Daiichi Nuclear Power Station (ARC-F) project was initiated in January 2019 for 3 years as one of near-term projects proposed by the Senior Expert Group on Safety Research Opportunities Post-Fukushima (SAREF) to follow the Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF project). It was supported by 12 countries. It addressed the following objectives: i) to resolve major remaining open issues identified in BSAF2, ii) to collect, compile and analyse publicly available data and information from the Fukushima Daiichi plant, and iii) to propose a next-phase project to further support management of the consequences of the accident at the plant site.

Three main tasks were conducted, consisting of activities relevant to refinement of analysis of accident scenarios and related fission product (FP) transport and dispersion, compilation and management of data and information and discussion for a next-phase project.

The first task included improvement of the accident scenario analysis, in-depth analysis for specific uncertain phenomena and backward source term analysis and offsite consequence analysis.

The plant modelling was refined which improved replication of available measured thermal hydraulics data. Analyses emphasised the significant impact of differences in material interaction models on core damage progression and that the remobilisation of FPs is an important phenomenon that needs to be investigated further. In-depth analysis for in-vessel melt progression, molten core-concrete interaction (MCCI), FP transport and source term, and hydrogen (H₂) transport and combustion were conducted. A code benchmark on MCCI for unit-1 was performed. Similarly to BSAF2, continuous concrete erosion was calculated with however narrower scatter. The most important models and conditions for further development were identified.

The work for FP transport and source term covered FP transport and behavior in the buildings, FP speciation, pool scrubbing, iodine chemistry and uncertainty analysis and variant calculations. Significant advances were made in the description of FP chemistry with a focus on Cs and I. The results will be used for enhancing understanding of the accident radiological consequences and for determining the causes of the major FP release events.

The H₂ transport and combustion were investigated for units 1 and 3. Unit 1 analysis implied the likelihood of an early H₂ combustion in the reactor well which might be a possible cause for the failure of the shield plugs. The analysis of H₂ combustion in unit 3 aimed at explaining the formation of orange-coloured flames and the rise of a black smoke plume during hydrogen explosion. The reason could be the combustion of organic gases generated by thermal decomposition of organic materials in the containment vessel.

Several improvements were achieved in techniques for backward source term. In particular, the identification and resolution of specific release episodes could be enhanced. The forward offsite consequence analysis was performed with different sets of weather data. Differences arose due to differences in weather data sets. Evaluation of weather data sets uncertainties is proposed as a future work.

Two databases storing publicly available information collected from websites of relevant Japanese organisations were built and provided to the project partners. They contain records based on documents and samples taken from the Fukushima Daiichi plant.

Finally, scopes of work and technical tasks of a next-phase project were deeply discussed among the partners of the ARC-F and Preparatory Study on Analysis of Fuel Debris (PreADES) projects and resulted in the launching of the FACE project. 

The final summary report of the project is near completion; plans are to have it completed end of 2022. 

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