Pilot study on thermal, physico-chemical, and mechanical behavior of concrete to understand the failure behavior of Fukushima Daiichi Nuclear Power Station reactor pressure vessel pedestals (Contract research); FY2024 Nuclear Energy Science & Technology and Human Resource Development Project

Collaborative Laboratories for Advanced Decommissioning Science; Tokai National Higher Education and Research System*

The Collaborative Laboratories for Advanced Decommissioning Science (CLADS), Japan Atomic Energy Agency (JAEA), has been conducting the Nuclear Energy Science & Technology and Human Resource Development Project (hereafter referred to "the Project") from FY2019. The Project aims to contribute to solving problems in the nuclear energy field represented by the decommissioning of the Fukushima Daiichi Nuclear Power Station (1F), Tokyo Electric Power Company Holdings, Inc. (TEPCO). 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 FY2023, this report summarizes the research results of the "Pilot study on thermal, physico-chemical, and mechanical behavior of concrete to understand the failure behavior of Fukushima Daiichi Nuclear Power Station reactor pressure vessel pedestals" conducted in FY2024. The present study aims to examine the mechanism of the collapse of only concrete with rebar remaining in the pedestal in the containment vessel (PCV) of 1F. In verifying concrete-specific factors, (1) high-temperature heating experiments were conducted on cement paste, aggregates, and their mixtures to obtain heating behavior. Phase composition changes and melting behavior under high-temperature conditions were analyzed using thermodynamic phase equilibrium calculations, and a numerical analysis method to reproduce water absorption behavior after high-temperature exposure was developed. In addition, (2) exposure experiments simulating the temperature and water injection history of the pedestal were conducted, and time-dependent expansion after heating and expansion behavior induced by water supply were evaluated. In the verification of special external environmental factors, (1) thermal conditions of concrete in the PCV during debris accumulation were evaluated by heat transfer analysis based on accident scenarios. In addition, (2) concrete collapse behavior with and without water vapor was evaluated by small-scale high-temperature holding tests, and concrete damage behavior due to reactions with simulated fuel debris was evaluated by high-temperature reaction tests.