Investigation of environment induced property change and cracking behavior in fuel debris (Contract research). FY2022 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 FY2022. 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. (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 FY2020, this report summarizes the research results of the 'Investigation of environment induced property change and cracking behavior in fuel debris' conducted from FY2020 to FY2022. The present study focuses on 'metallic debris', which is mainly composed of elements derived from fuel cladding and control rod sheaths. This is the final year of the three years project. The findings on their material properties are as follows. Six phases are precipitated in the metallic debris according to the state diagram of the Zr-Fe-B-C system. Under high-temperature oxidation conditions, an outer film composed mainly of Zr₃Fe is formed on the surface. An inner layer oxide film is also formed according to the chemical composition of the matrix phase of the debris. The growth of the inner film is accompanied by volumetric expansion, but since ZrB₂ grains are hard and do not oxidize, surface ridges and cracks occur. The phases constituting the metallic debris is corrosion resistant at about 300 ℃. Hydrogen absorption into the metallic debris is limited. There hardness and toughness are heterogeneous, but are categorized into two groups. (author)