Chemical forms of uranium evaluated by thermodynamic calculation associated with distribution of core materials in the damaged reactor pressure vessel

To suggest efficient process of the fuel debris treatment after the retrieval from the Fukushima Daiichi Nuclear Power Plant (1F), thorough investigation is indispensable on potential source of U in the fuel debris. The present study aims to estimate chemical forms of U in the in-vessel fuel debris, especially in the minor phases such as metallic phases, by performing the thermodynamic calculation under various conditions considering material relocation and changing environment during the accident progression in the 1F Unit 2. Input conditions for the thermodynamic calculation such as composition, temperature, and oxygen amount were set mainly based on the transient change in the core material distribution which had been calculated with severe accident analysis codes such as MAAP and MELCOR. The result showed that chemical form of U varied depending on the local amount of Fe and O. In regions of low steel content, the U-containing metallic phase was dominated by α-(Zr,U)(O), while regions of high steel content were dominated by Fe2(Zr,U) (Laves phase). A few percent of U transferred to the metallic phases were highly expected under reducing conditions. Therefore, those metallic phases should be one of the potential sources of U.