[M-AG39] Radioisotope migration: New development for dynamics study of radionuclides from 1F NPP accident

[MAG39-11] Dissolution behavior of iron-uranium oxide

*Ryutaro Tonna¹, Takayuki Sasaki¹, Yoshihiro Okamoto³, Taishi Kobayashi¹, Daisuke Akiyama², Akira Kirishima², Nobuaki Sato² (1.Kyoto University, 2.Tohoku University, 3.Japan Atomic Energy Agency)

Keywords:iron-uranium oxide, Dissolution behavior, X-ray diffraction

At Fukushima Daiichi Nuclear Power Plant, efforts are underway to retrieve fuel debris generated by severe accident. Since the retrieved fuel debris may be directly disposed deep underground, understanding of dissolution behavior of fuel debris and chemical speciation of dissolved nuclides in underground water, and their modeling are essential to improve the reliability of safety assessment for the disposal system. The chemistry of the fuel debris depends on the atmosphere and temperature at the time of its formation, and the formation of FeUO₄ phase has been suggested under an oxidative condition caused by the hydrogen explosion (D. Akiyama et al. 2019, V. Almjashev et al. 2011). In this compound, U was reported to take pentavalent state (X. Guo et al. 2016). However, the dissolution reaction of FeUO₄ has not been investigated so far. In this study, the dissolution reaction of FeUO₄ in water was investigated. FeUO₄ was synthesized by heating in an electric furnace, purified by immersed in HNO₃ solution to remove uranium oxide impurities, and then immersed in a solution of pH 2~8. After a certain period of time, pH and Eh were measured, and the dissolved Fe and U concentrations were determined by ICP-MS. It was interpreted that the dissolution of FeUO₄ was accompanied by the redox reactions of +3/+2 of Fe and +5/+6 of U. Characterization of the solid state of FeUO₄ before and after immersion will be also reported.