Dissolution behavior of iron-uranium oxide

Tonna, Ryutaro*; Sasaki, Takayuki*; Okamoto, Yoshihiro  ; Kobayashi, Taishi*; Akiyama, Daisuke*; Kirishima, Akira*; Sato, Nobuaki*

Since fuel debris recovered from the Fukushima Daiichi Nuclear Power Plant may be directly disposed of in deep geological strata, knowledge of dissolution reactions of fuel debris in water and chemical states of dissolved nuclides is essential for safety assessment of disposals. The chemical composition and physical properties of fuel debris depend on the atmosphere and temperature, but the formation of FeUO has been suggested at conditions under which air enters the reactor from outside. Uranium in FeUO is reported to be pentavalent, but no dissolution reaction in water has been investigated. In this study, FeUO was synthesized by heating at a predetermined temperature and oxygen pressure, immersed in nitric acid to remove unreacted uranium oxides, and then immersed in a solution of pH2-8. After the prescribed period, pH and Eh values were measured, and dissolved iron and uranium concentrations were measured by ICP-MS. X-ray absorption fine structure (XAFS) and powder X-ray diffraction (XRD) were used to evaluate the solid state before and after immersion. From these results, it was interpreted that the dissolution of FeUO was accompanied by a redox reaction between Fe(III)/Fe(II) and U(V)/U(VI) during dissolution.