Interpretation of dissolution behavior at the surface of uranium-zirconium oxide solid solutions

Tonna, Ryutaro*; Sasaki, Takayuki*; Okamoto, Yoshihiro  ; Kobayashi, Taishi*

The dissolution behavior of (U,Zr)O, the primary uranium solid phase in the fuel debris from the Fukushima Daiichi nuclear power plant accidents, was investigated thermodynamically and kinetically under atmospheric conditions. Cubic (U,Zr)O samples with a uniform solid solution of Zr were prepared using wet chemistry methods, and static batch immersion tests were conducted. In strongly acidic conditions, where the solubility of U and Zr exceeded their concentrations, congruent dissolution of both elements was observed with (U,Zr)O dissolving at the same rate as UO. In moderately acidic conditions, where the U solubility was higher than its concentration with Zr reaching a steady state at lower solubility, the U dissolution rate from (U,Zr)O decreased compared to UO. In the presence of oxalic acid, with increased Zr solubility due to the formation of complexes, the U dissolution rate from (U,Zr)O did not decrease. This indicates that Zr in (U,Zr)O formed a secondary solid phase on the solid surface under conditions of lower Zr solubility, which in turn suppressed the oxidative dissolution of U.