Dissolution behavior of uranium oxides with supercritical CO2 using HNO3-TBP complex as a reactant

Dissolution behavior of U3O8 and UO2 using supercritical CO2 medium containing HNO3-TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO3/TBP ratio of the HNO3-TBP complex and the concentration of the HNO3-TBP complex in the supercritical CO2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U3O8 when the HNO3/TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO3)(2)TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t(1/2)) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t1/2 was proportional to the logarithmic concentration of HNO3, C-HNO3, in the supercritical CO2. The slopes of the ln(l/(t1/2)) vs. In C-HNO3 plots for U3O8 and UO2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U3O8 and UO2 are different. A principle of the dissolution of uranium oxides with the supercritical CO2 medium is applicable to a method for the removal of uranium from solid matrices.