Extraction behaviors of U(VI) and selected fission product elements from HN0 3 solution into a supercritical C0 2 fluid containing tributylphosphate (TBP) were studied under equilibrium and dynamic extraction conditions. Uranium(VI) was found to be extracted in the supercritical C0 2 phase as a form of U0 2 (N0 3 ) 2 (TBP) 2 with a distribution ratio of, e.g., 2.2±0.3 for the extraction of 2 X10" 4 to 2 X 10" 2 M U(VI) from 3 M HN0 3 into supercritical C0 2 -0.3 M TBP mixture at 60°C and 15 MPa. The distribution ratio of U(VI) increased with an increase of HN0 3 concentration (0.1-6 M) or TBP (0.1-0.3 M), along with a decrease of pressure (10-40 MPa) at 60 °C, and with an increase of the temperature (40-80°C) at 15 MPa. A salting-out effect enhancing the distribution of U(VI) was observed by addition of a fairly high concentration of LiN0 3 to the aqueous phase. Results of dynamic extraction using a continuous flow of supercritical C0 2 -TBP through the aqueous nitric acid solution showed that >98% of U(VI) could be exctracted from 3 M HN0 3 +3 M LÍN0 3 solution into supercritical C0 2 -0.08 M TBP (60°C, 15 MPa). Main fission product elements such as lanthanides, Cs, Sr, Ba, Zr, Mo, and Pd were hardly extracted into the supercritical C0 2 phase under this condition, which suggests that the supercritical C0 2 extraction is a feasible method for the separation and recovery of uranium from these elements. Since rapid and complete removal of the C0 2 medium from the extracts is attained by gasification of C0 2 , the amount of the organic solvent waste generated in the extraction process can be largely minimized. Advantages of the supercritical C0 2 extraction in its application to the separation process in the field of nuclear technology are discussed.

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The extraction equilibrium of U(VI) between a nitric acid solution and a supercritical CO(2) phase containing tributyl phosphate (TBP) is formulated taking into account that (i) a distribution ratio of a metal extracted is a function of a distribution constant of each component involved in the extraction reaction, (ii) the distribution constant is defined as a ratio of solubilities of the component in both phases, and (iii) the solubility in the CO(2) phase is correlated with density of CO(2). A simple linear relationship between the distribution ratio, D(U), of U(VI) and density, ρ, of CO(2) is derived; log D(U) = a log ρ + A + B, in which a is a proportional constant implying the solvation characteristic of the solute in supercritical CO(2), A is a pressure-independent constant, and B is a variable determined by the distribution equilibrium of HNO(3). The equation derived was verified experimentally by the measurement of the distribution ratio of U(VI) and HNO(3) under various conditions of pressure and temperature. A novel concept of selective supercritical fluid extraction of metals by means of pressure-tuning or CO(2) density-tuning was proposed.

Dissolution behavior of U 3 O 8 and UO 2 using supercritical CO 2 medium containing HNO 3 -TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO 3 /TBP ratio of the HNO 3 -TBP complex and the concentration of the HNO 3 -TBP complex in the supercritical CO 2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U 3 O 8 when the HNO 3 /TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO 3 ) 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 t 1/2 was proportional to the logarithmic concentration of HNO 3 , C HNO 3 , in the supercritical CO 2 . The slopes of the ln(1/t 1/2 ) vs. ln C HNO 3 plots for U 3 O 8 and UO 2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U 3 O 8 and UO 2 are different. A principle of the dissolution of uranium oxides with the supercritical CO 2 medium is applicable to a method for the removal of uranium from solid matrices.