Dissolution behavior of uranium oxides with supercritical CO sub 2 using HNO sub 3 -TBP complex as a reactant

• Published in: Journal of nuclear science and technology Vol. 38; no. 12; pp. 1097 - 1102
• Main Authors: Tomioka, O, Meguro, Y, Enokida, Y, Yamamoto, I, Yoshida, Z
• Format: Journal Article
• Language: English
• Published: 01.12.2001
• ISSN: 0022-3131

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