Comparison of uranium dissolution rates from spent fuel and uranium dioxide

Abstract

Two similar sets of dissolution experiments, resulting from a statistical experimental design were performed in order to examine systematically the effects of temperature (25--75{degree}C), dissolved oxygen (0.002-0.2 atm overpressure), pH (8--10) and carbonate concentrations (2--200 {times} 10{sup {minus}4} molar) on aqueous dissolution of UO{sub 2} and spent fuel. The average dissolution rate was 8.6 mg/m{sup 2}{center_dot}day for UO{sub 2} and 3.1 mg/m{sup 2}{center_dot}day for spent fuel. This is considered to be an insignificant difference; thus, unirradiated UO{sub 2} and irradiated spent fuel dissolved at about the same rate. Moreover, regression analyses indicated that the dissolution rates of UO{sub 2} and spent fuel responded similarly to changes in pH, temperature, and carbonate concentration. However, the two materials responded very differently to dissolved oxygen concentration. Approximately half-order reaction rates with respect to oxygen concentration were found for UO{sub 2} at all conditions tested. At room temperature, spent fuel dissolution (reaction) rates were nearly independent of oxygen concentration. At 75{degree}C, reaction orders of 0.35 and 0.73 were observed for spent fuel, and there was some indication that the reaction order with respect to oxygen concentration might be dependent on pH and/or carbonate concentration as well as on temperature.