PNNL

Abstract

To evaluate the release of uranium from natural ore deposits, spent nuclear fuel repositories, and REDOX permeable reactive barriers, knowledge of the fundamental reaction kinetics associated with the dissolu-tion of uranium dioxide is necessary. Dissolution of crystalline uranium (IV) dioxide under environmental conditions has been studied for four decades but a cardinal gap in the published literature is the effect of pH and solution saturation state on UO2(cr) dissolution. To resolve these inconsistencies, UO2 dissolution experiments have been conducted under oxic conditions using the single-pass flow-through system. Experiments were conducted as a function of total dissolved carbonate ([CO3-2]T) from 0.001 to 0.1 M; pH from 7.5 to 11.1; ratio of flow-through rate (q) to specific surface area (S), constant ionic strength (I) = 0.1 M, and temperatures (T) from 23o to 60oC utilizing both powder and monolithic specimens. The results show that UO2 dissolution varies as a function of the ratio q/S and temperature. At values of log10 q/S > -7.0, UO2 dissolution becomes invariant with respect to q/S, which can be inter-preted as evidence for dissolution at the forward rate of reaction. The data collected in these experiments show the rate of UO2 dissolution increased by an order of magnitude with a 30° increase in temperature. The results also show the overall dissolution rate will increase with an increase in pH and decrease as the dissolved uranium concentration approaches saturation with respect to secondary reaction products. Thus, as the value of the reaction quotient, Q, approaches equilibrium, K, (with respect to a potential sec-ondary phase) the dissolution rate decreases. This decrease in dissolution rate was also observed when comparing measured UO2 dissolution rates from static tests where r = 1.7 ±0.14 × 10-8 mol m 2 s-1 to the rate for flow-through reactors where r = 3.1 ±1.2 × 10-7 mol m-2 s-1. Thus, using traditional static test methods can result in an underestimation of the true forward rate of UO2(cr) dissolution. These results illustrate the release of uranium from UO2 in the natural environment will be controlled by pH, solution saturation state, and the concentration of dissolved carbonate.