EXPERIMENTAL DETERMINATION OF UO2 (CR) DISSOLUTION KINETICS AT HIGH BICARBONATE CONCENTRATIONS

To evaluate the possibility of utilizing REDOX permeable reactive barriers for the remediation of uranium contamination, knowledge of the fundamental reaction kinetics associated with the dissolution of reduced uranium (IV) dioxide is necessary.  Four decades of existing UO_2+x (cr) (0 < x < 0.33) dissolution studies, focusing on spent nuclear fuel disposal, have generated numerous inconsistencies and unanswered questions.  A cardinal gap in our understanding is the relationship between UO_2+x (cr) dissolution rate and bicarbonate activity.  To resolve the inconsistencies we have conducted several UO₂ dissolution experiments under oxic conditions in the presence of [HCO₃^-] = 0.1 M; at pH = 8.0; flow rates (q) = 6, 24, 48, and 120 mL d^-1; geometric surface area (S) = 10^-3 m² g^-1, and temperatures (T) = 30 and 60^oC using a single pass flow-through apparatus.

The UO₂ was characterized using a suite of analytical and chemical techniques (XRD, XAS, SEM, and chemical digestion).  Analyses confirm that the UO₂ specimen is ~ 99.9% pure with trace amounts of Co₂O₃, CuO, NiO, and SrO, and dominated by the U^IV oxidation state with a U^IV-O oxygen bond length of approximately 2.2 Å in the first shell.  SEM images of powdered specimens show that the non-fractured particles are spherical and appear to be relatively porous.

Experiments with high flow rates (120-mL d^-1) achieve steady-state U release rates after the 4^th day, which corresponds to approximately 8 reactor volumes.  The elemental release rate of U from the UO₂ specimen increases by an order of magnitude with each 30-degree increase in temperature.  Measured log₁₀ rates (in mol m^-2 s^-1) at 30ºC are -7.45±0.07 and 60ºC are -6.53±0.06.   Additional experiments over a range of flow rates indicate that the dissolution rate is not only dependent on T, but also the ratio of flow to sample surface area (i.e., q/S). In all cases, rates become constant at high values of q/S, indicating the influence of chemical affinity on the dissolution kinetics.

U dissolution rates obtained in this study are 1 to 2 orders of magnitude larger than previously reported rates and may reflect their dependence on the ratio of q/S or on bicarbonate activity.  Our results highlight the importance of understanding the role between bicarbonate activity, solution saturation state, and dissolution kinetics.