The dissolution rate of unirratiated uranium dioxide under repository conditions: The influence of fuel and water chemistry, dissolved oxygen, and temperature

The dissolution rate of both unirradiated UO2 and spent fuel has been studied by numerous countries as part of the performance assessments of proposed geologic repositories. The effects of variables such as temperature, dissolved oxygen, and water and fuel chemistry on the dissolution rates of the fuel are necessary to provide a quantitative estimate of the potential dose over geologic time frames. The primary objective of this research was to determine the influence these parameters have on the dissolution rate of unirradiated UO2 under Yucca Mountain repository conditions and compare them to the current Yucca Mountain Model.;Fuels containing between 0 and 8 wt% Gd2O3-doped UO2 were tested in a single-pass flow-through setup. These tests have verified that in bicarbonate solutions as temperature increased the dissolution rate increased. However, the presence of silicate in the feedwater altered the system and lowered the dissolution rate at higher temperatures. Pure UO 2 samples exhibited a dependence on the dissolved oxygen concentration, which in the current experiments was varied from 3.0 to 8.7 ppm. The significance of this dissolved oxygen dependence increased with rising temperature. At 75ºC the powder samples had a maximum dependence of 0.7, although the fragment samples had a much larger dependence up to 2.2. For the case of the Gd2O3-doped samples, there was minimal oxygen dependence at any temperature. The Gd2O3-dopant stabilized the fuel matrix, which lowered the dissolution rates by over an order of magnitude at the higher dopant levels. This effect in lowering the dissolution rate was more pronounced at higher temperatures, and additional dopant continued to decrease the dissolution rate up to the 4 wt% Gd2O3-doped UO2 tested.;The dissolution rates for pure UO2 compared reasonably well with the Yucca Mountain Model for tests performed at 50ºC and 75ºC, but were found to be approximately half the values predicted by the model at 25ºC. After long time periods when the radiolytic field has diminished, additional benefits in dissolution reduction should exist due to constituents, such as Gd2O3 being present in spent fuel and silicate being present in the groundwater. Thus, the durability of spent fuel in Yucca Mountain may be greater than previously modeled.