3D Thermal-Chemical Reactive Transport Modeling of Fluid - UO 2 Reactions under Geological Repository Conditions

In this study, we investigated uranium dioxide (UO2) dissolution under geological repository conditions by applying a three-dimensional (3D) thermal-chemical reactive transport model. The transport of chemical species and thermal conduction in UO2 fuel pellets and chemical dissolutions of UO2 were considered. The mathematical and numerical formulations of the model are described in the paper. Fluid-UO2 reactions were modeled to demonstrate the validity of modeling reaction processes. UO2 dissolution under low (25°C) and high temperatures (250°C) was simulated, taking into account the changes in aqueous uranium species with temperature. The predicted lifetime of one UO2 pellet is greatly dependent on the temperature. To illustrate the effect of uranium species on reaction rates, numerical studies were conducted at the same temperatures but with different reaction types and chemical species. It was found that reactions that produce UCl40 enhance the dissolution rates of UO2 by consuming the Cl- in solutions. UO2 dissolution with varying pH values was also modeled. When pH increased to 6, the average dissolution rate of a UO2 fuel pellet was eight times slower than it was at pH=2. Dissolution simulations were carried out on the images of fractured UO2 pellets. The impact of microfractures on UO2 dissolution was illustrated. The developed model is able to quantify UO2 dissolution behaviors and identify key parameters controlling the physiochemical processes involved. The model can be used as a predictive tool for applications such as spent UO2 fuel sequestration, contaminant transport, and geothermal resources development.