Nuclear energy is produced when energy is released from a nucleus, the core of an atom. While it is a clean energy source in that there are no carbon emissions, the spent radioactive waste is a concern. Storage of spent fuel rods is a complicated process involving heat transfer and chemical reactions. It is critical to be able to predict the thermal-chemical processes to ensure safe application and prevent accidents. Spent nuclear fuel requires long-term storage, sometimes below ground where groundwater could invade the canisters and cause heat transfer and chemical reactions. There is limited understanding of this interaction as experimental studies are challenging and time consuming.
Researchers wanted to create a model that could simulate the dissolution of solid materials. In “3D Thermal-Chemical Reactive Transport Modeling of Fluid-UO2 Reactions under Geological Repository Conditions,” for the Journal of Energy Engineering, Min Liu, Qinjun Kang, Hongwu Xu, and Joshua White recommend a three-dimensional model. Their model combines the mass transfer of chemical species, heat transfer in fuel pellets and groundwater, and chemical dissolution of solid nuclear fuel. Learn more about how this research can be used to analyze solid material dissolution in place of performing complicated and time-consuming experiments at https://doi.org/10.1061/JLEED9.EYENG-4614. The abstract is below.
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 UCl04 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.
Learn more about applying the researchers' new model in the ASCE Library: https://doi.org/10.1061/JLEED9.EYENG-4614.