Modelling uranium dioxide corrosion under repository conditions: A pore-scale study of the chemical and thermal processes

Abstract This paper investigates the corrosion behaviors of spent uranium dioxide (UO2) fuel when placed in geological repositories. We performed pore-scale reactive transport simulations of the UO2 corrosion process in a defective fuel rod with different orientations of breach or fracture on its clad. It is found that the corrosion rate has strong dependency on the breach orientation. The highest corrosion rate of UO2 is calculated when the angle between the groundwater flow direction and the clad fracture reaches 180°. UO2 corrosion simulations with different flow rates are compared. The results show the higher flow rate accelerates the corrosion of UO2 fuel. The effect of pH on the corrosion process is also determined. It is indicated that higher pH can limit the corrosion of UO2 by reducing the reaction rate. The dependence of reaction rate and time on reactive surface area is explored. Spent fuel with lower surface area demonstrates longer lifetime under corrosion conditions. A numerical model coupling thermal conduction and chemical reactions is developed to assess the impact of temperature on the process of UO2 corrosion. The results show that higher temperature leads to larger corrosion rates for UO2. The predicted reaction rates are higher in comparison with the isothermal results. Hence, our results can help improve the fundamental understanding of UO2 corrosion in geological repositories for long-term storage of spent nuclear fuels and provide guidance for the safe operations and selection of appropriate repositories.