Analysis and Modeling of Oxide Reduction Processes for Uranium Oxides

This article describes a kinetic model for the reduction process of used oxide nuclear fuel with molten LiCl at 650°C. In this process, metal oxides, primarily UO₂, react to form metallic U and Li₂O. As UO₂ is gradually reduced, metallic U and Li₂O form in molten LiCl, in and through which Li₂O dissolves and diffuses. To capture the prescribed chemical and physical phenomena, two classical kinetic models are integrated sequentially to describe Li₂O formation, dissolution, and diffusion behaviors. The first part of the integrated model uses a shrinking core kinetics to explain the formation, dissolution, and diffusion of Li₂O while accounting for the expanding metallic uranium layer. The second part of the integrated model begins with the completion of Li₂O dissolution and deals with the diffusion of excess Li₂O in the metallic uranium layer. A Fick's second law without an external source describes the second part of the process behavior. The integrated model explains engineering scale kinetic measurement data from the literature with the effective Li₂O diffusion coefficient range between 2 and 6 (10⁻⁵cm²/sec).