Application of constrained Gibbs energy minimization to nuclear fuel thermochemistry

The modelling of the chemical composition of nuclear fuel is a complex problem due to the numerous fission products formed during fuel operation and their possibility to form various chemical compounds. The application of Gibbs energy minimization to model the chemical composition requires the assumption that the fuel is at local thermochemical equilibrium when the fuel is thermally in a steady state. The temperature through the pellet radius can vary 1000 K, with highest temperatures found in the pellet center. Depending on the local fuel temperature, the validity of the local equilibrium approximation varies, as diffusion and chemical kinetics limit the formation of the thermodynamically most stable species. At the fuel surface, the state of the fuel is farthest from equilibrium.

However, with constrained Gibbs energy minimization it is possible to investigate the thermochemically most favourable state of the fuel in cases where the composition of the fuel is not at equilibrium. As a first application on nuclear fuel, the effect of cesium iodide vaporization and radiolysis in a radiation field is investigated with constrained Gibbs energy minimization. A kinetic model for cesium iodide radiolysis is used to determine a steady state, which is then used to constrain the equilibrium calculated for a nuclear fuel pellet surface. The feedback from such an analysis can be used to further refine kinetic models, as the thermochemical equilibrium calculation yields information on the possibly important species that a kinetic model should consider.