MATHEMATICAL MODELLING OF THE FORMATION OF URANIUM HYDRIDE BLISTERS

Abstract

Uranium metal is used in the nuclear power and defence sectors. After use, it is extremely important to store uranium safely to prevent adverse effects on the environment. If uranium comes into contact with hydrogen, they react to form uranium hydride UH3. Uranium hydride is brittle and tends to crumble away from the metal as a powder. It is also pyrophoric and presents an issue in terms of safe storage. The metal often has a surface passivation layer (SPL) predominantly made of uranium dioxide UO2 which does not react with hydrogen and protects the metal from hydriding to an extent. Eventually hydride will form beneath the SPL. Hydride nucleates at distinct sites and grows into blisters or pits. After some time, some blisters cease to grow and some break through the SPL, exposing the surroundings to UH3. In this thesis, one- and two-dimensional multi-physics continuum models for the uranium-hydrogen system are developed and solved. The models incorporate hydrogen diffusion, hydriding reaction, and the effects of stress, deformation, temperature and the oxide layer. Simulated finite element method results for a one-dimensional model allow for comparisons between two different boundary conditions controlling hydrogen in-flux and instantaneous and time-dependent reaction kinetics with varying Damkohler values. Sensitivity analysis reveals that the parameters controlling hydrogen in-flux are highly influential on the extent to which the metal is hydrided. Results for two-dimensional finite element method models of hydriding are presented in two groups: reaction-diffusion-only models with and without an SPL, and thermoelastic models. An analysis of a reaction-diffusion-only model with no SPL reveals the evolution of a quasi-steady state that is a potential mechanism for the cessation of hydride growth. The introduction of an SPL shows that hydride is preferentially produced below regions where diffusivity in the SPL is higher. The results of the thermoelastic models reveal that restricting the non-uniform deformation of uranium induces large stresses in the metal. Difficulties encountered when combining the reaction-diffusion and thermoelastic models into a multi-physics model in two dimensions are discussed.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Richard Hewitt (Supervisor) & Andrew Hazel (Supervisor)