Internal Interface Strains Effects on UO2/U3O7 Oxidation Behaviour

TL;DR

This paper investigates how interface strains influence the oxidation behavior of UO2 to U3O7, focusing on crystallographic orientation, epitaxy relations, and mechanical stresses that lead to cracking during oxidation.

Contribution

It provides new insights into the crystallographic epitaxy relations and strain effects at the UO2/U3O7 interface, supported by FEM simulations of oxide layer growth.

Findings

Epitaxy relations between UO2 and U3O7 are determined using Bollmann's method.

Crystallographic compatibility conditions at the interface are analyzed.

A critical U3O7 layer thickness is identified through FEM simulations.

Abstract

The growth of a U3O7 oxide layer during the anionic oxidation of UO2 pellets induced very important mechanical stresses due to the crystallographic lattice parameters differences between UO2 and its oxide. These stresses, combined with the chemical processes of oxidation, can lead to the cracking of the system, called chemical fragmentation. We study the crystallographic orientation of the oxide lattice growing at the surface of UO2, pointing the fact that epitaxy relations at interface govern the coexistence of UO2 and U3O7. In this work, several results are given: - Determination of the epitaxy relations between the substrate and its oxide thanks to the Bollmann's method; epitaxy strains are deduced. - Study of the coexistence of different domains in the U3O7 (crystallographic compatibility conditions at the interface between two phases: Hadamard conditions). - FEM simulations of a multi-domain U3O7 connected to a UO2 substrate explain the existence of a critical thickness of U3O7 layer.