  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.
