Low-Temperature Air Oxidation of Uranium Dioxide Single Crystals

Accidental oxidation of uranium dioxide pellets is the most critical event which may affect the long-term storage of the spent nuclear fuel. Therefore the safety of a nuclear waste disposal lies on the determination of the mechanisms which lead to the alteration of UO₂ upon air oxidation. More precisely, the study of the formation of hyperstoichiometric oxides, intermediate fluorite-type U₃O₇/U₄O₈, or layered U₃O₈, is of prime importance since the formation of the latter one has to be firmly avoided. The experiments reported in this paper serve such a purpose with the study of the transformations (stoichiometry, depth and structure) occurring during the low-temperature (T = 180°C) air oxidation of uranium dioxide single crystals. Nuclear microanalysis techniques, more particularly Rutherford backscattering in channeling conditions, are perfectly adapted to this study since they allow to characterize the surface region (up to a depth of a few micrometers) of a solid material. Furthermore the influence of damage production on the oxidation behavior of the nuclear fuel is also investigated by external pre-irradiation of the UO₂ single crystals with high-energy heavy ions simulating fission fragment irradiation in the reactor.

The results show that air oxidation at T = 180°C leads to the formation and in-depth growth of a U₃O₇/U₄O₉ oxide layer; no layered U₃O₈ oxide was detected even at the largest oxidation time investigated (1200 hours). The growth kinetic of U₃U₇/U₄O₉ oxide exhibits a linear behavior, indicating that the rate of oxidation is interface-controlled. The linear oxidation rate extracted from the data is of the order of 1.8 × 10⁻¹³ m s⁻¹, i.e. close to the oxidation rate estimated for sintered pellets. Contrarily to previous works, our results show very little influence of the crystallographic orientation of the single crystals on the oxidation kinetics. More surprisingly, pre-irradiation of UO₂ samples with swift heavy ions, leading to the polygonization of the material, does not lead to significant changes in the oxidation behavior (stoichiometry or oxidation rate) of UO₂ single crystals.