Paper AC+SS-ThM5
The Oxidation of Uranium Dioxide at High Pressures in Pure Oxygen

Thursday, November 3, 2011, 9:20 am, Room 207

The oxidation of uranium dioxide has received much experimental and theoretical attention over the last several decades in large part because of its relevance to the operation and storage of uranium-based nuclear fuel. The oxidation process is inherently complicated, involving the formation of multiple different phases via distinct mechanisms even at relatively low temperatures. In the range of a few hundred degrees centigrade oxidation is generally assumed to be a two step process[1]: UO₂ → U₃O₇/U₄O₉ → U₃O₈. At low pressures the intermediate phases adopt crystal structures that are modifications of the UO₂ fluorite structure and are slightly denser. By contrast, U₃O₈ forms a considerably less dense orthorhombic structure (by some 23%). The large volume expansion resulting from the oxidation of UO₂ to U₃O₈ is a potentially serious concern in the event of oxidation of a fuel element, with consequent splitting of protective sheaths and the spalling of powder.

While attention has been focused on the oxidation of UO₂ at elevated temperatures, the associated experiments have all been performed at low partial pressures of oxygen. It is unclear how pressure affects the oxidation process – particularly in the context of the formation of U₃O₈, with and its large volume change with respect to UO₂. We have examined the oxidation of a nominal single crystal of UO₂ in pure oxygen at elevated pressures up to approximately 0.9 GPa (9000 atm) and temperatures of up to 450 °C. In-situ Raman scattering measurements were made as a function of temperature in order to monitor the oxidation. Recovered material was examined using electron based techniques including SEM, TEM, and electron diffraction and also using x-ray photoelectron spectroscopy.

Material synthesized under high pressure has a Raman spectrum that is different from both the UO₂ starting material and the common form of U₃O₈. Also, compared with common U₃O₈, we find that it has fewer crystalline defects and mostly adopts a hexagonal rather than orthorhombic form. Figure 1 of the supplemental document compares Raman spectra of UO₂ in oxygen with synthesized material. Fig. 2 compares electron diffraction obtained from recovered material with U₃O₈ synthesized at ambient pressure.

Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work is funded by Laboratory Directed Research and Development (LDRD) Program (10-SI-016) of Lawrence Livermore National Laboratory.