Rene Bes: Nuclear fuel study using XAS: benefits of HERFD for U valence states evaluation

The safe use and disposal of UO2 based nuclear fuels relies on the stability of their material properties, under extreme conditions of temperature and irradiation, and with constantly evolving chemical composition. Among them, the uranium valence state’s behavior is at the heart of safety assessment during the entire fuel lifecycle. First, the oxidation from UO2 to UO3 induces for example considerable reorganization of the crystal structure and results in a volume expansion of about 36% when reaching the U3O8 intermediate state, eventually leading to fuel cladding failure. Secondly, the uranium valence state U6+ and to a lesser extent U5+ are known to have solubility several orders of magnitude higher than U4+, which is a critical parameter during fuel final disposal, the UO2 matrix very slow dissolution acting as the ultimate barrier of radioactivity release into the environment. X-ray absorption spectroscopy (XAS) is a key technique to assess U valence states in nuclear fuels thanks to its elemental sensitivity. However, XAS applied to nuclear fuels suffers from the presence of almost all the periodic table in spent fuel, from the high radioactive background limiting the number of available beamlines, from the fact that transmission mode is often impossible to perform, and from the large core-hole broadening effects. I will discuss how High Energy Resolution Fluorescence Detected XAS (HERFD-XAS) can help overcoming those drawbacks through a selection of a few recent examples, and I will describe the main benefits of HERFD for U valence state evaluation.

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