Structural Characterisation of Advanced Cr & Mn Doped UO₂ Fuels Using X-Ray Absorption Spectroscopy

Advancements in UO₂ fuel technology are key in the generation of safe and efficient nuclear energy. The doping of UO₂ with transition metals, such as Cr and Mn, provides a method for improving properties, such as increased grain size, improved plasticity, and reduced pellet-cladding interactions. All these enhanced properties result in more accident tolerant and efficient fuels. Despite the implementation of Cr and Mn-doped UO₂ technology within the nuclear industry, there is a lack of fundamental understanding surrounding the local Cr/Mn coordination environment upon incorporation into the UO₂ matrix. Furthermore, the mechanisms by which Cr/Mn are accommodated within the UO₂ lattice are not well established. Here, a variety of spectroscopic techniques, including X-ray absorption spectroscopy (XANES, EXAFS, and HERFD-XANES) and Raman spectroscopy, provide significant insight into mechanisms behind the incorporation of Cr/Mn upon incorporation into the UO₂ structure. In the Mn-doped UO₂ system, Mn²⁺ is substituted onto the U⁴⁺ site, with vacancies and U⁵⁺ forming to maintain charge balance. A similar incorporation mechanism is observed in the Cr-doped UO₂ however, the UO₂ matrix accommodates Cr as a Cr²⁺on the U⁴⁺ site. Following further heat treatment (to simulate nuclear fuel synthesis), Cr is present as several different species (including, Cr²⁺ and Cr³⁺) in a variety of locations within the UO₂ fuel. The findings of these studies provide significant new insight into the fundamental chemistry behind doped UO₂fuels and offer new opportunities for advanced nuclear fuels to be further developed and implemented within the nuclear industry.