The research for radiation-tolerant materials with fluorite related structure is an area of intense research for developing reliable nuclear energy systems. These systems, and advanced fuels in particular, require accurate scientific data in order to ensure their safe behaviour throughout the fuel life cycle, from fabrication to end of life storage. The problem of understanding and developing a predictive capability for the evolution of fuels is challenging, even for phenomena apparently simple as oxidation. Uranium oxides have deceptively simple approximate chemical formulas that actually betray their extremely complex structural features and their propensity to form nonstoichiometric phases of composition UO2+x, where U atoms exhibit a mixed valence. We will summarize a multidisciplinary effort to characterize the structural features of laboratory prepared UO2+x systems (0