Thermodynamics of the O–U system. I – Oxygen chemical potential critical assessment in the UO2–U3O8 composition range

A critical assessment of oxygen chemical potential of UO2+x, U4O9 and U3O8 oxide non-stoichiometric phases as well as of diphasic related domains has been performed in order to build up primary input data files used in a further optimization procedure of thermodynamic and phase diagram data for the uranium–oxygen system in the UO2–UO3 composition range. Owing to the fact that original data are very numerous, more than 500 publications, a twofold process is used for the assessment – (i) first a critical selection of data is performed for each method of measurement together with a careful estimate of their uncertainties, (ii) second a reduction of the total number of data on the basis of a chart with fixed intervals of temperature and composition that allows a comparison to be made of the results from the various experiments. Results are presented for chemical potentials of oxygen with their associated uncertainties.