The behaviour of single atoms of molybdenum in urania

Computer simulation techniques are used to investigate the behaviour of single atoms of Mo in UO2+/-x. In UO2-x, Mo is calculated to be present as neutral atoms in bound Schottky trio sites. In UO2+x, most of the Mo is calculated to be in isolated uranium vacancy sites with Mo ionisation increasing with the O/U ratio. The behaviour near stoichiometric composition is more complex and is found to be very sensitive to changes in O/U ratio. An approximation to the free energy change associated with Mo incorporation in urania is plotted as a function of O/U ratio and Mo concentration. Although this plot is found to be in agreement with the observed insoluble character of Mo in urania, at high O/U ratios and very low Mo concentrations, Mo in solution may be preferred over Mo in the gaseous state.