A detailed model of the interaction of ruthenium and urania is developed and compared to experimental data. The mechanism involves physical solution of the metal in the grain boundaries of the ceramic followed by simultaneous diffusion and chemical reaction to produce URu3 intergranular inclusions. The process occurs only when the oxide is substoichiometric, the reduction being effected by oxygen absorption by the refractory metal crucible containing the specimen. Reaction ceases when the URU3 product in the grain boundary reaches a thickness that prevents removal of the other reaction product, oxygen. Fitting the model predictions to the isothermal ruthenium spreading data from a source plane of the metal held between oxide pellets provides quantitative estimates of the parameters of the model The theory also correctly predicts the shape and magnitude of ruthenium migration in UO2 in a temperature gradient, in which thermal diffusion does not appear to play a significant role.