Thermomigration of oxygen in (U, Pu)Osub(2-x) described by a cluster model

The theory of solid state oxygen thermomigration in hypostoichiometric mixed uranium-plutonium oxide is developed taking into account clustering of the oxygen vacancies. It is assumed that an oxygen vacancy tends to associate with a pair of plutonium cations, which are reduced to the trivalent state. Equations are derived for the steady state stoichiometry variation produced by a sufficiently long anneal in a temperature gradient. The variation of Arrhenius slope with stoichiometry is a natural consequence of the theory. Theoretical curves are a good fit to published experimental data, numerical values suggesting that the steady state oxygen distribution is largely controlled by the effective oxygen vacancy formation enthalpy. (Auth.)