Solid state transport as a mechanism of oxygen thermomigration in (U,Pu)O2±x

A general account is given of the theory of thermomigration as it applies to oxygen redistribution via the solid state in non-stoichiometric mixed uranium-plutonium oxide. This predicts that oxygen redistributes down the temperature gradient in hypostoichiometric material and up in hyperstoichiometric material. Some previous treatments are criticised. The evidence from laboratory experiments suggests that solid state transport is the dominant mechanism. A treatment, such as the cluster model, based on the manner in which oxygen deficiency or excess is accommodated in the structure, accounts naturally for the observed behaviour. The CO₂/CO route may be limited because of sluggish transfer of oxygen between the solid and the gas mixture. The H₂O/H₂ route could be important in reactor fuel pins during service, the behaviour then being different from that found in the out-of-pile work.