THERMAL DIFFUSION IN CLOSED OXIDE FUEL SYSTEMS

Several mechanisms for thermal diffusion in oxide fuels are presented and compared with experimental data. It is shown that gas phase transport along cracks and channels of interconnected porosity is an important aspect of the mechanism. The vapor phase transport path together with oxygen transport via solid phase diffusion provides a dynamically balanced flow of material at steady state which leads to a general relation, In x = Q*/(RT) + A, where x is the deviation from stoichiometry, T is the temperature and Q* is the heat of transport. The values of Q* are related to the partial molar enthalpies of the constituents involved in the transport mechanism which aid in the selection of a particular mechanism for a given set of conditions. The mechanism which best fits the experimental observations for hypostoichiometric urania is based on a dynamically balanced flow of UO2 in the gas phase and oxygen in the solid phase. For hypostoichiometric urania-plutonia solid solutions, the mechansim appears to be a dynamically balanced flow of H2O or CO2 in the gas phase and oxygen in the solid phase. For stoichiometric and hyperstoichiometric compositions, the mechanism proposed by Markin and Band seems appropriate. © 1969.