First Principles Model of CANDU Fuel Phase Equilibrium

Abstract

Many modeling projects on nuclear fuel rest on a quantitative understanding of the coexisting phases at various stages of burnup. Since the various fission products have considerably different abilities to chemically associate with oxygen, and the O/M ratio is slowly changing as well, the chemical potential (generally expressed as an equivalent oxygen partial pressure) is a function of burnup. Concurrently, well recognized small fractions of new phases such as inert gas, noble metals, zirconates, etc. also develop. To further complicate matters, the dominant UO₂ fuel phase may be non-stoichiometric and most of minor phases have a variable composition dependent on temperature and possible contact with D₂O in the event of a sheathing defect.

The treatment discussed is a melding of several thermodynamic modeling projects over the past few years dealing with isolated aspects of this cornerstone multicomponent system. To simplify the computations, the number of elements has been limited to twenty major representative fission products known to appear in spent fuel. The proportion of elements was generated using ORIGEN except for oxygen, which was inferred from the concentration of the others. Provision to study the disposition of very minor fission products is included within the general treatment but these are introduced only on an as needed basis for a particular purpose. The building blocks of the model are the standard Gibbs energies of formation of the many possible compounds expressed as a function of temperature. To this data is added mixing terms associated with the appearance of the component species in particular phases. Sample output will be compared to the available direct knowledge gathered from post irradiation examination.