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Many projects involving nuclear fuel rest on a quantitative understanding of the co-existing phases at various stages of burnup. Since the fission products have considerably different abilities to chemically associate with oxygen, and the metal-to-oxygen molar ratio is necessarily increasing, the chemical potential of oxygen 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 the minor phases themselves have a variable composition dependent on temperature and possible contact with the coolant in the event of a sheathing breach.A thermodynamic database has been in development to predict the phases in partially burned CANDU (CANada Deuterium Uranium) nuclear fuel containing the major fission products. The building blocks are the standard Gibbs energies of formation of the many possible compounds expressed as a function of temperature. To these data are added mixing terms associated with the appearance of the component species in particular phases. In operational terms, the treatment rests on the ability to minimize the Gibbs energy in a multicomponent system using the algorithms developed by Eriksson. The treatment, considered applicable in the range 300 to 2000 °C, is capable of handling non-stoichiometry in the UO2 fluorite phase, dilute solution behaviour of significant solute oxides, noble metal inclusions, a second metal solid solution U(Pd – Rh – Ru)3, zirconate, molybdate, and uranate solutions as well as other minor solid phases, and volatile gaseous species. The paper highlights the current capability of an ongoing project.

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The concentration of Al and Fe in molten zinc saturated with delta (~Fe 2 Zn 10 Al) or eta (~Fe 2 Al 5 Zn) intermetallic precipitates has been measured by melt equilibration for temperatures in the range of 450°C to 480°C. This information has generated a thermodynamic model for the phase equilibrium in the zinc-rich corner of the Zn-Al-Fe system for the range of compositions and temperatures of commercial interest in continuous galvanizing and galvannealing. The model is constrained by the reported activities of Al in the molten zinc phase and permits small compositional variance of the intermetallic solids. The resultant isothermal phase diagrams are compared with the work of others.

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