Ideal solution behavior and heats of fusion from the UO2-PuO2 phase diagram

Abstract Under equilibrium conditions, the binary system UO 2 -PuO 2 forms a continuous series of solid solutions, and has the familiar lens-shaped phase diagram, without the previously reported maximum in the melting point. The curves are reproduced with great accuracy by using the ideal solution relations and values of the heats of fusion for the pure constituents obtained from the two-component melting point data. The Epstein and Howland approximation, that the entropy of fusion of metals and oxides is equal to R per atom, is subjected to a rigorous statistical evaluation by writing ΔH i / T i = α i N i R . If the rule were exact, α i would be unity. For both the 56 elements and the 46 oxides for which reliable heat of fusion data are available, it was found that the mean value of the deviation constant is $ α = 1.18 ± 0.32 . For UO 2 and PuO 2 , the values are αuo 2 = 1.140 ± 0.001, αpuo 2 = 1.057 ± 0.001, and the corresponding heats of fusion are ΔHuo 2 = 21 200 cal / mole , ΔHPuo 2 = 16 800 cal / mole , with an estimated uncertainty of 1300 cal/mole. Using these constants, the Lyon and Baily phase diagram can be derived theoretically, with deviations no greater than the experimental uncertainties.