High-Temperature Thermodynamic Properties of Uranium Dioxide

Abstract

Effusion measurements have been carried out with uranium dioxide over a temperature range from 1600° to 2800°K and a pressure range from 10^—8 to 6 mm. Four different isotopic compositions, three different effusion cells and two different apparatuses were used. The results are concordant and demonstrate that the oxide vaporizes congruently in this range. The cubic (fluorite) lattice parameter of the congruently vaporizing composition, UO_2.00, is 5.4588±0.0002kX.‡ All higher oxides will yield UO_2.00 when heated above 1300°C in a vacuum of 10^—6 mm Hg. A least‐squares analysis of the data yields for the total volatility, calculated as the dioxide, the equation, $$\log \mathrm{p\hspace{0.17em}(}\mathrm{mm}\text{)=13.298−}\frac{{\text{3.7195×10}}^4}{T}+\frac{{\text{3.5162×10}}^6}{T^2}+\frac{{\text{2.6178×10}}^9}{T^3},$$ and reveals a pronounced positive curvature at the higher temperatures. It is doubtful that this curvature arises from a deviation of the flow rate from that predicted by kinetic theory, and hence it must be caused by two vaporization processes. The volatility of a uranium and uranium dioxide mixture shows that no gaseous suboxides are important. The principal vapor species at the lower temperatures (1600–2000°K) is undoubtedly UO₂ for which the vapor pressure is given by $$\log \mathrm{p\hspace{0.17em}(}\mathrm{mm}\text{)=(−33,115±371)/T−4.026}\log \text{T+25.686±0.205.}$$ The heat and entropy of sublimation are 137.1±1.7 kcal/mole and 36.4±0.9 eu at 1800°K and 159.6 kcal/mole and 49.4 eu at 298°K. The heat of sublimation at absolute zero is estimated to be 160 kcal/mole with an estimated uncertainty of 5 kcal/mole. This value leads to 14.4 ev for the dissociation energy for gaseous UO₂. The positive curvature cannot be caused by solid monoxide or gaseous UO₃ on the basis of existing thermodynamic data. It is postulated that the curvature is caused by a dimer U₂O₄(g) which becomes important at the high temperatures. Its vapor pressure is represented by $$\log \mathrm{p\hspace{0.17em}(}\mathrm{mm}\text{)=(−38\hspace{0.17em}221/T)+14.056.}$$ The heat and entropy of dissociation of the dimer are 88.9 kcal/mole and 16.6 eu at 2450°K. Crude absolute entropy calculations indicate this entropy change to be reasonable.