Equation of State of UO2

Abstract

An international project supported by INTAS (International Association for Promotion of Cooperation with Scientists from the New Independent States of the former Soviet Union) was started in 1994 with the intent of constructing an equation of state (EOS) for liquid and gaseous UO2, which fully reproduces the comprehensive thermodynamic database for this compound. The new equation of state was devised for applications encompassing hypo- and hyper-stoichiometric compositions. A so-called “chemical model” was used for the theoretical description of liquid urania. This model is based on the thermodynamic perturbation theory (TPT) modified in order to account for the specific properties of the system investigated. It describes, in a unified formalism, a multicomponent mixture of chemically reactive, strongly interacting neutral and charged molecules and atoms. Comparisons of the predicted equilibrium vapor pressures with literature data provided an initial validation of the model up to temperatures of the order of 5500 K. A further, positive result is the fairly good agreement of the predicted heat capacity with experimental values, which extend up to 8000 K. A characteristic feature of non-congruentvaporization in UO2±x is the production of a very high maximum vapor pressure (P max∼1 GPa) as well as a substantial oxygen enrichment of the vapor phase over boiling UO2 ((O/U)max∼7). The critical point of a truly non-congruent phase transition in UO2 was also calculated. This point essentially differs from that defined for a gas–liquid phase transition in simple liquids; in particular, the equation (∂P/∂V) c ∼(P/V)≠0 applies here. The predicted critical parameters are: T c≈10120 K, P c≈965 MPa, ρ c≈2.61 g·cm−3.