Equation of state of UO2

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 constructi ng an equation of suite (EOS) for liquid and gaseous UO2, which fully repro duces the comprehensive thermodynamic database for this compound. The new e quation 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 thermo dynamic perturbation theory (TPT) modified in order to account for the spec ific properties of the system investigated. It describes, in a unified form alism, a multicomponent mixture of chemically reactive, strongly interactin g neutral and charged molecules and atoms. Comparisons of the predicted equ ilibrium vapor pressures with literature data provided an initial validatio n of the model up to temperatures of the order of 5500 K, A further, positi ve result is the fairly good agreement of the predicted beat 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 max imum vapor pressure (P-max similar to 1 GPa) as well as a substantial oxyge n enrichment of the vapor phase over boiling UO2 ((O/U)(max)similar to7). T he critical point of a truly non-congruent phase transition in UO2 Was also calculated. This point essentially differs from that defined for a gas-liq uid phase transition in simple liquids; in particular, the equation ((o) ov er capP/(o) over capV)(c) similar to (P/V) not equal 0 applies here. The pr edicted critical parameters are: T-c approximate to 10120 K, P-c approximat e to 965 MPa, rho (c) approximate to 2.61 g.cm(-3).