  The paper presents a model for liquid uranium dioxide, obtained by improving
a simplified ionic model, previously adopted to describe the equation of state
of this substance [1]. A "chemical picture" is used for liquid UO2 of
stoichiometric and non-stoichiometric composition. Several ionic species are
considered here: U(5+), U(4+), U(3+), O(2-) and O(-). The ions are described as
charged hard-spheres of different diameters. Coulomb interaction of ions is
taken into account according to the modified Mean Sphere Approximation (MSA).
The main result of the new model is the appearance of natural "plasma"
equivalent, which, from the theory, is directly related to the definition of
oxygen potential in liquid UO(2+x). The features of the model make it possible
to describe non-congruent phase equilibrium (and evaporation) in uranium
dioxide, as well as other relevant phenomena characterising the phase
equilibrium in chemically active matter. First calculation results are
discussed.
