  Some uncertainties are discussed on the high-temperature phase boundaries and
critical point parameters for gas-liquid phase transition in silica (SiO2). The
thermal and caloric phase diagrams are compared and examined as being predicted
by various theoretical approaches, such as the quasi-chemical representation,
the wide-range semi-empirical equation of state and the ionic model under
direct molecular dynamic simulation. The theoretical predictions are confronted
with handbook recommendations and scanty experimental data on the equilibrium
vapor composition over SiO2 boiling. Validity of conventional semi-empirical
rules is tested for the theoretically predicted SiO2-phase diagrams. The
non-congruence of gas-liquid phase transition in SiO2 is considered for this
matter to be used as a modeling body to study the non-congruent evaporation in
uranium dioxide and other uranium-bearing fuels at both existing and
perspective nuclear reactors.
