IMPERFECT VAPOR GAS-MIXTURES AND HOMOGENEOUS NUCLEATION

We study the imperfect behaviour of vapour-gas mixtures and the associated total pressure dependence of the homogeneous nucleation rate of liquid droplets from a vapour. A pressure dependent rate of homogeneous nucleation has an important bearing on the analysis of aerosol formation during a severe reactor accident. It is found from a statistical mechanical approach that interactions between gas molecules and vapour monomers or sub-critical clusters perturbs the equilibrium between vapour and condensate and thereby alters the nucleation rate. The effect can be expressed in terms of second virial coefficients. Estimates of the effect lead to a nucleation rate reduction with increasing pressure, the magnitude depending on the carrier gas used. The largest effect considered involves a six-fold reduction in rate for n-nonane nucleation in argon at 273 K and a supersaturation of 10, when the pressure is raised by 1 atm. The reduction is rather less significant in a carrier gas of helium.