ENERGY FLUCTUATIONS IN HOMOGENEOUS NUCLEATION THEORY FOR AEROSOLS

For the analysis of the release of fission product aerosols from reactor cores in a severe accident, it is necessary to understand the process of homogeneous nucleation of droplets from a supersaturated vapour, including the dependence on system pressure, which can vary greatly depending on the type of accident. A model of the effect of carrier gas on the nucleation process is presented, based on the changes to the energy distribution of nucleating vapour molecule clusters induced by collisions with carrier gas molecules. The rate of nucleation is altered since the cluster decay rate is strongly-energy dependent. The approach is compared with previous treatments of the problem, illustrating the importance of using an equilibrium cluster energy distribution which goes beyond a Gaussian approximation, and clarifying previous confusion between cluster temperature and cluster energy in the literature. Calculations of nucleation rates are performed for n-nonane in argon and water vapour in air, and show that an analytic approximation gives a reasonable approximation to numerical results. However, the resulting pressure dependence of the nucleation rate is weaker than has been observed experimentally suggesting that additional mechanisms operate.