Aerosol modelling in containment

The wet atmosphere of an LWR containment under severe accident conditions has the potential significant scrubbing of radiotoxic aerosol particles. In the absence of sprays, two mechanisms are potentially significant: particle growth (and consequent accelerated settling) by condensation of water vapour and Stefan flow to containment surfaces. The degree to which these phenomena play a role is governed by the thermodynamics of the containment system coupled with the behaviour of the aerosol. Some preliminary results are presented of calculations with the QUICKM code that is designed to explore this coupled thermodynamics/aerosol behaviour in as mechanistic a fashion as possible. The calculations were performed for a simulation experiment of the NSPP series at Oak Ridge National Laboratory and on a Surry TLMB' sequence treated in the Battelle Memorial Institute Report BMI-2104. They show unexpected sensitivity to the numerical treatment of the coupled models and to the degree of coupling. The results are also sensitive to assumptions on the transition of two-phase flow from the pressurized primary system to containment. Because the saturation ratio is a sensitive function of gas temperature, its prediction is particularly uncertain. Since the saturation ratio drives steam condensation on particles, the conclusion from our work to date is that further analysis is needed before any prediction of water vapour condensation on particles is believable. (author)