Modeling of Diffusive Gravitational Aerosol Deposition in CONTAIN

The CONTAIN code is an integrated code for predicting the containment behavior (chemical, physical, and radiological) in a severe accident. It models the thermal hydraulics as well as the aerosol and fission products behavior inside the containment. There are four aerosol deposition mechanisms modeled in the code: settling, diffusion to surfaces, thermophoresis, and diffusiophoresis. In general, the settling and diffusion are the most important. A comparison of the CONTAIN deposition rate expression with a general and more accurate rate expression, however, shows that for most geometries, the code expression overestimates the deposition of small particles, mainly because of an inadequate assumption regarding the dependence of the thickness of the boundary layer on particle size. For some specific geometries, the expression can also overestimate deposition of large particles. The general and more accurate expression is implemented in the CONTAIN code for the cubic and spherical geometries for a test problem. The original and the modified versions of the CONTAIN code are found to yield different results for the suspended aerosol mass. The differences depend on other aerosol processes such as coagulation and also on geometry.