Assessing Spatial Effects in Aerosol Transport Models

Abstract

Fission product retention in the containment building of nuclear reactors is an important issue in the prediction of the radiological source term from postulated severe accidents. The fission products are generally postulated to be released into the containment as noble gases, or as aerosols. Computer codes currently available to model the behaviour of aerosols in containment employ the "well-mixed" assumption and are believed to be well advanced. One of the questions that remains, however, is the influence of actual spatial inhomogeneities on the source term. In this paper I consider a spatial aerosol dynamic due to gravitational settling. It is formulated by a one-dimensional space-time model including coagulation and gravitational removal. The model equations are solved using a finite element method with linear basis functions in a logarithmic particle size coordinate and a Dirac delta weighting function (i.e. collocation), and orthogonal collocation on finite elements with Hermite polynominal bases in the spatial coordinate. This method yields a set of time-dependent, ordinary differential equations with a tri-diagonal, block-diagonal coefficient matrix. I present the results for the special case of a constant coagulation kernel and compare them with the prediction of a corresponding well-mixed model.