[en] There are two aspects to aerosol behavior modeling. One concerns the mathematical modeling of individual behavior mechanisms, the other the technique employed in solving the model. The latter is of interest because the high concentrations of particulate matter expected in large nuclear reactor containments present problems of scale that are not readily mastered numerically. Thus numerical schemes that agree on the laboratory scale sometimes diverge on the large scale. It is therefore important to validate a code's solution approach as well as its mechanistic models. This paper addresses the simplifying assumptions of the HAARM-3 solution technique by comparing HAARM-3 with a reference code, QUICK, that minimizes simplifying assumptions at the expense, however, of computer storage space and running time. The accuracy of the QUICK code has been successfully tested against alternate methods. The comparison shows remarkable agreement between the two codes for many accident scenarios of interest. It also shows order of magnitude divergence in some cases. As a possible explanation it is suggested that, in a sense, HAARM-3 represents a first order correction to a monodisperse model and that some divergence of its predictions relative to those of a more accurate model must be expected when particle size distributions are, or become, very disperse. The second part of this paper addresses the assumption of uniform aerosol mixing that is fundamental to most existing nuclear aerosol behavior models. To quantify the significance of this assumption, a computer code, the ZONE code, has been developed at Battelle. This code accounts for spatial inhomogeneities using a cell model approach. Predictions of the ZONE code for severe sodium fire accidents are presented and compared with corresponding predictions using the QUICK code. The comparison shows the generally conservative prediction of complete mixing models during the sodium burn period. Complete mixing is predicted by the ZONE code immediately after the fire ceases and for the following period, the predictions of the ZONE code coincide with those of QUICK