The LIMIT computer code is utilized to study the behavior of containment atmospheres following severe core damage accidents. The degree of heterogeneity in passive entity mixing, especially mass stratification, is assessed. Two scenarios are analyzed using a two-dimensional Cartesian 84-node mesh computational domain as the baseline geometry. Both calculations include steam and liquid fields and involve hydrogen injection. Some stratification is observed after source injections are terminated. The formation of the stable mass gradients is caused principally by heat removal to structures located in the lower regions. Preliminary results conclude that in the absence of sprays or fan coolers, stratification can occur depending on the location and heat capacity of energy absorbing/liberating structures.