[en] One of the most important aspects of the demonstration of the safety of liquid metal cooled fast reactors (LMFBR) is the assurance of adequate cooling of the reactor and critical structures during all potential or hypothetical events. To this end, a design philosophy has developed in which alternate heat removal systems are included in a plant in order to provide independent, diverse, and redundant cooling from the normal heat transport system. Recent studies have examined various aspects of natural convective flow in such systems and preliminary conclusions from these efforts have indicated that the basic phenomena are reasonably well understood and predictable in most cases. However, in certain situations where significant thermal stratification occurs, the resulting buoyancy-driven flow patterns can become quite complex and as a result, the confidence in computer simulations diminishes. The purpose of this paper is to present the results of an experimental and analytical study of one class of such problems in which the development of natural convective flow requires a transition through an unstably stratified condition. The experiments were conducted in the Experimental Breeder Reactor II (EBR-II) and the analytical tool used was the NATDEMO system simulation code