The thermohydraulics of the LMFBR upper plenum under buoyancy - affected condition has been investigated. It was found that under low flow, thermal stratified conditions, reduced-scale sodium simulation cannot accurately predict the full-scale LMFBR upper plenum behavior. However, water is an adequate test fluid for prediction of full-scale LMFBR thermohydraulic behavior. It was also found that the threshold Peclet number above which convection dominates the heat transfer is about 10;A three-dimensional transient computer code PLENMIX which is a simplified version of COMMIX-1A has been used for the analysis. It was found that the PLENMIX code can accurately predict temperature transients. Comparisons were made with measured temperature distributions in a plenum simulation test. It was also shown that the adequacy of scaling laws can be determined by the PLENMIX code in a plenum simulation.