[en] This paper describes a best-estimate computational code system which has been developed at PNC for the objective to offer methodologies to analyze any thermohydraulic phenomena occurring in LMFBRs. This code system includes plant-wide, multi-dimensional in-vessel, and single-subassembly codes. Considerable efforts have been devoted to improve and verify the analytical capabilities required for evaluating natural circulation decay heat removals. Applications of these codes to the analysis of natural circulation tests are also presented. The results indicate some key points for predicting natural circulation phenomena, e.g., inter-assembly heat transfer, thermal stratification in plena, and low-flow and low-heat-flux phenomena. (author)