[en] The feasibility study for fast breeder reactors (FBRs) including related nuclear fuel cycle systems has been started from the 1999 fiscal year by Japan Nuclear Cycle Development Institute (JNC). Phase 1 studies were finished at the end of March, 2000. Various options of FBRs plant systems was studied and concept of Lead-Bismuth Eutectic (LBE) cooled FBRs have been selected as one of these options. In the United States, the LBE cooled reactor has been examined by Generation IV. Plant dynamics analyses on 2 type of LBE-cooled reactors, forced circulation type which designed by JNC and natural circulation type which was designed by University of California, Berkeley, have been performed to understand the basic thermal-hydraulic characteristics of the reactors. As a result of the analysis on JNC forced circulation reactor, it has been clarified that hot coolant remains in the upper plenum by the thermal stratification in case of a manual trip condition. And the characteristics of pump coast down influences core exit high-temperature in case of a loss of power condition. In addition, as a result of analysis on the natural circulation reactor, the flow-redistribution effect in ductless core channels by the buoyancy force has been evaluated for a candidate duct core channels. (author)