Describes existing gas-cooled fast breeder reactor (GCFR) designs with emphasis on the core cooling and associated systems. Factors affecting the convective heat transfer and pressure drop in the core are examined, particularly the effects of surface roughness on the enhancement of heat transfer, the effects of the variation of the coolant's thermophysical properties with temperature and composition, and the influence of cross flow and cross mixing. The characteristics of other components of the cooling system, the helium circulator, and the steam generator are examined with particular emphasis on off-normal operation. Existing computer codes for the prediction of both normal and abnormal operations are described and compared. Two new codes, developed under this project, are described, and some preliminary results are given of their application to GCFR thermal-hydraulic performance. Problem areas pertinent to cooling-system performance and safety are summarized, and recommendations are made for additional research in areas that need attention to assure economical design and safe operation. 163 pp. Ref.