[en] Thermal fatigue is one of the critical problems in designing the upper core structure (UCS) of fast breeder reactors due to the mixing of hot and cold sodium. A complex thermal buffer structure was introduced for the prevention of high-cycle thermal fatigue in the designing of prototype reactor based on the safe life design concept. The design based on the damage tolerance concept is expected to contribute to the rationalization of UCS of the demonstration reactor. An evaluation method for crack propagation under high-cycle thermal fatigue has been developed using simplified equations, and has been verified with finite element analysis in this study. Test equipment to simulate thermal striping has also been developed to confirm the applicability of the evaluation method. (author)