[en] A joint research plan is in progress for optimization of FBR reactor design by the groups of Thermal-hydraulics and Structure-materials in JNC, to develop a methodology for analysis and evaluation of thermal striping, temperature stratification, thermal loading, and structural integrity of reactor components. As for thermal striping three items of findings are as follows: (1) Based on present design survey, subjects are extracted to evaluate thermal-hydraulics and structural strength. It is urgently important to develop LES(Large Eddy Simulation) and its benchmark test, which is expected now to be most reliable and applicable to industries near future. It is also important to improve estimation accuracy for metal surface temperature response and to collect high frequency fatigue data. (2) High frequency fatigue due to thermal striping is considered to be controlled with crack generation lifetime. It is necessary to collect 10⁹-10¹⁰ cycles fatigue data under strain control. The crack propagation evaluation requires new findings on behavior around lower limiting values. (3) Multi-coordinate stress, thermal stress, peak strain, random load, and small crack and plural crack propagation behavior were studied as factors influencing crack propagation due to thermal striping. Influences of frequency, average stress, primary stress on crack propagation were modelled, and their sensitivity analyses were made. (Yokoo, H.)