Plan and status of research and development (R&D) were described on coated fuel particle (CFP) and fuel compacts for the core of small-sized high-temperature gas-cooled reactor (HTGR) HTR50S at second step of phase I (second core of HTR50S). Specifications of existing CFPs for high burnup (HTR50S2type-CFPs) were adopted as specifications of CFPs, to reduce R&D. HTR50S2-type-CFPs were fabricated based on technology developed in High Temperature Engineering Test Reactor (HTTR) project. The first irradiation test of HTR50S2-type-CFPs is now being carried out. In addition, R&D for fuel compact with high packing fraction is planned, because volume fraction of UO 2 kernel to whole of HTR50S2-type-CFP is rather smaller than that of the HTTR-type-CFP. We would aim to complete the proof of nuclear/thermal design of second core of HTR50S on integrity of fuel, feeding back results of R&Ds and nuclear/thermal design to each other. In addition, we describe outline of R&D plans for core of HTR50S in phase II and practical HTGR in Japan in future, naturally safe HTGR.

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