[en] Capsule HRB-15A successfully irradiated a variety of LHTGR fuel particle types to peak exposures of 6.5 x 10²⁵ n/m² (E > 29 fJ)/sub HTGR/ fast fluence, 29% FIMA fissile burnup and 6.4% FIMA fertile burnup, and a time-average irradiation temperature of 1100⁰C. The capsule was irradiated in HFIR for 6 months. The HRB-15A fuel rods were in excellent condition following irradiation. Candidate TRISO-coated LEU UC/UO₂ fuel particles performed adequately for the HTGR steam cycle application. The SiC coatings on the two UC₀₅O₁₅ TRISO batches were least retentive of Ag-110m during irradiation. Severe degradation of the SiC layers by fission product attack had occurred in 70% of these particles with much deeper penetration than in other TRISO batches. Large diameter (nominal 600 μm kernel) ThO₂ particles were judged unacceptable for use in an all-TRISO separable fuel system because of high OPyC and SiC failure levels. ZrC barrier layers in two ZrC-TRISO-coated fissile particle batches showed superior resistance to fission products relative to SiC. However, one of the ZrC-TRISO batches was poorly retentive of Ag and Eu, and fuel rods containing both batches had the highest end-of-life fission gas release values. Performance models for defective particles were shown to be overly conservative. Failure rate of OPyC coatings on TRISO particles was found to increase with the particle diameter