[en] This is the final report on the high-temperature, gas-cooled reactor fuels project that was pursued at the Los Alamos Scientific Laboratory from early 1973 to mid-1977. This work followed two broad paths: (1) ZrC was utilized in various ways to improve the temperature capability of fission-product-retaining coated nuclear fuel particles and (2) techniques were developed for fabricating fuel rods by extruding the coated particles in high-density graphite matrices. The key to the ZrC coating project proved to be the ZrCl₄ powder feeder development. The ZrCl₄ reacted with H₂ and CH₄ or C₃H₆ to produce chemically vapor-deposited (CVD) ZrC. This CVD ZrC was used on nuclear fuel particles for replacing the SiC in the TRISO-II design, for making ZrC-C graded coats, and for making ZrC-alloyed isotropic carbon coats. Fuel particles with all of the described coats have been very successful in high-temperature, full-fluence (8 x 10²¹ n cm^-2) irradiation tests. High-temperature diffusion experiments with cesium showed that ZrC is comparable to SiC as a diffusion barrier. Extruded fuel rods with matrix densities of 1.6 to 1.7 g cm^-3 and with up to 45-vol percent coated particles were made without difficulty. All fuel rods tested in high-temperature, full-fluence irradiations had excellent results