It was demonstrated that the in-block carbonization process step of high-temperature gas-cooled reactor (HTGR) fuel element refabrication can produce acceptable HTGR fuel rods with respect to the performance of Biso-coated thoria fuel particles. The rods had the desired pitch-coke yield (30 ± 5%), and the defective fraction of fuel particles was below the desired maximum limit of 1 × 10−4. The pitch-coke yield was controlled by the heating rate of the carbonization cycle, and the defective fraction of fuel particles was controlled by both the heating rate of the carbonization cycle and the particle crushing strength. The heating rate and the particle crushing strength can vary considerably, and the process will still produce an acceptable product.