[en] As a research related to the development program of VHTR in JAERI, process studies have been carried out on head-end step in the fuel reprocessing. The VHTR is planned to use TRISO-coated low enriched UO₂ particles embedded in graphite matrix in the form of hollow cylinder called 'fuel compact'. The compacts are encased in graphite sleeve to make fuel rod, and then the rods are fabricated into prismatic fuel block. For this 'pin-in-block' type fuel, the modified Purex process would be essentially adaptable or the head-end treatment. Although the development except f of the head-end process has extensively been made in USA (2) and FR of Germany (3) on the process of '(Crush)-Burn-Grind-(Reburn)-Leach', various technical problems still remain there relating to the handling of a large volume of graphite containing ¹⁴C and highly radioactive powders, and to the treatment of the associated off-gas and solid wastes. The process principles of major steps in the proposed head-end flowsheet were experimentally shown. For the graphite burn with CO₂ gas, an active catalyst of mixed Co-Na was found to promote the burning of graphite at 850 deg. C. Bench-scale experiments in a fixed bed and fluidized-bed show that the development of advanced reactor with high heat supply is the most important problem. For the decomposition of CO gas, an iron oxide was selected as an effective catalyst