PURPOSE: To enhance the heat transmission characteristics between helium gas and a graphite block significantly and to prevent the heat transmission characteristics from deteriorating significantly due to the laminar flow phenomenon of helium gas caused by heating.

CONSTITUTION: A graphite block 13 is provided with holes 16 for loading a plurality of uranium fuel in addition to a plurality of helium gas channels 17. Each helium gas channel 17 is provided with protrusions or irregularities 19 on the inner wall face thereof so that the heat transmission surface area is increased thus enhancing the heat transmission characteristics between helium gas 15 and the graphite block 13. The protrusions or the irregularities 19 also servo to accelerate turbulence of helium gas 15 flowing through the channel 17 thus preventing the heat transmission characteristics from deteriorating significantly due to the laminar flow phenomenon of helium gas (phenomenon where the heat transmission characteristics deteriorates extremely due to transition of turbulent fluid to laminar state).