The fuel type of the HTTR is the pin-in-block type where the fuel rods are inserted in a hexagonal graphite block as shown in Fig. 4.18. The coated particle fuels are mixed with graphite powder and pressed into an annular cylinder, which is called a fuel compact. Fourteen fuel compacts are inserted into a graphite sleeve to form a fuel rod. Thirty-three fuel rods are loaded into a hexagonal graphite block (36 cm in width and 58 cm in height), which forms a fuel block. Helium gas coolant flows downward in the spaces of a few milli­meters between the fuel rods and wall of cooling holes in the fuel block. A fuel compact contains about 14 g of uranium which corresponds to about 13,000 coated particle fuels.

Three dowel pins are provided at the top of the fuel block and three dowel sockets are provided at its bottom. They are used for positioning when piling up the blocks. Holes for loading burnable poison are provided below each dowel pin. The burnable poison rod consists of B4C pellets and it compensates burnup reactivity.