A fuel assembly consists of fuel rods and a hexagonal graphite block, 360 mm in across width of flats and 580 mm in length, as shown in figure. The fuel assembly has three dowels on the top and three mating sockets at the bottom to align the fuel assemblies. TRISO coated fuel particles with UO2 kernel, about 6 wt% in average enrichment and 600 µm in diameter, are dispersed in the graphite matrix and sintered to form a fuel compact. Fuel compacts are contained in a fuel rod, 34 mm in outer diameter and 577 mm in length. Fuel rods are inserted into vertical holes made in the graphite block. Helium gas coolant flows through gaps between the holes and the rods.