PURPOSE: To allow all of fuel rods in a reactor core to uniformly approach respective heat limits to enhance the power density of the core.

CONSTITUTION: Respective fuel rods 40 are individually surrounded by cylindrical channels 50 and a decelerating and cooling material is allowed to flow through the channels and the regions between the channels. The fuel rods surrounded by the channels are pref. arranged at a triangular pitch and a predetermined number of the fuel rods are set to one group so as to make handling easy to be bundled in a hexagonal shape. Spacers 80 having blades are arranged between the respective fuel rods and the channels surrounding them not only to keep the intervals between the fuel rods and the channels but also to control the flow in the channels.