Abstract: The nuclear fuel is made up of at least one bed (11) of substantially spherical particles (1′) having a diameter of between 0.5 and 5 mm. The structure for holding the fuel assembly (10) comprises a casing (8) of prismatic shape and at least one cage (9) placed inside the casing (8) and containing at least one bed (11) of nuclear fuel particles. The end nozzles (12, 13) of the casing are each traversed by at least one opening for the passage of water, the cage or cages comprising porous walls traversed by openings of a size smaller than the diameter of the fuel particles (1′) and placed such that the bed or beds of fuel particles (11) are traversed by cooling water from the nuclear reactor entering into the fuel assembly casing (8) via the first end nozzle (12) and leaving the fuel assembly via the second end nozzle (13).