PURPOSE: To obtain a flux trap neutron absorbing apparatus to maximize the available fuel storage space by disposing it between nuclear fuel assembly storage cans in a nuclear fuel storage pool.

CONSTITUTION: The flux trap neutron absorbing apparatus 54 comprises separate pockets 50 extended vertically along the opposed sidewalls 40A of storage cans 40 adjacently disposed at an interval and disposed at an interval, and plates 48 of thermal neutron absorbing materials provided in the pockets 50. The plates 48 are disposed at an interval, and a fast neutron moderating region is formed therebetween. The apparatus 54 further comprises a slab 60 of metal hydride provided in the region between the plate 48 and the pocket 50 of the adjacent sidewall 40A, and a canister 56 disposed in the region to contain the slab 60. The thermal neutron absorbing material of the plate is normally boron carbide.