Abstract: A rack is provided for either temporary or permanent storage of spent nuclear fuel assemblies. The rack consists of a checkerboard array of substantially square cells. Alternate cells in each row include neutron-absorbing poison material, either in the cell wall or otherwise, while the other cells are storage cells for the fuel assemblies. For temporary storage, the poison cells also contain a moderator which may be water or which may be a solid hydrogen compound. For permanent storage fuel assemblies that have decayed to a lower level of radioactivity, the moderator is eliminated so that all the cells can contain spent fuel assemblies, and the entire rack structure is encased in concrete for shielding.

Abstract: An improved reactor and fuel assembly design is disclosed wherein a liquid moderated thermal reactor is initially run with all or a portion of its fuel assemblies in an undermoderated boiling water mode to take advantage of increased conversion ratio at lower H/fuel ratios, and after a suitable period of operation, the neutron spectrum for all or a portion of the undermoderated boiling assemblies is shifted to lower energies to increase reactivity by converting a number of fuel assemblies to a nearer optimum moderated pressurized or non-boiling mode. The increased reactivity allows for continued operation of the modified assembly. The improved reactor and fuel assembly design results in improved fuel utilization and neutron economy and reduced control requirements for the reactor. The design may be augmented by reducing the number of fuel rods comprising the assembly after a suitable period of operation, as disclosed in the applicants copending application filed Nov. 1, 1977, Ser. No. 847,524.

Abstract: An improved reactor and fuel assembly design is disclosed wherein a light water reactor is initially run with undermoderated fuel assemblies to take advantage of increased conversion ratio, and after a suitable period of operation, the neutron spectrum for the undermoderated assemblies is shifted to lower energies to increase reactivity by withdrawing a number of fuel rods from the assemblies. The increased reactivity allows for continued operation of the modified assembly, and the fuel rods which are removed are used to construct similar assemblies which are also capable of continued operation. The improved reactor and fuel assembly design results in improved fuel utilization and neutron economy and reduced control requirements for the reactor.

Abstract: A maximum density storage rack is provided for long term or semipermanent storage of spent nuclear fuel assemblies. The rack consists of storage cells arranged in a regular array, such as a checkerboard array, and intended to be immersed in water. Initially, cap members are placed on alternate cells in such a manner that at least 50% of the cells are left open, some of the caps being removable. Spent fuel assemblies are then placed in the open cells until all of them are filled. The level of reactivity of each of the stored fuel assemblies is then determined by accurate calculation or by measurement, and the removable caps are removed and rearranged so that other cells are opened, permitting the storage of additional fuel assemblies in a pattern based on the actual reactivity such that criticality is prevented.

Abstract: A rack is provided for the storage of spent nuclear fuel assemblies in a water-filled storage pit. The rack consists of an array of storage cells containing the fuel assemblies and poison boxes consisting of water-filled enclosures having walls incorporating a neutron-absorbing material. The poison boxes function as neutron traps, and the storage cells and poison boxes are arranged in such a manner that each poison box is immediately adjacent at least one storage cell, and adjacent storage cells are in contact on at least one side of each cell. This results in maximizing the number of fuel assemblies that can be stored in a given space.