Abstract: A storage box insert for spent nuclear fuel rods has a structure mounted near its lower end to form a support for the rods within the storage box. The support structure comprises five separate steps, or louvers, slanted toward one interior corner of the box. Rods are stacked on the steps and the riser spaces between the steps provide passageways for cooling fluid.

Abstract: A box insert for receiving nuclear fuel is formed from a plurality of vertically extending plates arranged as an open-ended polygonal container having a smaller cross-sectional area than the opening where the box is to be located in the fuel storage rack. Each plate has a flat portion forming a respective side of the container and an integral tab portion rigidly projecting outwardly from the longitudinal edge of the plate. The adjacent tabs of each plate are connected, thereby giving the container rigidity and providing the container with a plurality of outwardly projecting ribs. When the boxes are located in the rack, the ribs fit into the corners of openings and maintain the container in proper spaced relation relative to the side walls of the opening.