Modular radial neutron reflector

Abstract

Modular neutron reflector assemblies removably insertable into the irregular space in a nuclear reactor between a generally cylindrical array of square fuel assemblies comprising the core and the cylindrical core barrel surrounding the core, include an elongated enclosure having a strongback adjacent the core barrel and flat plates adjacent the fuel assemblies. The strongback is thicker than the steel plates by an amount which results in the strongback and plates reaching approximately the same temperature to minimize thermal stresses despite the 8 to 20 times greater nuclear heating at the core side of the enclosure as opposed to that at the core barrel side. The enclosure is suspended by a single point mounting on the core barrel which permits vertical, radial, and circumferential expansion but a pin depending from the lower end of the enclosure slides in a vertical bore in the reactor lower core support plate to limit lateral movement while allowing unrestrained vertical expansion and contraction. A separately removable reflector unit in each enclosure comprises a bundle of closely packed stainless steel circular rods with smaller diameter rods inserted in the interstices between the larger rods to generate a 0.90-0.92% volume fraction of stainless steel while also providing a distributed array of small longitudinal passages for reactor coolant. Selected rods are suspended from a cover plate on the module with the remaining rods suspended from a horizontal plate which is in turn, suspended from the selected rods so that the rods are all free to expand and contract vertically.