Abstract: A fuel assembly for a pressurized water nuclear reactor incorporating fluid moderator spectral shift control means. During the first part of the fuel cycle when there is excess reactivity, neutron moderation may be decreased by replacing a portion of the water within the core with a less effective moderator such as heavy water. During the life of the fuel, the heavy water is gradually replaced with regular water. The fuel assembly incorporates the necessary means and apparatus to effectuate such control.

Abstract: A nuclear reactor in which the bottom nozzle of each fuel assembly is a casting having leg-like pins which are cast as part of the bottom nozzle. The pins extend from the corners of the bottom nozzle of each fuel assembly. The transverse cross section of each pin has the shape of a circular sector whose arc depends on the number of apeces of the bottom nozzle, typically 90.degree. for a square nozzle and 120.degree. for a hexagonal nozzle. The fuel assemblies are mutually nested side-by-side so that a group of contiguous pins extends from each region where corners of fuel elements converge. The sides of the pins of each group are in engagement so that the pins of the group form a continuous circularly cylindrical surface. The core support plate has a plurality of holes. The boundaries of the holes are circular cylinders. Each group of pins extend into a hole in the core support plate.

Abstract: A fuel assembly for a nuclear reactor comprises a 5.times.5 array of guide tubes in a generally 20.times.20 array of fuel elements. The guide tubes are arranged to accommodate either control rods or water displacer rods. The fuel assembly also comprises a plurality of Inconel and Zircaloy grids arranged to provide stability of the fuel elements and guide tubes while allowing the flow of reactor coolant therebetween.