Abstract: A displacer rod for use in a mechanical spectral shift pressurized water nuclear reactor comprises a sectioned substantially hollow low neutron absorbing metal rod capable of displacing reactor coolant without absorbing a high amount of neutrons. The tubular sections can be made of stainless steel or Zircaloy tubing and may contain pellets for weighting the displacer rod.

Abstract: An arrangement for controlling the moderation of a nuclear reactor which includes multiple fuel assemblies each of which includes top and bottom nozzles with fuel rods and control rod guide thimbles located therebetween. Certain ones of said assemblies include at least one tube initially filled with a gas. One end of the tube rests on the lower nozzle upper surface while the upper end terminates at a point just below the lower plate in the upper nozzle. A spike immovably fixed in the lower plate of the upper nozzle is directed downwardly toward the end of the tube. As the tube expands or grows during operation in a reactor, and at a point where the criticality factor may proceed to less than one, the expanded tube will be pierced by the spike and thus permit the escape of gas from the tube which is then replaced by water coolant circulating through the reactor.

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.