Reactor and fuel assembly design for improved fuel utilization in liquid moderated thermal reactors

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. 847,5