Nuclear reactor

A nuclear reactor is described including a core having fuel assemblies, upper internals, and neutron-flux adjusting rods and means for moving the neutron-flux adjusting rods between the fuel assemblies and the upper internals. The upper internals includes guide means for guiding the neutron-flux adjusting rods, including columnar guides for guiding certain of the neutron-flux adjusting rods. Each columnar guide coextensive generally vertical guide sections aligned to receive the certain neutron-flux adjusting rods, the guide means also including generally horizontal plate sections connected to the guide sections and positioned generally vertically along the upper internals. Each plate section nested in the manner of a ''jig-saw'' puzzle and secured to, adjacent plate sections at the generally vertical level of each plate section to form an integrated composite plate at each level. These extend laterally substantially throughout the upper internals, each plate section having perforations patterned to pass others of the neutron-flux adjusting rods, the plate sections being oriented each at its generally vertical level with their perforations for the others of the neutron-flux adjusting rods aligned so that the composite plates serve as guides for the others of the neutron-flux adjusting rods