Patent Number: 046831169
Section: abstract

A nuclear reactor in which control rods are inserted in the thimbles of only certain of the fuel assemblies (which may be called controlled assemblies) in the core and the remainder of the assemblies (which may be called non-controlled assemblies) are provided with hollow structural members containing burnable poison which form these non-controlled assemblies into integrated units. The hollow structural members are formed with end plugs welded to the lower end and are open at the top. The end plug of each member is secured to the bottom nozzle of the non-controlled fuel assembly. A skeleton is formed of the bottom nozzle, the structural members secured to it and a plurality of grids spaced along the structural members. A bulge tool is then inserted in each hollow member and it is on each side of each grid. Neutron absorbers are then inserted in each hollow member and an upper end plug is welded to it. The fuel rods are then inserted in the assembly and the upper end plugs are secured to the top nozzle.. Typically, there are 24 thimbles in a controlled assembly but not all thimble locations in a non-controlled assembly contain neutron-absorber structural members. Typically, there are eight structural members in a non-controlled assembly. In the other positions there are fuel rods. In reactors where there are water displacement control rods which extend over several assemblies, the so-called non-controlled assemblies may have the necessary thimbles for the water displacement rods.