Patent Number: 046577254
Section: claims

1. A nuclear reactor having a core and water coolant flowing therethrough, said core consisting of prismatic assemblies of identical cross-section arranged vertically and side by side and each comprising a cluster of vertical fuel rods spaced to permit circulation of the water coolant, said fuel assemblies comprising: a plurality of first fuel assemblies whose fuel rods contain initially contain enriched uranium only, certain of the fuel rods being substituted which guide tubes in the cluster and said fuel rods being sufficiently spaced for the neutron energy spectrum within said furst fuel assemblies to be thermal at least when there is water within said guide tubes, and  a plurality of second fuel assemblies in which all fuel rods initially contain recovered plutonium only and are so spaced that the neutron energy spectrum within said second fuel assemblies is higher than thermal,  said first and second fuel assemblies being distributed throughout the reactor core. 2. A nuclear reactor as claimed in claim 1, wherein said plurality of first fuel assemblies includes fuel assemblies whose guide tubes are arranged for receiving spectral shift control rods only and fuel assemblies whose guide tubes include guide tubes arranged to receive reactor reactivity control rods and guide tubes arranged to receive spectral shift control rods. 3. A nuclear reactor as claimed in claim 1, wherein said fuel assemblies are of hexagonal cross-section and wherein each of said second fuel assemblies is surrounded by first fuel assemblies, in the central part of said core. 4. A nuclear reactor as claimed in claim 1, wherein the distance between adjacent fuel rods in said second fuel assemblies is substantially equal to one third of the distance between the fuel rods in the first fuel assemblies. 5. A nuclear reactor as claimed in claim 1, wherein said first fuel assemblies and second fuel assemblies are distributed evenly throughout the core.