Patent Number: 051165678
Section: claims

1. A nuclear reactor comprising: a reactor vessel;  a reactor core having a fuel bundles arranged in a lower two-dimensional array, said reactor core also having fuel bundles arranged in an upper two-dimensional array, each of said fuel bundles containing vertically extending fuel rods, each of said fuel rods having a fuel section containing fissionable fuel and a plenum section for accumulating gaseous fission byproducts, said fuel rods in fuel bundles in said lower array having plenum sections below their fuel sections, said fuel rods in fuel bundles in said upper array having plenum sections above their fuel sections; and said fuel bundles in said upper array on the average being less spent than the fuel bundles in said lower array; and  circulation means for circulating heat transfer fluid so that it flows axially through said bundles in said lower array toward and through said bundles of said upper array. 2. In a boiling-water reactor having a vessel for containing recirculating water having a water phase and a steam phase, a nuclear reactor core comprising multiple fuel bundles, said fuel bundles being arranged in an upper matrix and a lower matrix, each of said fuel bundles in said upper matrix being stacked on one of said fuel bundles in said lower matrix, said core being positioned within said vessel so that said coolant flows upwardly through said core from said lower matrix toward said upper matrix, said fuel bundles sharing common external dimensions so that any one bundle can be positioned in any position in either matrix. 3. A nuclear reactor as recited in claim 2 wherein each of said fuel bundles contains plural fuel rods, each of said fuel rods having a plenum end for accumulating gaseous fission byproducts, the plenum ends of said fuel rods in said upper matrix being oriented upward, the plenum ends of said fuel rods in said lower matrix being oriented downward. 4. A nuclear reactor as recited in claim 2 wherein the fuel bundles in said upper matrix are less spent than the fuel bundles in said lower matrix. 5. A nuclear reactor as recited in claim 1 wherein each fuel bundles of said upper array is stacked on a respective fuel bundle of said lower array.