Patent Number: 051456354
Section: claims

1. A high conversion water reactor fuel assembly having a coolant inlet end and a coolant outlet end, comprising fuel rod housing means;  an array of uranium-plutonium mixed fuel rods supported in said housing means, the fuel rods extending axially to define an effective fuel region of the fuel assembly of axially-varying plutonium enrichment, in which the fuel rods are closely spaced side-by-side to provide a high fissile material conversion ratio in operation with water coolant flowing from the upstream end to the downstream end, and  the effective fuel region comprising  (a) an axially upstream half having a first average fissile plutonium enrichment value, and  (b) an axially downstream half having a second average fissile plutonium enrichment value lower than said first average fissile plutonium enrichment value of the upstream half.  first and second end portions adjacent the upstream end and downstream end respectively, the end portions having a low level of fissile plutonium enrichment, and  a central portion extending between said end portions and having a higher level of fissile plutonium enrichment than said end portions.  (a) providing a fuel assembly comprising housing means and a closely-spaced array of uranium-plutonium mixed fuel rods extending axially side-by-side in said housing means to form an effective fuel region of the fuel assembly,  the effective fuel region having an axially-varying plutonium enrichment, with a first axial half of a first average fissile plutonium enrichment value and a second axial half of a second average fissile plutonium enrichment value lower than the first average fissile plutonium enrichment value;  (b) positioning the fuel assembly in the reactor core with said first axial half towards a coolant supply for the core and the second axial half towards a coolant discharge for the core, and  (c) passing water coolant through the core from said supply to said discharge at high void fraction and high conversion ratio of fissile material in the fuel assembly.  fuel rod housing means;  an array of uranium-plutonium mixed fuel rods mounted in said housing means, the fuel rods extending axially to define an effective fuel region of the fuel assembly;  said effective fuel region comprising (a) a downstream end portion comprising uranium-plutonium mixed oxide fuel having a low enrichment of fissile plutonium;  (b) a downstream central portion comprising uranium-plutonium mixed oxide fuel having an intermediate enrichment of fissile plutonium;  (c) an upstream central portion comprising uranium-plutonium mixed oxide fuel having a high enrichment of fissile plutonium, and  (d) an upstream end portion comprising uranium-plutonium mixed oxide fuel having a low enrichment of fissile plutonium,  whereby a downstream half of the axial length of the fuel assembly has a lower average enrichment of fissile plutonium than an upstream half of the axial length thereof.  fuel rod housing means;  an array of uranium-plutonium mixed fuel rods mounted in said housing means, the fuel rods extending axially to define an effective fuel region of the fuel assembly;  said effective fuel region comprising  (a) a longer, downstream end portion comprising uranium-plutonium mixed oxide fuel having a low fissile plutonium enrichment;  (b) a shorter, upstream end portion comprising uranium-plutonium mixed oxide fuel having a low enrichment of fissile plutonium, and  (c) a central portion between said upstream and downstream end portions and comprising uranium-plutonium mixed oxide fuel having a high enrichment of fissile plutonium; whereby a downstream half of the axial length of the fuel assembly has a lower average enrichment of fissile plutonium than an upstream half of the axial length thereof. 2. A fuel assembly as claimed in claim 1 in which the fuel rods in the effective fuel region are closely spaced side-by-side to provide an effective water to fuel volume ratio less than 0.4. 3. A fuel assembly as claimed in claim 2 in which said second average fissile plutonium enrichment value is at least 0.05% lower than said first average fissile plutonium enrichment value. 4. A fuel assembly as claimed in claim 3 in which said second average fissile plutonium enrichment value is at least 0.1% lower than said first average fissile plutonium enrichment value. 5. A fuel assembly as claimed in claim 2 in which the effective fuel region comprises 6. A fuel assembly as claimed in claim 5 in which said central portion has a substantially uniform higher level of fissile plutonium enrichment along its length, and said first end portion is shorter than said second end portion whereby to provide said lower average fissile plutonium enrichment of the axially downstream half of the effective fuel region. 7. A fuel assembly as claimed in claim 5 in which the central portion has an upstream region and a downstream region, the upstream region having a highest fissile plutonium enrichment which is substantially axially uniform, and the downstream region having a less high fissile plutonium enrichment which is substantially axially uniform, to provide said lower average fissile plutonium enrichment of the axially downstream half of the effective fuel region. 8. A fuel assembly as claimed in claim 7 in which the downstream region of the central portion is substantially shorter than the upstream region of the central portion. 9. A method of operating a high conversion water reactor core using uranium-plutonium mixed fuel, comprising the steps of 10. A method as claimed in claim 9 in which in step (c) the effective volume ratio of water coolant to fuel in the effective fuel region is less than 0.4. 11. A method as claimed in claim 10 in which in step (c) the conversion ratio of fissile material is at least 0.6. 12. A method as claimed in claim 11 in which said conversion ratio is at least 0.8. 13. A method as claimed in claim 12 in which said conversion ratio is about 1. 14. A water reactor fuel assembly having an axially upstream end and an axially downstream end, and comprising 15. A fuel assembly as claimed in claim 14 in which the fuel rods in the effective fuel region have an effective water to fuel volume ratio of less than 0.4. 16. A fuel assembly as claimed in claim 15 in which, in each of the upstream central portion and downstream central portion, fissile plutonium enrichment is substantially uniformly axially distributed. 17. A fuel assembly as claimed in claim 15 in which the upstream central portion is substantially longer than the downstream central portion. 18. A water reactor fuel assembly having an axially upstream end and an axially downstream end, and comprising 19. A fuel assembly as claimed in claim 18 in which the fuel rods in the effective fuel region have an effective water to fuel volume ration of less than 0.4. 20. A fuel assembly as claimed in claim 19 in which, in said central portion, the high enrichment of fissile plutonium is distributed substantially uniformly along the axial length of said central portion. 21. An improved method of operating a light water cooled, light water moderated reactor having a reactor core comprising at least one fuel assembly having uranium-plutonium mixed fuel rods, said fuel assembly having an effective axial fuel region formed by said fuel rods, by passing light water coolant through said core from an upstream end to a downstream end thereof at an effective water:fuel volume ratio less than 0.4 in said effective fuel region, and converting fissile material in said fuel assembly at a conversion ratio of at least 0.8, wherein the improvement comprises providing an axial non-uniformity of plutonium enrichment in said fuel assembly whereby a downstream axial half of said assembly comprises uranium-plutonium mixed oxide fuel having an average plutonium enrichment at least 0.05% less than an average plutonium enrichment of an upstream axial half of said assembly. 22. A nuclear reactor comprising a reactor core and means for passing coolant through said core from an upstream end to a downstream end thereof, the core comprising at least one fuel assembly as claimed in claim 2.