Patent Number: 
Section: claims

1. A fuel assembly for a pressurized water nuclear reactor, comprising:fuel rods which are arranged at nodes of a substantially regular network having a polygonal outer contour, the fuel rods containing uranium which is enriched in isotope 235 and not containing any plutonium before the assembly is used in a reactor, wherein the rods are distributed in at least:a first central group which is constituted by fuel rods which have a first level of enrichment e1 in uranium 235; andan outer peripheral layer of fuel rods distributed in:a second group of fuel rods that extend along faces of the outer contour of the network and that have a second level of enrichment e2 in uranium 235 strictly less than the first level of enrichment e1; anda third group of fuel rods that are arranged at corners of the outer contour of the network and that have a third level of enrichment e3 in uranium 235 that is strictly less than the second level of enrichment e2wherein the outer peripheral layer of fuel rods defines an outer periphery of the fuel assembly. 2. The fuel assembly according to claim 1, wherein first central group has rods which contain a neutron contaminant. 3. The fuel assembly according to claim 1, wherein the second group extends, for each of the faces of the outer contour of the network of fuel rods, from one corner to the other of the face in question, and the third group comprises only the fuel rods that are arranged in the corners of the outer contour of the network of fuel rods. 4. The fuel assembly according to claim 1, wherein the second level of enrichment e2 is between e1—0.8% and e1—0.2%. 5. The fuel assembly according to claim 1, wherein the third level of enrichment e3 is between e1—1.8% and e1—0.6%. 6. The fuel assembly according to claim 1, wherein the first level of enrichment e1 is between 3% and 6%. 7. The fuel assembly according to claim 1, wherein the fuel rod network has a square outer contour. 8. A nuclear reactor core, comprising:at least two fuel assemblies, wherein each of the at least two fuel assemblies comprises fuel rods which are arranged at nodes of a substantially regular network having a polygonal outer contour, the fuel rods containing uranium which is enriched in isotope 235 and not containing any plutonium before the assembly is used in a reactor, wherein the rods are distributed in only:a first central group which is constituted by fuel rods which have a first level of enrichment e1 in uranium 235; andan outer peripheral layer of fuel rods distributed in:a second group of fuel rods that extend along faces of the outer contour of the network and that have a second level of enrichment e2 in uranium 235 strictly less than the first level of enrichment e1; anda third group of fuel rods that are arranged at corners of the outer contour of the network and that have a third level of enrichment e3 in uranium 235 that is strictly less than the second level of enrichment e2wherein the outer peripheral layer of fuel rods defines an outer periphery of the fuel assembly. 9. The nuclear reactor core according to claim 8, wherein the first central group has rods which contain a neutron contaminant. 10. The fuel assembly according to claim 1, further comprising a skeleton, the skeleton having a lower tie plate, an upper tie plate and guide tubes for receiving rods of a control rod cluster, the guide tubes connecting the lower tie plate and the upper tie plate. 11. The nuclear reactor core according to claim 8, wherein each of the at least two fuel assemblies further comprises a skeleton, the skeleton having a lower plate, upper plate and guide tubes, the guide tubes connecting the lower plate and the upper plate. 12. The fuel assembly as recited in claim 1 wherein the fuel assemblies are in a 17 by 17 configuration. 13. The fuel assembly as recited in claim 1 wherein the fuel assemblies are at least in a 14 by 14 configuration. 14. The fuel assembly as recited in claim 1 wherein the fuel assemblies are in a 14 by 14 configuration. 15. The fuel assembly as recited in claim 1 wherein the fuel assemblies are in a 15 by 15 configuration. 16. A fuel assembly for a pressurized water nuclear reactor, comprising:fuel rods which are arranged at nodes of a substantially regular network having a polygonal outer contour, the fuel rods containing uranium which is enriched in isotope 235 and not containing any plutonium before the assembly is used in a reactor, wherein the rods are distributed in only:a first central group which is constituted by fuel rods which have a first level of enrichment e1 in uranium 235; andan outer peripheral layer of fuel rods distributed in:a second group of fuel rods that extend along faces of the outer contour of the network and that have a second level of enrichment e2 in uranium 235 strictly less than the first level of enrichment e1; anda third group of fuel rods that are arranged at corners of the outer contour of the network and that have a third level of enrichment e3 in uranium 235 that is strictly less than the second level of enrichment e2wherein the outer peripheral layer of fuel rods defines an outer periphery of the fuel assembly. 17. The fuel assembly according to claim 16, wherein first central group has rods which contain a neutron contaminant. 18. The fuel assembly according to claim 16, wherein the second group extends, for each of the faces of the outer contour of the network of fuel rods, from one corner to the other of the face in question, and the third group comprises only the fuel rods that are arranged in the corners of the outer contour of the network of fuel rods. 19. The fuel assembly according to claim 16, wherein the second level of enrichment e2 is between e1—0.8% and e1—0.2%. 20. The fuel assembly according to claim 16, wherein the third level of enrichment e3 is between e1—1.8% and e1—0.6%. 21. The fuel assembly according to claim 16, wherein the first level of enrichment e1 is between 3% and 6%. 22. The fuel assembly according to claim 16, wherein the fuel rod network has a square outer contour.