Patent Number: 
Section: claims

1. A method for operating a nuclear reactor in order to produce electricity, the reactor comprising a core that is loaded with assemblies that comprise nuclear fuel rods, at least one nuclear fuel rod being of a type comprising:a cladding of recrystallized zirconium-based alloy, comprising, by mass,from 0.8 to 1.3% of niobium,between 1000 ppm and 1700 ppm of oxygen,between 0 and 35 ppm of sulphur,between 0 and 7000 ppm in total of at least one of iron, chromium and vanadium,between 0 and 2% of tin,between 0 and 70 ppm of nickel,between 0 and 100 ppm of carbon, andbetween 0 and 50 ppm of silicon,a balance being constituted by zirconium, with an exception of inevitable impurities, andpellets of nuclear fuel based on uranium oxide, the pellets being stacked inside the cladding, the method comprising:controlling operation of the reactor, during a class 2 transient power occurrence, such that at least one of:(a) a linear power density of the nuclear fuel rod remains lower than a limit linear power density, the limit linear power density being greater than 430 W/cm and less than or equal to 444 W/cm, and(b) a variation of the linear power density of the nuclear fuel rod remains lower than a limit variation, the limit variation being greater than 180 W/cm and less than or equal to 253 W/cm. 2. The method as recited in claim 1 further comprising initiating a corrective action or activating an alarm when the linear power density exceeds the limit linear power density or the variation of the linear power density exceeds the limit variation. 3. The method according to claim 1, wherein the limit linear power density is greater than 440 W/cm. 4. The method according to claim 1, wherein the limit variation is greater than 200 W/cm. 5. The method according to claim 4, wherein the limit variation is greater than 220 W/cm. 6. The method according to claim 1, wherein the alloy further comprises:between 5 and 35 ppm of sulphur by mass. 7. The method according to claim 1, wherein the alloy comprises between 0.03 and 0.25% in total of at least one of iron, chromium and vanadium. 8. The method according to claim 1, wherein the recrystallized zirconium alloy has been subjected to at least one annealing operations at temperatures of less than 600° C. 9. The method according to claim 1, wherein an inner side of the cladding is pressurized, before use, to a pressure of less than 20 bar. 10. The method as recited in claim 1 wherein the controlling includes using a deformation energy density for ensuring that the linear power density of the nuclear fuel rod remains lower than the limit linear power density, or the variation of the linear power density of the nuclear fuel rod remains lower than the limit variation. 11. The method according to claim 1, wherein the pellets include the metal oxide Cr2O3.  12. The method according to claim 11, wherein the pellets comprise from 1200 to 2000 ppm by mass of Cr2O3.  13. The method according to claim 12, wherein the pellets comprise from 1450 to 1750 ppm by mass Cr2O3.  14. A method for operating a nuclear reactor in order to produce electricity, the reactor comprising a core that is loaded with assemblies that comprise nuclear fuel rods, at least one nuclear fuel rod being of a type comprising:a cladding of recrystallized zirconium-based alloy, comprising, by mass,from 0.8 to 1.3% of niobium,between 1000 ppm and 1700 ppm of oxygen,between 0 and 35 ppm of sulphur,between 0 and 7000 ppm in total of at least one of iron, chromium and vanadium,between 0 and 2% of tin,between 0 and 70 ppm of nickel,between 0 and 100 ppm of carbon, andbetween 0 and 50 ppm of silicon,a balance being constituted by zirconium, with an exception of inevitable impurities, andpellets of nuclear fuel based on uranium oxide, the pellets being stacked inside the cladding, the pellets including at least one metal oxide for increasing a thermal creep of the pellets, the method comprising:controlling operation of the reactor, during a class 2 transient power occurrence, such that at least one of:(a) a linear power density of the nuclear fuel rod remains lower than a limit linear power density, the limit linear power density being greater than 430W/cm and less than or equal to 620 W/cm, and(b) a variation of the linear power density of the nuclear fuel rod remains lower than a limit variation, the limit variation being greater than 180 W/cm and less than or equal to 450 W/cm. 15. The method as recited in claim 14 further comprising initiating a corrective action or activating an alarm when the linear power density exceeds the limit linear power density or the variation of the linear power density exceeds the limit variation. 16. The method as recited in claim 14, wherein the limit linear power density is greater than 440 W/cm. 17. The method as recited in claim 14, wherein the limit variation is greater than 200 W/cm. 18. The method as recited in claim 14, wherein the limit variation is greater than 220 W/cm. 19. The method as recited in claim 14, wherein the alloy further comprises:between 5 and 35 ppm of sulphur by mass. 20. The method as recited in claim 14, wherein the alloy comprises between 0.03 and 0.25% in total of at least one of iron, chromium and vanadium. 21. The method as recited in claim 14, wherein the recrystallized zirconium alloy has been subjected to at least one annealing operations at temperatures of less than 600° C. 22. The method as recited in claim 14, wherein the limit linear power density is greater than 590 W/cm. 23. The method as recited in claim 14, wherein the limit linear power density is greater than 600 W/cm. 24. The method as recited in claim 14, wherein the limit linear power density is greater than 610 W/cm. 25. The method as recited in claim 14, wherein the limit variation is greater than 430 W/cm. 26. The method as recited in claim 14, wherein the limit variation is greater than 440 W/cm. 27. The method as recited in claim 14, wherein the metal oxide is Cr2O3.  28. The method as recited in claim 27, wherein the pellets comprise from 1200 to 2000 ppm by mass of Cr2O3.  29. The method as recited in claim 28, wherein the pellets comprise from 1450 to 1750 ppm by mass Cr2O3.  30. The method as recited in claim 14, wherein an inner side of the cladding is pressurized, before use, to a pressure of less than 20 bar. 31. The method as recited in claim 17 wherein the controlling includes using a deformation energy density for ensuring that the linear power density of the nuclear fuel rod remains lower than the limit linear power density, or the variation of the linear power density of the nuclear fuel rod remains lower than the limit variation.