Patent Number: 
Section: claims

1. A method for determining at least one threshold value of at least one operating parameter of a nuclear reactor comprising a core in which fuel assemblies are loaded, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the method being implemented by an electronic determination system and comprising:determining a first threshold value of a respective operating parameter for an operation of the nuclear reactor at a first power;determining a second threshold value of a respective operating parameter for an operation of the nuclear reactor at a second power,the operation at the lower power of the first and second powers being an operation continued for a duration of at least 8 hours over a 24-hour sliding window; anddetermining a third threshold value of the respective operating parameter for an operation of the nuclear reactor at a third power, the third power being between the first power and the second power,the nuclear reactor being in a thermomechanical deconditioning mode of the fuel rods, the first power being greater than the second power, the nuclear reactor being intended to operate successively at the first power, the third power and the second power during the thermomechanical deconditioning mode of the fuel rods, the third power corresponding to a power level in the thermomechanical deconditioning mode. 2. The method according to claim 1, wherein the first power preferably being substantially equal to the rated power of the nuclear reactor. 3. The method according to claim 1, wherein the second power is substantially equal to a target power for a sustained operation at intermediate power of the nuclear reactor. 4. The method according to claim 1, wherein the respective operating parameter is selected from the following group: a temperature deviation in the core, a linear power in the fuel rods and a neutron flux variation in the core. 5. The method according to claim 1, wherein the method further comprises:operating the nuclear reactor while verifying that the value of the operating parameter is below the respective threshold value of the operating parameter among the first, second and third threshold values for the operation of the nuclear reactor at the corresponding power among the first, second and third powers. 6. A non-transitory computer readable medium comprising a computer program comprising software instructions which, when executed by a computer, carry out the method according to claim 1. 7. The method according to claim 1, wherein the power level corresponds to an operating duration of the nuclear reactor at said power greater than one hour. 8. The method according to claim 7, wherein the first power is substantially equal to a target power for a sustained operation at intermediate power of the nuclear reactor. 9. The method according to claim 7, wherein the second power is substantially equal to the rated power of the nuclear reactor. 10. The method according to claim 1, wherein each threshold value of the respective operating parameter is determined via a calculation of a PCI margin. 11. The method according to claim 10, wherein the calculation of the PCI margin comprises:ii) simulating at least one operating transition of the nuclear reactor,iii) calculating a value reached by at least one physical property during the operating transition in at least part of a cladding of a fuel rod, andiv) determining, as the PCI margin, a deviation between a maximum value reached by the value, calculated at ii), during the operating transition and a technological limit of the fuel rod. 12. The method according to claim 11, wherein the transition simulated at ii) is a transition chosen from among the group consisting of:an excessive load increase,an uncontrolled withdrawal of at least one group of control clusters,a fall of one of the control clusters, andan uncontrolled boric acid dilution. 13. The method according to claim 11, wherein the physical property is chosen from among the group consisting of:a constraint or a constraint function in the cladding; anda deformation energy density in the cladding. 14. An electronic system for determining at least one threshold value of at least one operating parameter of a nuclear reactor comprising a core in which fuel assemblies are loaded, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the electronic system comprising:a first determination module configured to determine a first threshold value of a respective operating parameter for an operation of the nuclear reactor at a first power;a second determination module configured to determine a second threshold value of a respective operating parameter for an operation of the nuclear reactor at a second power,the operation at the lower power of the first and second powers being an operation continued for a duration of at least 8 hours over a 24-hour sliding window; anda third determination module configured to determine a third threshold value of the respective operating parameter for an operation of the nuclear reactor at a third power, the third power being between the first power and the second power,the nuclear reactor being in a thermomechanical deconditioning mode of the fuel rods, the first power being greater than the second power, the nuclear reactor being intended to operate successively at the first power, the third power and the second power during the thermomechanical deconditioning mode of the fuel rods, the third power corresponding to a power level in the thermomechanical deconditioning mode. 15. A method for determining at least one threshold value of at least one operating parameter of a nuclear reactor comprising a core in which fuel assemblies are loaded, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the method being implemented by an electronic determination system and comprising:determining a first threshold value of a respective operating parameter for an operation of the nuclear reactor at a first power;determining a second threshold value of the respective operating parameter for an operation of the nuclear reactor at a second power,the operation at the lower power of the first and second powers being an operation continued for a duration of at least 8 hours over a 24-hour sliding window; anddetermining a third threshold value of the respective operating parameter for an operation of the nuclear reactor at a third power, the third power being between the first power and the second power,the nuclear reactor being in a thermomechanical reconditioning mode of the fuel rods, and the first power is less than the second power, the nuclear reactor being intended to operate successively at the first power, the third power and the second power during the thermomechanical reconditioning mode of the fuel rods, the third power corresponding to a power level in the thermomechanical reconditioning mode. 16. The method according to claim 15, wherein the power level corresponds to an operating duration of the nuclear reactor at said power greater than one hour. 17. A method for determining at least one threshold value of at least one operating parameter of a nuclear reactor comprising a core in which fuel assemblies are loaded, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the method being implemented by an electronic determination system and comprising:determining a first threshold value of a respective operating parameter for an operation of the nuclear reactor at a first power;determining a second threshold value of the respective operating parameter for an operation of the nuclear reactor at a second power,the operation at the lower power of the first and second powers being an operation continued for a duration of at least 8 hours over a 24-hour sliding window; anddetermining a third threshold value of the respective operating parameter for an operation of the nuclear reactor at a third power, the third power corresponding to a power level and being between the first power and the second power,the method further comprising operating the nuclear reactor while verifying that the value of the operating parameter is below the respective threshold value of the operating parameter among the first, second and third threshold values for the operation of the nuclear reactor at the corresponding power among the first, second and third powers,during the operation of the nuclear reactor, after a thermomechanical deconditioning followed by a thermomechanical reconditioning of the fuel rods leading to a local combustion rate of the fuel rods, a minimum operating duration of the nuclear reactor at its rated power before a new thermomechanical deconditioning of the fuel rods is the duration corresponding to reaching a target PCI margin,the target PCI margin corresponding, starting from a PCI margin existing before the thermomechanical deconditioning of the fuel rods, to a PCI margin that would have resulted from an operation at rated power of the reactor until obtaining the same local combustion rate. 18. An electronic system for determining at least one threshold value of at least one operating parameter of a nuclear reactor comprising a core in which fuel assemblies are loaded, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the electronic system comprising:a first determination module configured to determine a first threshold value of a respective operating parameter for an operation of the nuclear reactor at a first power;a second determination module configured to determine a second threshold value of a respective operating parameter for an operation of the nuclear reactor at a second power,the operation at the lower power of the first and second powers being an operation continued for a duration of at least 8 hours over a 24-hour sliding window; anda third determination module configured to determine a third threshold value of the respective operating parameter for an operation of the nuclear reactor at a third power, the third power being between the first power and the second power,the nuclear reactor being in a thermomechanical reconditioning mode of the fuel rods, and the first power is less than the second power, the nuclear reactor being intended to operate successively at the first power, the third power and the second power during the thermomechanical reconditioning mode of the fuel rods, the third power corresponding to a power level in the thermomechanical reconditioning mode.