A method for determining at least one threshold value (ΔT max, P linmax, (dΦ/dt) max) of at least one operating parameter (ΔT, P lin, dΦ/dt) 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 the following steps: - 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 said 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; and - determining a third threshold value of said respective operating parameter for an operation of said nuclear reactor at a third power, the third power corresponding to a power level and being between the first power and the second power; characterized in that the nuclear reactor is in a thermomechanical deconditioning mode of the fuel rods, and the first power is greater than the second power, or in a thermomechanical reconditioning mode of the fuel rods, and the first power is less than the second power, in the thermomechanical deconditioning mode of the fuel rods, the first power is substantially equal to the rated power of the nuclear reactor, and the second power is substantially equal to a target power for a sustained operation at intermediate power of the nuclear reactor, in the thermomechanical reconditioning mode of the fuel rods, the first power is equal to a target power for a sustained operation at intermediate power of the nuclear reactor, and the second power preferably being substantially equal to the rated power of the nuclear reactor. the method being implemented by an electronic determination system and comprising the following steps: - 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 said nuclear reactor at a second power; - 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 said 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; and - determining a third threshold value of said respective operating parameter for an operation of said nuclear reactor at a third power, the third power corresponding to a power level and being between the first power and the second power; - determining a third threshold value of said respective operating parameter for an operation of said nuclear reactor at a third power, the third power corresponding to a power level and being between the first power and the second power; characterized in that the nuclear reactor is in a thermomechanical deconditioning mode of the fuel rods, and the first power is greater than the second power, or in a thermomechanical reconditioning mode of the fuel rods, and the first power is less than the second power, in the thermomechanical deconditioning mode of the fuel rods, the first power is substantially equal to the rated power of the nuclear reactor, and the second power is substantially equal to a target power for a sustained operation at intermediate power of the nuclear reactor, in the thermomechanical reconditioning mode of the fuel rods, the first power is equal to a target power for a sustained operation at intermediate power of the nuclear reactor, and the second power preferably being substantially equal to the rated power of the nuclear reactor.