Patent Number: 
Section: claims

1. A method for calculating a pellet-cladding interaction margin associated with a loading pattern of a nuclear reactor comprising a core in which fuel assemblies are loaded according to the loading pattern, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets,the method being implemented by an electronic system and comprising the following steps:b) calculating a reference principal pellet-cladding interaction margin for a reference loading pattern of the fuel assemblies in the core,c) calculating a reference secondary pellet-cladding interaction margin for the reference loading pattern,d) calculating a modified secondary pellet-cladding interaction margin for a modified loading pattern of the fuel assemblies in the core,e) calculating a modified principal pellet-cladding interaction margin for the modified loading pattern, depending on a comparison of the modified secondary pellet-cladding interaction margin with the reference secondary pellet-cladding interaction margin;the method further comprising controlling the state of the power balance of the nuclear reactor by using the calculated principal pellet-cladding interaction margin for a considered loading pattern of the fuel assemblies in the core to avoid rupture by pellet-cladding interaction of the claddings present in the core,wherein neutronic calculations and thermodynamic calculations are done to calculate each pellet-cladding interaction margin, andwherein the neutronic calculations and the thermomechanical calculations are coupled to calculate a corresponding principal pellet-cladding interaction margin, the thermomechanical calculations being uncoupled from the neutronic calculations to calculate a corresponding secondary pellet-cladding interaction margin. 2. The method according to claim 1, wherein the method further comprises the following step:f) determining a limit value to trigger an emergency stop and/or an alarm from the calculated principal pellet-cladding interaction margin and for the considered loading pattern of the fuel assemblies in the core. 3. The method according to claim 1, wherein when the modified secondary pellet-cladding interaction margin is greater than or equal to the reference secondary pellet-cladding interaction margin, the modified principal pellet-cladding interaction margin is equal to the reference principal pellet-cladding interaction margin, andwhen the modified secondary pellet-cladding interaction margin is less than the reference secondary pellet-cladding interaction margin, the modified principal pellet-cladding interaction margin is less than the reference principal pellet-cladding interaction margin. 4. The method according to claim 3, wherein, when the modified secondary pellet-cladding interaction margin is less than the reference secondary pellet-cladding interaction margin, the modified principal pellet-cladding interaction margin is equal to the reference principal pellet-cladding interaction margin reduced by a corrective factor depending on the deviation between the modified secondary pellet-cladding interaction margin and the reference secondary pellet-cladding interaction margin. 5. The method according to claim 4, wherein the corrective factor depends on a ratio between the modified secondary pellet-cladding interaction margin and the reference secondary pellet-cladding interaction margin and is strictly between 0 and 1. 6. The method according to claim 1, wherein step b) comprises the following sub-steps:b1) simulating at least one operating transient of the nuclear reactor,b2) calculating the value reached by at least one physical quantity during the operating transient in at least part of a cladding of a fuel rod, andb3) determining, as reference principal pellet-cladding interaction margin, the deviation between the maximum value reached by the value calculated in sub-step b2) during the transient and a technological limit of the fuel rod. 7. The method according to claim 6, wherein the transient simulated in sub-step b1) is a transient 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. 8. The method according to claim 6, wherein the method comprises, before step b), the following step:a) determining a rupture value of the physical quantity characterizing a rupture of the cladding. 9. The method according to claim 8, wherein step a) includes:subjecting previously irradiated fuel rods to experimental nuclear power ramps,calculating the values reached by the physical quantity in at least one cladding broken during a power ramp, andselecting the rupture value as being the minimum value from among the calculated values reached. 10. The method according to claim 6, wherein the physical quantity is chosen from among the group consisting of:a constraint or a constraint function in the cladding; anda deformation energy density in the cladding. 11. A non-transitory computer-readable medium including a computer program comprising software instructions which, when executed by a computer, carry out a method according to claim 1. 12. An electronic system for calculating a pellet-cladding interaction margin associated with a loading pattern of a nuclear reactor comprising a core in which fuel assemblies are loaded according to the loading pattern, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets, the electronic system comprising:a first calculating module configured to calculate a reference principal pellet-cladding interaction margin for a reference loading pattern of the fuel assemblies in the core;a second calculating module configured to calculate, on the one hand, a reference secondary pellet-cladding interaction margin for the reference loading pattern, and on the other hand, a modified secondary margin for a modified loading pattern of the fuel assemblies in the core; anda comparison module configured to compare the modified secondary pellet-cladding interaction margin with the reference secondary pellet-cladding interaction margin,the comparison module further being configured to calculate a modified principal pellet-cladding interaction margin for the modified loading pattern, depending on the comparison of the modified secondary pellet-cladding interaction margin with the reference secondary pellet-cladding interaction margin,the electronic system configured for controlling the state of the power balance of the nuclear reactor by using the calculated principal pellet-cladding interaction margin for a considered loading pattern of the fuel assemblies in the core to avoid rupture by pellet-cladding interaction of the claddings present in the core,wherein neutronic calculations and thermodynamic calculations are done to calculate each pellet-cladding interaction margin, andwherein the neutronic calculations and the thermomechanical calculations are coupled to calculate a corresponding principal pellet-cladding interaction margin, the thermomechanical calculations being uncoupled from the neutronic calculations to calculate a corresponding secondary pellet-cladding interaction margin. 13. A method for calculating a pellet-cladding interaction margin associated with a loading pattern of a nuclear reactor comprising a core in which fuel assemblies are loaded according to the loading pattern, the fuel assemblies comprising fuel rods each including nuclear fuel pellets and a cladding surrounding the pellets,the method being implemented by an electronic system and comprising the following steps:a) determining a rupture value of the physical quantity characterizing a rupture of the cladding,b) calculating a reference principal pellet-cladding interaction margin for a reference loading pattern of the fuel assemblies in the core,c) calculating a reference secondary pellet-cladding interaction margin for the reference loading pattern,d) calculating a modified secondary pellet-cladding interaction margin for a modified loading pattern of the fuel assemblies in the core,e) calculating a modified principal pellet-cladding interaction margin for the modified loading pattern, depending on a comparison of the modified secondary pellet-cladding interaction margin with the reference secondary pellet-cladding interaction margin;the method further comprising controlling the state of the power balance of the nuclear reactor by using the calculated principal pellet-cladding interaction margin for a considered loading pattern of the fuel assemblies in the core to avoid rupture by pellet-cladding interaction of the claddings present in the core;wherein step b) comprises the following sub-steps:b1) simulating at least one operating transient of the nuclear reactor,b2) calculating the value reached by at least one physical quantity during the operating transient in at least part of a cladding of a fuel rod, andb3) determining, as reference principal pellet-cladding interaction margin, the deviation between the maximum value reached by the value calculated in sub-step b2) during the transient and a technological limit of the fuel rod;wherein step a) includes:subjecting previously irradiated fuel rods to experimental nuclear power ramps,calculating the values reached by the physical quantity in at least one cladding broken during a power ramp, andselecting the rupture value as being the minimum value from among the calculated values reached;wherein each of steps c) and d) includes, for each fuel assembly, the following sub-steps:i) simulating an evolution of the operation of the nuclear reactor by applying, to the fuel rods, a nuclear power ramp from the nil power,ii) calculating the values reached by a physical quantity in the claddings of the fuel rods,iii) comparing the values reached to the rupture value,iv) determining a power at break equal to:I) the power associated with the rupture value, if a minimum value from among the values reached calculated in sub-step ii) is equal to the rupture value, orII) infinity, if no value, from among the values reached calculated in sub-step ii), is equal to the rupture value,v) evaluating a power margin by difference between the power at break determined in sub-step iv) and an estimated maximum power in the fuel assembly,the corresponding secondary pellet-cladding interaction margin, calculated during each of steps c) and d), being equal to the minimum margin from among the power margins evaluated for the fuel assemblies in sub-step v).