Patent Number: 
Section: claims

1. A method for designing a nuclear fuel assembly which is intended to be positioned in a nuclear reactor, the assembly comprising a plurality of guide tubes and a control cluster which comprises a plurality of control rods and a support for the control rods, the control rods and the guide tubes extending in parallel with a longitudinal direction, each of the control rods being received in a guide tube in order to form pairs comprising guide tubes/control rods, each of the guide tubes comprising a lower damping portion which comprises at least a section of reduced inside diameter, the lower damping portion configured to contain a fluid for damping a fall of the control rod which is received in the guide tube, the section of reduced inside diameter surrounding the control rod with a radial passage gap when the control rod is introduced in the guide tube, the method comprising:calculating an expected falling speed of the control rods upon entry into the lower damping portions when the control cluster falls in an event of a shutdown of the nuclear reactor;calculating, based on the falling speed, a progression of the falling speed of the control rods in the lower damping portions;calculating, based on the progression of the falling speed of the control rods in the lower damping portions, a maximum elevated pressure produced in the fluid contained in the lower damping portions;calculating, based on the maximum elevated pressure, a maximum circumferential stress produced in the lower damping portions; anddesigning the guide tubes as a function of the maximum circumferential stress so that a maximum stress admissible by the guide tube is not exceeded. 2. A method for designing a nuclear fuel assembly which is intended to be positioned in a nuclear reactor, the assembly comprising a plurality of guide tubes and a control cluster which comprises a plurality of control rods and a support for the control rods, the control rods and the guide tubes extending in parallel with a longitudinal direction, each of the control rods being received in a guide tube in order to form pairs comprising guide tubes/control rods, each of the guide tubes comprising a lower damping portion which comprises at least a section of reduced inside diameter, the lower damping portion configured to contain a fluid for damping a fall of the control rod which is received in the guide tube, the section of reduced inside diameter surrounding the control rod with a radial passage gap when the control rod is introduced in the guide tube, the method comprising:calculating an expected falling speed of the control rods upon entry into the lower damping portions when the control cluster falls in an event of a shutdown of the nuclear reactor;calculating, based on the falling speed, a progression of the falling speed of the control rods in the lower damping portions;calculating, based on the progression of the falling speed of the control rods in the lower damping portions, a maximum elevated pressure produced in the fluid contained in the lower damping portions;calculating, based on the maximum elevated pressure, a maximum circumferential stress produced in the lower damping portions; anddesigning the guide tubes as a function of the maximum circumferential stress; andverifying, using the maximum circumferential stress, that a maximum stress admissible by the guide tube has not been exceeded. 3. The method according to claim 1, wherein the calculating, based on the falling speed, of a progression of the falling speed of the control rod in the lower damping portion, is performed using a higher value for the radial passage gap and the step of calculating, based on the progression of the falling speed of the control rod in the lower damping portion, a maximum elevated pressure produced in the fluid contained in the lower damping portion, is performed using a lower value for the radial passage gap. 4. The method according to claim 3, wherein the higher value is the maximum statistical value for the passage gap. 5. The method according to claim 3, wherein the lower value is the minimum statistical value for the passage gap. 6. The method according to claim 2, wherein the calculating, based on the falling speed, of a progression of the falling speed of the control rod in the lower damping portion, is performed using a higher value for the radial passage gap and the step of calculating, based on the progression of the falling speed of the control rod in the lower damping portion, a maximum elevated pressure produced in the fluid contained in the lower damping portion, is performed using a lower value for the radial passage gap. 7. The method according to claim 6, wherein the higher value is the maximum statistical value for the passage gap. 8. The method according to claim 6, wherein the lower value is the minimum statistical value for the passage gap.