Patent ID: 7627073

Claim:
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; and designing the guide tubes as a function of the maximum circumferential stress so that a maximum stress admissible by the guide tube is not exceeded.