Patent Number: 
Section: claims

1. A method for evaluating at least one quantity relative to the distortion of a nuclear fuel assembly, the method comprising the following steps:placing the nuclear fuel assembly in a volume of water bounded by a free upper surface;placing a camera outside the volume of water, above the free upper surface;taking at least one image of at least one lateral face of the nuclear fuel assembly with the camera while the camera is above the free upper surface of the volume of water; andgraphically analyzing the at least one image and deducing the at least one quantity relative to the distortion of the nuclear fuel assembly therefrom. 2. The evaluation method according to claim 1 wherein the at least one image taken by the camera shows the entire lateral face of the nuclear fuel assembly. 3. The evaluation method according to claim 1 wherein the nuclear fuel assembly comprises: a plurality of longitudinally elongated nuclear fuel rods; and a plurality of grids for maintaining the nuclear fuel rods in position, the plurality of grids being distributed longitudinally along the nuclear fuel rods, the at least one quantity relative to the distortion of the nuclear fuel assembly being respective shifts of the grids in a transverse plane perpendicular to a longitudinal direction. 4. The evaluation method according to claim 3 wherein the camera has an optical axis forming an angle between 10° and 40° relative to a vertical at a time of a shot. 5. The evaluation method according to claim 3 wherein the camera and the nuclear fuel assembly are spaced apart from each other when the image is taken by a distance between 1 meter and 4 meters in a horizontal plane. 6. The evaluation method according to claim 3 wherein the nuclear fuel assembly comprises an upper and a lower maintenance grid situated near opposite ends of the nuclear fuel rods; and a plurality of intermediate maintenance grids distributed between the upper and the lower maintenance grids, wherein the step for graphic analysis comprises the following sub-steps:materializing, on the image, at least one substantially longitudinal reference line extending from the upper or lower maintenance grid to the other upper or lower maintenance grid; anddetermining, on the image, a transverse shift of each of the intermediate grids relative to the at least one reference line. 7. The evaluation method according to claim 6 wherein the upper, lower and intermediate maintenance grids or the fuel rods near the maintenance grids can have respective visual references substantially aligned longitudinally when the upper, lower and intermediate maintenance grids are not shifted transversely, the reference line passing through the visual references relative to the upper and lower maintenance grids, the transverse shift of each intermediate maintenance grid being determined by estimating, on the image, the transverse shift between the visual reference relative to the intermediate maintenance grid and the reference line. 8. The evaluation method according to claim 3 wherein at least one image is taken of each of the two lateral faces perpendicular to each other of the nuclear fuel assembly, and the respective shifts of the maintenance grids are determined in two directions perpendicular to each other and perpendicular to the longitudinal direction. 9. The evaluation method according to claim 1 wherein the nuclear fuel assembly comprises upper and lower caps, wherein the quantity relative to the distortion of the nuclear fuel assembly is the rotation of the upper and lower caps relative to each other around a longitudinal direction. 10. The evaluation method according to claim 9 wherein the camera presents an optical axis forming an angle between 1° and 10° relative to a vertical when the image is taken. 11. The evaluation method according to claim 9 wherein, when the image is taken, the camera and the nuclear fuel assembly are spaced apart from each other by a distance smaller than 1 meter in a horizontal plane. 12. The evaluation method according to claim 9 wherein the upper and lower caps have determined respective geometric lines normally parallel to each other when the upper and lower caps do not have any rotation relative to each other around the longitudinal direction, wherein the step for graphic analysis of the image comprises the following sub-steps:determining, in the image, the two geometric lines; anddetermining, in the image, a relative angle between the two geometric lines. 13. The method according to claim 1 wherein the camera has a determined optical axis, and a shielding window being placed on the free surface and inserted on the optical axis of the camera.