Patent Number: 
Section: claims

1. A method for controlling positions of a plurality of nuclear fuel assemblies in relation to an upper core plate in a nuclear reactor core, the method comprising of the following steps:choosing a reference point in reactor internals or in a reactor vessel;determining positions of S shaped holes of the nuclear fuel assemblies relative to the reference point, each S shaped hole being intended to cooperate with a corresponding centering pin of the upper core plate;acquiring positions of the centering pins of the upper core plate relative to the reference point;directly comparing the position of each of the S shaped holes relative to the reference point and the position of the respective centering pin relative to the reference point and deducing therefrom whether each of the individual nuclear fuel assemblies is correctly positioned in relation to the upper core plate. 2. The method as recited in claim 1 wherein the reference point is a guide pin integrally attached to a lining of the core, the guide pin being adapted so as to cooperate with a notch of the upper core plate in order to position the upper core plate in relation to a lining of the core. 3. The method as recited in claim 1 wherein the positions of the S shaped holes relative to the reference point are determined by taking images of the nuclear fuel assemblies, and determining with the aid of said images the positions of the S shaped holes relative to the reference point. 4. The method as recited in claim 3 wherein each image is adapted to provide positions of the S shaped holes of at least one given nuclear fuel assembly and at least one S shaped hole of a nuclear fuel assembly adjacent to the given nuclear fuel assembly. 5. The method as recited in claim 3 wherein an overall image of the plurality of nuclear fuel assemblies is developed from the images of the nuclear fuel assemblies, the overall image providing the positions of all of the S shaped holes of the plurality of nuclear fuel assemblies. 6. The method as recited in claim 5 wherein the comparison of the positions of the S shaped holes and the positions of the pins is carried out by comparing the overall image of the plurality of nuclear fuel assemblies to a theoretical image of the upper core plate providing the positions of all of the centering pins corresponding to all the S shaped holes of the plurality of nuclear fuel assemblies. 7. The method as recited in claim 6 wherein each nuclear fuel assembly is considered to be correctly positioned in relation to the upper core plate if the comparison of the position of each S shaped hole of said nuclear fuel assembly with the position of the corresponding pin indicates that the S shaped hole and the pin have a distance between them that is less than a predetermined limit. 8. The method as recited in claim 7 wherein the predetermined limit is 8 millimeters. 9. The method as recited in claim 3 wherein the images are taken by a digital image capturing apparatus moved by a machine for loading the nuclear fuel assemblies. 10. The method as recited in claim 1 wherein the plurality of nuclear fuel assemblies comprise at least one quarter of the nuclear fuel assemblies of the core. 11. The method as recited in claim 10 wherein the plurality of nuclear fuel assemblies comprise at least half of the nuclear fuel assemblies of the core. 12. An assembly for controlling positions of a plurality of nuclear fuel assemblies in relation to an upper core plate in a nuclear reactor core, the control assembly comprising:a determiner for determining the positions of S shaped holes of nuclear fuel assemblies relative to a reference point, each S shaped hole being provided to cooperate with a corresponding centering pin of the upper core plate, the reference point being chosen in reactor internals or in a reactor vessel;a determiner for determining the positions of the centering pins of the upper core plate relative to the reference point; anda comparator for directly comparing the position of each of the S shaped holes relative to the reference point and the position of the respective centering pin relative to the reference point, and deducing therefrom whether each of the individual nuclear fuel assemblies is correctly positioned in relation to the upper core plate.