Patent Number: 
Section: claims

1. A fuel assembly for a PWR nuclear reactor wherein an upper nozzle arranged above the fuel assembly comprises:an adapter plate constituting a lower structure of the upper nozzle, and being formed in a square shape whose two diagonal lines orthogonal to each other pass through a center and diagonal corners of a passage surface of the adapter plate;a plurality of upright side walls extended in an upright direction along sides of the periphery of the adapter plate;an overhang projected into a center area of a space above the adapter plate from the upper part of the side wall; andapertures for attaching control rod guide tubes and passage holes, which are formed in a passage surface of the adapter plate,wherein the passage holes in the passage surface of the adapter plate include: one or more of first passage holes located on an area where coolant impinges upon the overhang, the first passage holes being generally arranged line-symmetric with respect to one diagonal line of said two diagonal lines serving as a symmetric axis, and second passages holes which are not at least partially covered by the overhang, andwherein first ligaments around the first passage holes which are completely covered by the overhang projected from the side walls and are located on an outer side of those of the attaching apertures which are located on the outer peripheral side of the passage surface, are set to be larger than second ligaments around the second passage holes located on opposite transverse sides of those of the attaching apertures,further wherein one or more elongated holes constituting the first passage holes are extended such that a longitudinal direction of the one or more elongated holes extends in parallel to the corresponding side wall located in areas that are line-symmetrical with respect to the one diagonal line. 2. A fuel assembly for a PWR nuclear reactor as set forth in claim 1, wherein passage holes for rectifying the coolant flow are arranged in an adjacent area of the attaching apertures which are located along the symmetric axes. 3. A fuel assembly for a PWR nuclear reactor as set forth in claim 1, wherein elongated passage holes for rectifying the coolant flow are arranged at positions on an inner side of the passage surface closer to a center of the passage surface than positions where the overhang is provided. 4. A fuel assembly for a PWR nuclear reactor as set forth in claim 1, wherein said first passage holes are a plurality of elongated holes provided along and parallel to the side walls. 5. A nuclear reactor core comprising a plurality of the fuel assemblies as recited in claim 1 arranged evenly in the nuclear reactor core. 6. A fuel assembly for a PWR nuclear reactor as set forth in claim 4, wherein the first passage holes which are formed at positions where the coolant impinged upon the overhang, are elongated so as to have a length which is longer than that of the passage holes which are located at positions on an inner side of the passage surface closer to a center of the passage surface than positions where the overhang is provided. 7. A fuel assembly for a PWR nuclear reactor as set forth in claim 6, wherein one elongated hole is provided as the first passage hole along each of the side walls located in the areas that are line-symmetrical with respect to said one diagonal line such that said one elongated hole is parallel to the corresponding side wall so that said one elongated hole is provided along each of the side walls.