Patent Number: 048511870
Section: summary

The invention relates to a nuclear reactor fuel assembly including a fuel assembly box having sides, an upper end and corners, a head plate disposed in the fuel assembly box, fuel rods containing nuclear fuel being disposed in the fuel assembly box and guided in leadthroughs formed in the head plate, at least some of the fuel rods being secured to the head plate in the leadthroughs, a corner bolt standing on top of the head plate, a cross bar disposed inside one of the corners at the upper end of the fuel assembly box on the corner bolt, an angle element having an outer surface and being adapted to the fuel assembly box, two leaf springs each being disposed on the outer surface of the angle element at a respective one of the sides of the fuel assembly box and extending in longitudinal direction of the fuel assembly box, a screw bolt firmly screwed to the fuel assembly box and the angle element at the corner bolt, the screw bolt having an expansion shaft with a reduced diameter, a bolt head having two ends and being disposed on top of and supported on the angle element, a constriction disposed between the ends of the bolt head defining a coaxial bolt head shaft with a reduced diameter, the constriction being disposed in a bore formed in the angle element between the ends of the bolt head, and a transverse pin being disposed in the bore and engaging the constriction. A nuclear reactor fuel assembly of this type is known from European Patent Application No. 0 142 778, corresponding to U.S. application Ser. No. 120,725, filed Nov. 13, 1987. In this known fuel assembly, the transverse pin, which engages the constriction location between two ends of the bolt head, is secured at both ends on the angle element, and acts as a retainer in order to associate the screw bolt with the angle element in captive fashion. The screw bolt will break only at the expansion shaft and nowhere else whenever expansion forces in the screw bolt occurring in the loosening direction become too great, such as when the fuel assembly is used in a boiling water reactor. Meanwhile the constriction location of the bolt head, or in other words the bolt head shaft having a reduced diameter, is not stressed at all by these expansion forces. The transverse pin secured with both ends to the angle element thus prevents the fragments of the screw bolt from being able to become detached from the fuel assembly, when the expansion shaft breaks. Nevertheless, the screw bolt is rotatable about the longitudinal axis thereof. There is no provision, however, for replacement of the screw bolt, such as after the fuel assembly has been placed in the nuclear reactor. It is accordingly an object of the invention to provide a nuclear reactor fuel assembly, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which permits easy replacement of the screw bolt. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor fuel assembly, comprising a fuel assembly box having sides, an upper end and corners; a head plate disposed in the fuel assembly box; fuel rods containing nuclear fuel being disposed in the fuel assembly box and guided in leadthroughs formed in the head plate, at least some of the fuel rods being secured to the head plate in the leadthroughs; a corner bolt standing on top of the head plate; a cross bar disposed inside one of the corners at the upper end of the fuel assembly box on the corner bolt; an angle element having an outer surface and being adapted to the fuel assembly box; two leaf springs each being disposed on the outer surface of the angle element at a respective one of the sides of the fuel assembly box and extending in longitudinal direction of the fuel assembly box; a screw bolt firmly screwed to the fuel assembly box and the angle element at the corner bolt, the screw bolt having an expansion shaft with a reduced diameter, and a bolt head having two ends and being disposed on top of and supported on the angle element, the bolt head having an outer surface with an annular recess formed therein between the ends of the bolt head defining a coaxial bolt head shaft with a reduced diameter, the coaxial bolt head shaft being disposed in a bore formed in the angle element between the ends of the bolt head; and a transverse pin being disposed in the bore and having one end protruding into the annular recess in the bolt head. The transverse pin protruding with one end into the annular recess or constriction location of the bolt head is removable from the angle element whenever the screw bolt is to be replaced. In accordance with another feature of the invention, the transverse pin extends in radial direction of the bolt head shaft. In accordance with a further feature of the invention, the transverse pin is a screw. In accordance with an added feature of the invention, one of the leaf springs is firmly screwed to the angle element with the screw. In accordance with a concomitant feature of the invention, the expansion shaft has a smaller diameter than the coaxial bolt head shaft. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a nuclear reactor fuel assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.