Patent Number: 046831150
Section: summary

The invention relates to a nuclear reactor fuel assembly having a grid-shaped spacer with square grid meshes, wherein mutually parallel rods, particularly fuel rods containing nuclear fuel, are arranged, respectively, in a grid mesh, the spacer having flat outer straps extending transversely to the rods, and an intermediate strip extending parallel to the rods between two of the respective outer straps, the intermediate strip being inclined to the two outer straps. Such a nuclear reactor fuel assembly is already in use. It has one end part formed of a head member, and another end part formed of a base member. These two end parts are connected to one another by control rod guide tubes. Each of the control rod guide tubes is detachably connected by one end thereof, respectively, to a head plate of the head member and by the other end thereof, respectively, to a base plate of the base member, for example, by a threaded connection. The control rod guide tubes pass perpendicularly through the head and base plates. The head and base plates are rectangular and, ordinarily, are square. The control rod guide tubes are guided, respectively, in a square grid mesh through several rectangular, likewise ordinarily square, grid-shaped spacers which generally have a like spacing from one another as viewed in direction of the longitudinal axis of the control rod guide tubes and therefore also in direction of the fuel assembly, and which are aligned one with the other and with the head and base plates. Fuel rods, which contain nuclear fuel in hermetically sealed cladding tubes, are guided or extended through the square grid meshes of the spacer which are not occupied by the control rod guide tubes. These fuel rods are neither fastened to the head plate of the head member nor to the base plate of the base member, but rather have play in longitudinal direction between the head and the base plates and can therefore expand or elongate freely in direction of the longitudinal axis thereof i.e. in longitudinal direction of the fuel assembly. In the reactor core of a pressurized-water nuclear reactor in particular, a series of such nuclear reactor fuel assemblies of identical construction are arranged closely adjacent one another with parallel longitudinal axes in a checkerboard patters. During the loading and unloading of the nuclear reactor with the individual nuclear reactor fuel assemblies, adjacent nuclear reactor fuel assemblies might become hooked together in the reactor core on the outer straps of the spacers, which results in the destruction of these outer straps and makes replacement of the nuclear reactor fuel assemblies impossible. Nuclear reactor fuel assemblies which may have become greatly curved or bent due to operating stresses in the reactor core especially have a tendency to getting hooked together. For this reason, two edges of the outer straps of the spacers, namely the two edges perpendicular to the rods, i.e. to the control rod guide tubes and the fuel rods of the nuclear reactor fuel assembly, have already been provided with slightly inwardly-inclined rejection tabs, at which, when loading and unloading the reactor core, the corresponding edges of the outer straps of the spacers which are perpendicular to the rods of laterally adjacent nuclear reactor fuel assemblies moving relative to one another and to the first-mentioned reactor fuel assembly can slide off, so that the respective fuel assemblies are prevented from getting hooked at these edges. Such inwardly-inclined rejected tabs, however, cannot be arranged at the corners of a spacer of the nuclear reactor fuel assembly at which the corner rods of the nuclear reactor fuel assembly, mostly fuel rods, are located in a square corner grid mesh between two orthogonal or mutually perpendicular outer straps because they degrade thereat the thermohydraulic conditions for the coolant in the nuclear reactor to such a degree that the corner rods are not cooled sufficiently and may therefore become damaged. The danger therefore exists that diagonally adjacent nuclear reactor fuel assemblies in the checkerboard pattern in the reactor core become hooked with the corners of the spacers thereof during loading and unloading of the nuclear reactor. Even chamfering or forming inclinations in these corners does not provide the desired reliable protection against getting hooked, in extreme cases. In is accordingly an object of the invention to provided a nuclear reactor fuel assembly which cannot become hooked with a diagonally adjacent nuclear reactor fuel assembly in the nuclear reactor at the corners of the spacers therefore during the loading and unloading of the nuclear reactor. With the foregoing and other objects in view, there is provided in accordance with the invention, a nuclear reactor fuel assembly having a grid-shaped spacer with square grid meshes, wherein mutually parallel rods are arranged, respectively, in a grid mesh, the spacer having flat outer straps extending transversely to the rods and an intermediate strip extending parallel to the rods between two of the respective outer straps, the intermediate strip being inclined relative to the two outer straps, comprising a rejection rise formed at the outside of the intermediate strip and extending in direction of a diagonal of a grid mesh located at a corner between the two outer straps, the rejection rise being disposed transversely to the two outer straps and being inclined downwardly towards two respective ends of the intermediate strip in longitudinal direction of the rods. The rejection rises in the checkerboard pattern of diagonally adjacent nuclear reactor fuel assemblies can, respectively, engage the corner rod of the other nuclear reactor fuel assembly and force the two diagonally adjacent nuclear reactor fuel assemblies diagonally apart. During the relative movement of the two diagonally adjacent nuclear reactor fuel assemblies in longitudinal direction during the loading or unloading of the nuclear reactor, the ends of the intermediate strips at the spacers of the diagonally adjacent nuclear reactor fuel assemblies are therefore pushed away from one another to such an extent that they can no longer be hooked together if they should finally slide over one of the chamfers or inclinations at the rejection rises of the diagonally adjacent nuclear reactor fuel assembly. In accordance with another feature of the invention, the rejection rise is located on a spring strip suspended from the ends of the intermediate strip. In accordance with a further feature of the invention, the rejection rise is located on a fishplate on one of the outer straps, the fishplate forming the intermediate strip and being secured at an inner side thereof to the other outer strap. This simplifies the fabrication of the spacer. In accordance with an additional feature of the invention, the two outer straps are inclined at respective corners starting from edges thereof and extending towards the intermediate strip, and form chamfers of like inclination. In accordance with an added feature of the invention there is provided a rectilinear inclination continued from the respective outer straps into the intermediate strip and forming a pointed cutout at the ends of the intermediate strip. In accordance with a concomitant feature of the invention, the rejection rise is formed by a tab cut out of said fishplate on the one outer strap and is disposed in the plane of this one outer strap, the tab being formed at lateral edges thereof with respective pointed cutouts, the points of said cutouts being located on a bending edge extending parallel to the rods, said tab being bent perpendicularly at the bending edge into a plane wherein the other outer strap is disposed, the tab being secured to the outside of the other outer strap. Other features which are considered as characteristics for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in 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.