Patent Number: 054815797
Section: summary

TECHNICAL FIELD This invention relates generally to boiling water nuclear reactors and, more specifically, to a latching and lifting mechanism facilitating removal of fuel rods from fuel bundle assemblies within such reactors. BACKGROUND It is well known that fuel rods in boiling water reactor (BWR) fuel bundles are supported on a lower tie plate and extend upwardly to an upper tie plate. Between the upper and lower tie plates, the fuel rods are supported laterally by a plurality of spacers. In some BWR, bundle designs, the upper and lower tie plates are directly connected to each other by several special fuel rods, called tie rods. These tie rods have lower end plugs which are threadably secured in the lower tie plate. The tie rods also have threaded upper end plugs which extend through the upper tie plate. Conventional nuts are used on these upper end plugs to secure the tie rods to the upper tie plate. Special lock washers are used to prevent rotation of the nuts. Specifically, the washers are formed with tabs which are bent upward to lock the nuts in position and thus prevent rotation. The upper tie plate typically includes a handle extending upwardly from its base. To lift the fuel bundle, a grapple engages this upwardly extending handle. The weight of the fuel rods is transmitted to the lower tie plate, through the tie rods to the upper tie plate, and then to the handle. Occasionally, a fuel bundle must be disassembled partway through its service life or at the end of its use. At such time, the lock washer tabs are bent to free the nuts, and the nuts and washers are then removed. Thereafter, the upper tie plate can be removed followed by extraction of one or more fuel rods as necessary. The invention here relates to a different constructions for transferring the fuel bundle weight to the upper tie plate, and for latching and unlatching the upper tie plate from the fuel bundle to facilitate removal of fuel rods from the bundle. DISCLOSURE OF THE INVENTION In accordance with this invention, highly reliable latching and lifting mechanisms are provided for fuel bundles in boiling water reactors. More specifically, the invention here relates to BWR fuel bundles which incorporate a pair of large water rods, each occupying a space which would otherwise be occupied by four fuel rods. The water rods typically have lower end plugs inserted into the lower tie plate and upper end plugs inserted into the upper tie plate. These end plugs are free to slide vertically within holes in the tie plates. The present invention utilizes these water rods to lift the fuel bundle. To this end, the lower water rod end plugs are rigidly attached to the lower tie plate by suitable means (e g, by threaded attachment), but the specific manner of attachment to the lower tie plate forms no part of this invention per se. A latching mechanism (three embodiments are disclosed) is utilized to attach the upper end plugs to the upper water rod tie plate Thus, when the fuel bundle is lifted, the load is transferred from the fuel rods to the lower tie plate, through the water rods to the latching mechanism and then to the upper tie plate and finally to the lifting handle. In an exemplary embodiment, the upper tie plate is formed with an enlarged double boss for receiving the end plugs for the pair of water rods. This double boss is formed integrally with the tie plate and is connected to the other fuel rod bosses in the upper tie plate by a series of relatively narrow webs similar to those webs which interconnect the various fuel rod bosses. This enlarged double boss is formed with a centering hole located between the pair of bosses and adapted to receive a center post utilized to center and secure a latching bar on the double boss. The double boss area is also formed with a forwardly extending projection midway between the bosses projecting normal to a line between the boss centers. The projection has a vertical hole on a center axis which is parallel to and located forwardly of the centering hole. At the same time, the upper ends of the water rods are provided with end plugs having upper ends formed with axially extending cut-outs which, in the assembled orientation, face each other. A latching bar in accordance with this first exemplary embodiment is formed with a vertically extending central through bore adapted to receive the centering pin, enabling the latching bar to be properly oriented on the double boss of the upper tie plate. The latching bar is also formed with lower ends laterally projecting in opposite directions and, as explained below, rotatable into and out of the cut-outs provided on the end plugs of the water rods. This latching bar is also formed with a forwardly extending projection containing a through hole, the center line or axis of which may be aligned with the projection hole in the enlarged double boss. The lower surface of the latching bar is provided with a centrally located, elongated rib extending from front to back in a direction perpendicular to the oppositely and outwardly extending lower end portions of the latching bar. With the latching bar in place on the double boss, and after the upper tie plate is lowered into position such that the end plugs (including the axial cut-outs) of the water rods project through the double boss of the upper fie plate, the latching bar may be rotated such that the oppositely extending lower end portions move into the cut-out areas of the end plugs. Springs are located on the end plugs so that when the lower tie plate is lowered into position as described above, the springs will exert a biasing force in the opposite direction. Accordingly, when the latching bar is rotated into position, dove, award pressure on the lower-tie plate is released, causing the tie plate and latching bar to be resiliently biased upwardly against shoulders formed in the end plugs. As a result, the upper tie plate, water rods and lower tie plate are now rigidly connected. A locking pin is then used to secure the latching bar against rotation relative to the tie plate, thereby preventing accidental rotation of the latching bar out from its locking position. At the same time, the locking pin is designed to prevent accidental withdrawal of the pin itself from the latching bar through the utilization of a compression ring described in greater detail below. In an alternative embodiment of the invention, the latching bar and locking pin are modified so that the locking pin is spring loaded in a downward direction, again preventing accidental withdrawal of the pin. In a third exemplary embodiment of the invention, the locking pin is provided with a pair of spring fingers which, in combination with a pair of adjacent stops, lock the latching bar against rotation and also prevent accidental withdrawal of the pin. In a fourth exemplary embodiment, the water rod end plugs are modified to include 360.degree. grooves, and the latch bar is of a "double horseshoe-type" where oppositely facing recesses on the latch bar are pivoted into locking engagement with the reduced diameter grooves in the end plugs The advantage of this approach is that the 360.degree. circumferential grooves on the end plugs makes the angular orientation of the water rods unimportant, at least insofar as the reliability of the locking mechanism per se is concerned. By making the radial position unimportant, the tolerances in the water rod will not need to be as tight, thus enhancing the manufacturability of the water rods. The anti-rotation feature of the water rods in this embodiment may be achieved by keying in the lower or upper tie plate with shaped holes. Another advantage of the above described "double horseshoe type" latch bar is that additional surface engagement of the locking surfaces can be achieved. In a variation of this fourth embodiment, the oppositely facing recesses in the latch bar are generally square, and the 360.degree. grooves or cut-outs in the water rod end plugs are also squared at the base of the respective grooves. This arrangement not only provides even more surface locking engagement, but also provides locked radial positioning of the water rods. This, then, simplifies the tie plate manufacturing in that no shaped holes for keyed positioning of the water rod end plugs is required. Additional objects and advantages of the invention will become apparent from the detailed description which follows.