Patent Number: 053393422
Section: summary

The invention relates to a fuel assembly for a boiling water reactor, having a bundle of fuel rods being disposed approximately parallel to one another, being retained in a skeleton and being inserted together with the skeleton into a fuel assembly case. The skeleton contains an upper tie plate on the upper end of the bundle that is retained at a handle, a lower tie plate at the lower end of the bundle, and as a rule a plurality of spacers. The spacers are disposed at a plurality of axial positions of the bundle and include ribs extending transversely to the bundle and laterally supporting the fuel rods through springs and/or knobs. In order for the lower tie plate to be joined to the upper tie plate and the handle, the skeleton includes at least one support element, and corner rods of the bundle and/or other selected fuel rods, which are screwed on both ends to the plates, can be used for the support element. In accordance with Published International Application WO 91/13441, corresponding to U.S. application Ser. No. 924,056, filed Aug. 28, 1992, a central water tube can also be used as a support element, on which the spacers are secured and in which the lower tie plate and the upper tie plate with the handle are retained on both ends the tube through screw fasteners. Once the fuel rods have been introduced, the skeleton and the bundle of fuel rods form an insert that can be inserted into or removed from the fuel assembly case that laterally surrounds the bundle and is open at the top. By lifting the handle, the skeleton with the fuel rods and the fuel assembly case slipped onto it can be removed from the core lattice of the boiling water reactor for inspection or repair purposes after an operating cycle, or for replacement purposes. The invention also relates to a locking means, which can also be used with other fuel assemblies, that releasably hold the fuel assembly case on the head part of the fuel assembly. It is accordingly an object of the invention to provide a fuel assembly for a boiling water reactor with a redundant support structure and a locked fuel assembly case, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which assures that the skeleton and the fuel rod bundle can be lifted and moved on the handle even if there is no longer any load-bearing connection between the lower tie plate and the upper tie plate in the skeleton as a result of damage, such as torn or broken screw fasteners of the support rods or the water tube. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water reactor, comprising approximately mutually parallel fuel rods in a bundle having upper and lower ends; a skeleton holding the bundle and having a handle, an upper tie plate retained on the handle at the upper end of the bundle, a lower tie plate at the lower end of the bundle, and at least one support element joining together the lower tie plate and the upper tie plate; a fuel assembly case in which the skeleton and the bundle are inserted; and redundant support means (in particular support means disposed outside the fuel assembly skeleton) holding the lower tie plate, the fuel assembly case and the upper tie plate together, when the handle is lifted. In accordance with another feature of the invention, there is provided a base part secured to the fuel assembly case, the lower tie plate being inserted into the base part, and the redundant support means including a releasable upper stop engaging the fuel assembly case and the upper tie plate or the handle. In accordance with a further feature of the invention, the fuel assembly case has a lower part disposed at the lower tie plate, and the redundant support means include an upper stop engaging the fuel assembly case and one of the upper tie plate and the handle, the upper stop defining a maximum distance between the handle and the lower part of the fuel assembly case, at least when the handle is lifted. In accordance with an added feature of the invention, there is provided a base plate carrying the lower tie plate, the fuel assembly case having an upper part disposed at the upper tie plate, the redundant support means including a releasable lower stop engaging the fuel assembly case and at least one of the lower tie plate and the base plate, and the lower stop defining a maximum distance between the lower tie plate and the upper part of the fuel assembly case, at least when the fuel assembly case is lifted. In accordance with an additional feature of the invention, the fuel assembly case has a recess formed therein, the skeleton has a stop surface opposite the recess, at least the upper stop is retained in the recess and against the stop surface in an installed state of the fuel assembly, for instance by frictional forces or spring forces, and at least the upper stop is movable by counter forces with a tool far enough to displace the fuel assembly case relative to the skeleton, or to displace the upper tie plate with the handle relative to the fuel assembly case, for disassembly of the fuel assembly. In accordance with yet another feature of the invention, the lower stop is a screw passing through the fuel assembly case and the base part transversely to the axis of the fuel assembly, the screw having a head being countersunk in the fuel assembly case from the outside and a bore formed in the screw being accessible from inside the base part and facing toward the head, the bore being flared open after the screw has been screwed into the base part. In accordance with yet a further feature of the invention, the upper stop is resiliently retained on the skeleton and reaches through a window 16 formed in the fuel assembly case in a direction transverse to the axis of the fuel assembly. With the objects of the invention in view, there is also provided a fuel assembly installed in a boiling water reactor, comprising a bundle of approximately mutually parallel fuel rods; a lower end having a base part and a lower tie plate forming a lower stop for the fuel rods; an upper end having an upper tie plate forming an upper stop for the fuel assembly, having a laterally protruding distance piece, and having a handle connected to the upper tie plate and to the distance piece; spacers for laterally fixing the fuel rods in place; a fuel assembly case laterally surrounding the bundle with the spacers, the lower tie plate and the upper tie plate; means for retaining the lower end at the fuel assembly case; and a locking spring being retained by the upper end and having a locking element reaching through a window formed in the fuel assembly case. In accordance with yet an added feature of the invention, the tension spring, the locking spring and the tension spring having restoring forces, the upper end being displaceable toward the lower end counter to the restoring force of the tension spring, and the locking element being movable out of the window counter to the restoring force of the locking spring, with the upper end displaced. In accordance with yet an additional feature of the invention, the fuel assembly case has an upper part, the locking spring has a spring element resting flatly on the upper part of the fuel assembly case in the direction of the lower end, the spring element is secured to the upper end and has a lower end engaging the window from above in hook-like fashion. In accordance with again another feature of the invention, the locking spring has an engagement surface, and the upper end has a disassembly bore formed therein leading to the engagement surface from above. In accordance with again a further feature of the invention, the locking element is a locking bar being held by the upper end, being displaceable counter to the restoring force of the locking spring, and protruding into another window formed in the upper end. In accordance with again an added feature of the invention, the other window in the upper end is a recess in which the locking spring and the locking bar are supported, and the locking bar has an end being supported in the recess and pressed outward. In accordance with again an additional locking element is a latch being held by the upper end and pivotable approximately at right angles to the axis of the fuel assembly counter to the restoring force of the locking spring. In accordance with still another feature of the invention, the fuel assembly case has corners, the pivotable latch has a joint part, the upper end has a frame part resting inside one of the corners of the fuel assembly case, the frame part has a recess formed therein, and the locking spring and the joint part are supported in the recess. In accordance with still a further feature of the invention, the recess forms a joint socket, the joint part and the joint socket have corresponding approximately hemispherical or cylindrical shapes, and the joint part rolls on the joint socket. In accordance with still an added feature of the invention, the corner of the fuel assembly case is formed by two adjacent walls toward which the recess is open, and the latch is laterally introduced into the recess, has considerable play in all directions in the recess and is retained by the locking spring. In accordance with a concomitant feature of the invention, the locking element has a forked tongue engaging corresponding windows formed in the walls of the fuel assembly case meeting one another in the corner of the fuel assembly case. 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 fuel assembly for a boiling water reactor with a redundant support structure and a locked fuel assembly case, 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.