Patent Number: 053393422
Section: description

Referring now to the figures of the drawing in detail, and first, particularly, to FIG. 1 thereof, there is seen a preferred exemplary embodiment of a fuel assembly which includes a support element in the form of a central coolant tube (water tube) WR, that forms a flow channel for liquid coolant and has openings 1, 2 at the bottom and the top and extends from a lower tie plate LTP to an upper tie plate UTP. The bottom plate or lower tie plate LTP is screwed to a lower end piece of the water tube WR. An upper end piece also has a screw fastening, and in this case a bolt 3 of the tube WR reaches through an opening in the cover plate or upper tie plate UTP and in a handle H mounted on it. A protruding end of the bolt 3 has a thread on which a nut is mounted that forms a stop for the handle and the upper tie plate UTP. In FIG. 1, this nut does not become visible until the handle and the upper tie plate are pressed against a compressed tension spring 4 inserted between the water tube and the upper tie plate. Once this spring is relieved, the nut vanishes in a profiled recess in the handle H in which this nut is secured against being unintentionally turned and loosened. At various axial positions of the water tube, spacers SP are held between stops 5 and 6. Ribs of this spacer SP form voids in which fuel rods FR are supported. Upper end caps of these fuel rods are constructed as bolts and are loosely guided in bores 7 in the upper tie plate UTP. The coolant tube, spacer, lower and upper tie plates and fuel rods accordingly form an insert, which can be held at the handle H and inserted from above or below into a fuel assembly box or case or water channel WC that is open at the bottom and top. In the process this insert with the lower tie plate LTP comes to rest on an upper edge of a base part FT on which the lower tie plate can be set or even welded. A skeleton for the bundle of fuel rods includes the handle H, the upper tie plate UTP, the lower tie plate LTP and the support element or water tube WR. If the lower tie plate LTP is welded to the base plate FT, then it also forms part of the skeleton onto which the water channel WC can be slid from above. The entire, fully installed fuel assembly can be lifted with the handle. A distance d between the upper surface of the upper tie plate UTP and the bottom of the lower tie plate LTP is substantially constant with this construction of the load-bearing skeleton. In other words, the distance d varies only as a result of the thermal expansion of the water tube WR, and this construction also enables a thermal expansion of the fuel rods FR to take place. If the load-bearing skeleton is destroyed, for example if the water tube WR breaks or if one of the screw fasteners that keep the handle H on the upper tie plate and keep the lower tie plate LTP on the water tube WR (or load-bearing fuel rods if applicable) tears, then the lower tie plate held by the base part FT and the fuel rods resting on it can no longer be removed from the core by lifting the handle H. The distance d accordingly increases to the extent by which the handle H is lifted. The invention therefore provides a redundant support structure that, in addition to the support means of the skeleton, assures that the distance d will not drop below a specific maximum value. This redundant support structure may be formed in a simple manner by the water channel WC and by corresponding stops or retaining means between the water channel and the two plates UTP and LTP, with these stops defining the maximum value d. To that end, suitable fastening means for the water channel and the lower tie plate, or for the base part FT carrying the lower tie plate, are provided on a lower edge of the cluster or bundle of fuel rods. These fastening means may be a stop retained in the channel wall, which laterally engages the inside of the lower tie plate LTP or fits over this lower tie plate at its lower surface. By way of example, this stop may be a spring clip, for instance, that is secured to the base part and initially engages only a bore oriented toward the water channel, so that the lower edge of the water channel resting on the base part and on the lower tie plate can be slipped onto the base part and the lower tie plate. In that case, the lower edge of the water channel has a corresponding bore as well, into which the spring clip snaps due to its spring force once the water channel has been slipped on, so that the spring clip then forms a stop that engages recesses in the base part (or the lower tie plate) and in the water channel and so that when the water channel is lifted, a maximum value for the distance d between the lower tie plate and the upper edge of the water channel that rests on the upper tie plate UTP is defined. Naturally, a corresponding spring clip may be secured to the outside of the base part FT or to the outer surface of the side of the water channel, so that once the water channel has been slipped on it will engage corresponding recesses in the water channel and in the base part FT (or in the lower tie plate LTP) and be held in that position. In FIG. 1, a screw SCR is provided as the lower stop. Once the case has been installed, this screw SCR is screwed into a threaded bore 8 in the base part FT, through a corresponding hole 9 that is shaped for receiving the head of the screw and is provided in the lower edge of the case. Since this screw is seated at a point of the fuel assembly that is virtually unstrained thermally, and mechanically as well if the supporting skeleton is intact, then it is sufficient for retention of this stop to adapt the threaded bore 8 to the screw profile with adequate play, so that the screw can be inserted into the threaded bore 8 with only a little exertion of force, and the resultant frictional forces can hold this screw. Since the lateral position of the lower tie plate LTP resting on the base part is also fixed because of the retention of the water channel on the base part, it is possible in this construction to dispense with a weld connection between the lower tie plate and the base part. Unless it is necessary to make the fuel rods inserted into the skeleton laterally accessible for inspection purposes, the lower tie plate can be fixed to the water channel and to the base part by welding them together, instead of having to provide a screw or a corresponding stop. A stop disposed between the water channel and the plate and retained by a spring clip in the described way is shown in FIG. 1 as a retainer for the upper tie plate UTP. Typically, the handle and the upper tie plate have a frame that on one hand fits over the upper edge of the water channel WC for retaining it and on the other hand serves as a bearing surface for fuel assemblies adjacent the frame in the reactor core. According to FIG. 1, a spring clip or locking spring SG, having a free end which reaches through a recess 11 in the water channel and rests with a locking element in the form of a profiled part 13 on a stop surface 12 of the upper tie plate UTP, is secured to the outer surface of a frame or laterally protruding distance piece FM by lock means 10. Through the use of the compressed tension spring 4, the upper tie plate UTP has been pushed so far upward that the profiled part 13 of the spring clip SG is retained captive between the stop surface 12 and the case wall even if the spring clip SG itself should break. Alternatively or in addition, FIG. 1 shows a screw 14, with which the frame FM, water channel WC and handle H are screwed together in this position, in the manner already described for the screw SCR serving as a lower stop. This construction assures that even if the load-bearing parts of the supporting skeleton break (for instance if there is a broken water tube WR), a maximum value for the distance d between the upper tie plate UTP and the lower edge of the water channel WC itself (and therefore the bearing of the lower tie plate LTP resting there) will not be exceeded even if the fuel assembly is lifted at the handle H. In order to remove the coolant tube, the upper tie plate UTP is pressed against the tension spring 4, so that the profiled part 13 of the spring clip and the recess 11 in the fuel assembly case adapted to it face one another, the profiled part 13 can be bent outward by a tool through the recess 11, and the fuel assembly case WC can be pulled off. The screws SCR and 14 are loosened in the process. When the fuel assembly is installed, the procedure is correspondingly the reverse. The spring clip is first bent outward far enough by a suitable tool that the fuel element case can be slipped into the skeleton until, when the spring clip is released, with the upper tie plate pressed downward, the profiled part can snap through the opening 11 into its terminal position and lock in the terminal position specified by the profiled part 12 when the spring 4 is relaxed. A corresponding spring clip that locks into place in a recess in the fuel assembly case can naturally be secured to the upper tie plate UTP or to the top of the frame FM instead of to the side of the frame. It is equally possible to provide a stop that is held on the fuel assembly case by spring forces or frictional forces, which engages a corresponding opening in the frame and/or in the handle and/or in the upper tie plate. Accordingly, while the profiled part 13 forms an upper stop for the fuel assembly case, with the stop being resiliently held on the skeleton and reaching transversely to the fuel assembly axis through a window (recess 11) in the case, the lower stop is advantageously constructed as a screw that reaches through the fuel assembly case and the base part. As is seen in FIG. 2, this screw is approximately transverse to the fuel assembly axis and has a head SCH that is countersunk from the outside into the fuel assembly case, while a threaded part has a bore SCB that is oriented toward the screw head SCH and is accessible from the interior of the base part. This bore SCB is slightly flared open once the screw has been screwed into the base part. As a result, the threaded part of the screw is firmly pressed into the contrary thread of the base part, producing a connection between the case and the base part that is releasable only by damaging the screw. In this way, the screw itself is secured against being lost. It is practical to reinforce the base part in the region of the screw, and FIG. 2 shows that bypass bores BP, through which coolant can be introduced into the region of the fuel rods, can be disposed inside this reinforced part. A locking element that locks resiliently in detent fashion into corresponding receiving points of the fuel assembly case and of the upper part (upper end of the skeleton), and that is therefore simple to release but is secured in captive fashion on the fuel assembly, can advantageously be used as the upper stop that keeps the fuel assembly case supportingly on the upper part. This kind of locking means can also be used if the fuel assembly does not include any skeleton that would form an integrated insert which would be removable from the fuel assembly case along with the fuel rods. The invention therefore relates to fuel assemblies for a boiling water reactor that contain a cluster or bundle of fuel rods FR that are approximately parallel to one another, spacers SP for lateral fixation of the fuel rods, a lower end with a base part FT and a lower tie plate LTP as a stop for the fuel rods FR, an upper end with an upper tie plate UTP as a stop for the fuel assembly and a handle H connected to the upper tie plate and to the frame or distance piece FM, and a fuel assembly case WC laterally surrounding the cluster or bundle along with the spacers SP, the lower tie plate and the upper tie plate. Through the use of screw fasteners or other retaining means, the lower end is held on the fuel assembly in the installed state, and the lower tie plate serves as a lower stop for the fuel rods. The upper tie plate on the upper end correspondingly acts as an upper stop for the fuel rods, and this upper end includes a handle that is connected to the upper tie plate and to a distance piece laterally protruding beyond it. FIGS. 3 and 4 show two longitudinal sections through an exemplary embodiment, in which a spring with a locking element reaching through a window of the fuel assembly case is held on the upper end. In FIG. 3, the fuel assembly case WC, which has a polygonal and in particular a square cross section, and the handle H, can be seen. The handle forms a component that is cast in one piece with the upper tie plate UTP and a frame having a distance piece DP, or is held together in some other way. The distance piece DP fits over the upper edge of the fuel assembly case WC on at least two sides and forms a bearing surface on which adjacent fuel assemblies rest, in the core structure. It may be advantageous to hold a distance spring DS on the head by means of a fastening screw HS, in order to support the fuel assembly against adjacent fuel assemblies. In the case of assembly and disassembly, the upper end with the upper tie plate and the distance piece can be displaced toward the lower end, counter to the restoring force of the tension spring 4 that was already shown in FIG. 1, into a position 15 that is represented by a phantom line. As a result, a locking element 17 that reaches through a window 16 can be moved out of the window 16. This locking element 17 is retained on the upper part of the fuel assembly by a locking spring or spring element 18. It can therefore be bent out of the window counter to the restoring force of this locking spring. As FIG. 4 shows, the locking spring is secured on the upper part due to the fact that one end of the spring is screwed to the distance piece DP by a retaining screw 19. The locking spring 18 has a spring part 20 resting flatly on the upper part of the water channel WC in the direction of the lower end, and the locking spring 18 has a lower hook-like end 21 which engages the window 16 in hook-like fashion from above as an upper stop. Accordingly, if the upper end of the fuel assembly case is lifted at the handle H, then the case is suspended in the hook-like end 21 of the locking spring. Advantageously, this kind of locking spring 18 that is constructed as a leaf spring has an engagement surface in the form of a strap 22 which may be disposed laterally, for instance, so that the spring can be bent out of the window 16. FIG. 3 shows that the frame of the upper part also has disassembly bores 23 leading to this engagement surface 22, and a pin for moving the spring could be introduced through these bores. An advantageous spring shape is also shown in FIG. 5, wherein a spring has a middle part 24 bent into a U and it has a flat spring part 25 pointing upward, with a locking element 26. Ends 27 of legs of the spring may be suspended from or screwed or welded to the upper part, while the locking element 26 engages the window of the fuel assembly case. Instead of leaf springs, which sometimes tend to break, helical springs can also be used as the locking springs- in particular, a locking bar that is held by the upper part, is displaceable counter to the restoring force of the helical locking spring, and protrudes into the window of the fuel assembly case can be used as the locking element. FIG. 6 shows this kind of structure, in which the distance piece DP forming the frame of the upper part has a window or recess 28 formed therein, in which a helical locking spring 29 is supported in such a way that one end 30 of a locking element in the form of a bar that is located in the recess, is pressed outward. In this outwardly pressed position, another end 31 of the locking bar engages the inside of the window 16 of the water channel WC. Once again, a disassembly bore 32 makes it possible to press the locking bar back by the insertion of a pin, thereby compressing the spring 29 and freeing the window 16 in the water channel. In the preferred embodiment, the locking element is constructed as a latch that is held by the upper part and is pivotable approximately perpendicularly to the fuel assembly axis, counter to the restoring force of the locking spring. Such an embodiment is shown in FIG. 7 in a plan view of the upper part of the fuel assembly, in FIG. 8 in a cross section through the handle, and in FIG. 9 in a longitudinal section through the upper part. In this case, the upper part is provided in multiple parts and includes a hoop of the handle H extending diagonally across the rectangular cross section of the fuel assembly and merging with frame parts that rest on opposite corners of the water channel WC from the inside and through which frame parts another frame part that fits over the upper edge of the fuel assembly case is screwed on by means of fastening screws HS. This other frame part carries the distance springs DS, acting as the distance piece DP, on which the upper parts of adjacent fuel assemblies are supported. The upper tie plate UTP has through-flow openings 40 for the coolant and guide openings 41 for the upper cap pieces of the fuel rods and is secured to a slightly eccentric central case CC, which is cast onto the handle. It can be seen from FIG. 8 that a frame part 42 which rests from the inside on one corner of the fuel assembly case, has a recess 43 formed therein, in which a joint element 44 of a latch and a locking spring 45 are supported. In the case of installation, the locking spring 45 can be inserted through a bore 46 and compressed enough to permit the locking element, of which only the joint part 44 is visible in FIG. 8, to be inserted into the recess 43 from the side. This type of recess, support and installation may also be provided for the locking bar of FIG. 6. The locking element may then include a tongue that reaches through the opening 46 diametrically into a corresponding window in the corner of the fuel assembly case. However, FIG. 9 shows that the locking element has a forked tongue 47, so that it can engage corresponding windows of abutting case walls in the corner of the fuel assembly case. The recess 43 has a lower part which forms an approximately hemispherical or partially cylindrical joint socket 48. The joint part 48 accordingly likewise has a hemispherical or partially cylindrical end, with the radius of curvature of the joint part being less than the radius of curvature of the socket. Upon a pivoting motion of the locking element, this half-round profile of the joint part accordingly rolls along the socket. It can be seen from FIGS. 6 and 8 that the locking element can be seated in the recess 43 with considerable play to all sides, because its final position is fixed once the locking spring presses the locking element outward and the locking bars or tongues engage the inside of the window. This makes installation easier and lessens the danger of the locking element sticking to the frame part after the fuel assembly has been in use for a relatively long period in a nuclear reactor because of corrosion or soiling, which would prevent it from being pushed back counter to the restoring force of the locking spring when the fuel assembly is being dismantled. In particular, such factors are virtually no threat to the pivoting motion of the latch of FIGS. 7 through 9, because the generous play means that it can easily be broken loose and can roll along the joint socket even over layers of corrosion and dirt. When the fuel assembly is installed, the resilient locking parts automatically snap into the windows and openings that are provided for retaining the fuel assembly case on the upper part of the fuel assembly, as soon as the upper part is inserted into the case. In the case of disassembly, the screws 19 in FIG. 4 and HS in FIGS. 3 and 7 that are involved in this fastening, need not be loosened. If the connection is constructed as a resiliently supported locking bar or latch, then there is also no danger that leaf springs might break or that parts of the lock might loosen and be lost in the flow of coolant, where they could cause considerable disruption to the system. Naturally, corresponding resilient locking means may also be provided as a releasable connection between the lower end of the fuel assembly case and the lower end of the fuel assembly, but in this region of the fuel assembly they are unnecessary in many cases because of the low thermal and mechanical strain.