Patent Number: 053274717
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a completely assembled fuel assembly having a longitudinal axis being indicated by reference symbol AX, in a view which is not to scale. The fuel assembly is surrounded laterally by a fuel assembly case or jacket WC, which is open at the top and bottom. A fuel assembly head or cap HD and a foot or base part FT are located there. The foot part FT is positioned in the core of a reactor on a base grid by means of a bracket FTa. A transition piece 2 forms a flow channel that leads from an inlet opening 3a to coolant inlets 3 in a bottom plate 1 that covers the foot part or the lower end of the fuel assembly. A lower edge WCa of the fuel assembly case WC is supported against and largely sealed off from the foot part FT and its bottom plate 1 by means of a sealing spring 4. A coolant tube or "water tube" WR extends axially through the interior and preferably the center of the case WC. The coolant tube WC has one respective end piece WRa, WRb and respective openings 5a, 5b for the passage of coolant (water) at its lower end upper ends. The fuel assembly head HD and the bottom plate 1 are supports for the coolant tube WR. Spacers SP are mounted on the coolant tube at predetermined axial positions between retaining means in the form of stop bodies 90, 91. The spacers SP are at right angles to the tube WR and include support ribs. These support ribs form a grid with meshes or holes, which may be formed, for instance, from sheets that are welded together or from lengthwise and crosswise ribs that meet one another at right angles. Supported on these ribs are a number of fuel rods FR. Each of the fuel rods FR is parallel to the case, passes through the meshes of a plurality of spacers and carries closure caps FRa, FRb at the bottom and top. The fuel assembly head HD has a cover plate 6, which covers the fuel assembly case WC on the top and has coolant outlets 7. A handle 9 is disposed on the upper surface of the cover plate, and the cover plate 6, the handle 9 and the upper end piece WRb of the coolant tube are held together by a connecting part 8 that is constructed as a stop body. The fuel rods FR are firmly clamped substantially within the meshes of the spacers. The bottom plate 1 and the cover plate 6 serve only as stops that prevent major axial motions of the rods. The closure caps of the fuel rods therefore have no thread with which they would be screwed to the plates. Instead, the fuel rods stand on the bottom plate with their lower closure caps, and are also only loosely guided by the upper closure caps in appropriate receiving positions on the cover plate. The nuclear reactor fuel assembly of FIG. 2 has a fuel assembly head with a cover plate 6 and a fuel assembly foot part FT, each having a rectangular (in this case square) cross section, an elongated coolant tube WR with a cross-shaped, round or rectangular (in this case square) cross section, and an elongated fuel assembly case WC, which likewise has a rectangular (square) cross section. The coolant tube WR is disposed centrally in gridlike spacers SP, which are located inside the fuel assembly case and are spaced apart from one another, as seen in the longitudinal direction of the coolant tube. One fuel rod FR containing nuclear fuel reaches through each mesh or space of these spacers SP. These fuel rods loosely engage leadthroughs in the fuel assembly head and the fuel assembly foot with bolts that are formed on both ends by closure caps FRa, FRb. As can be seen in FIG. 4, all of the fuel rods are fixed in helical springs 95 which are biased for pressure. These springs are seated on the bolt on the lower surface or underside of the fuel assembly head and are supported on that lower surface and on the applicable fuel rod FR. While the fuel assembly head and foot are formed of stainless steel, the elongated fuel assembly case WC is manufactured from a zirconium alloy. The fuel assembly case WC is open on both ends and is fitted at one end over the cover plate 6 and at the other end over the fuel assembly foot FT. The fuel assembly case accordingly engages the fuel assembly head and foot laterally. On the non-illustrated upper end, the fuel assembly case has sheet-metal strips pointing inward at the corners, which are firmly screwed to stay bolts 97 on the top of the cover plate 6 in the head. The coolant tube WR having the rectangular or square cross section is likewise formed of a zirconium alloy. The sides of the cross section of the coolant tube WR are parallel to the respective adjacent side of the cross section of the fuel assembly case. The ribs of the spacers intersect one another at right angles and are each likewise parallel to two mutually parallel sides of the cross section of the coolant tube WR. All of the sides of the coolant tube cross section are spaced apart from the respective adjacent side of the cross section of the fuel assembly case WC by the same distance. The coolant tube is closed on the upper end with a first end piece WRb and on the lower end with a second end piece WRa. Both end pieces are likewise made of a zirconium alloy and are firmly welded to the coolant tube. The upper end piece WRb has two flow openings 5b for the coolant extending longitudinally of the coolant tube, which are best seen in FIG. 4, while the second end piece on the lower end is provided with a flow opening for the coolant which extends longitudinally of the coolant tube but cannot be seen in FIG. 2. The first end piece WRb on the upper end of the coolant tube has two stay bolts 120 and 121, which have different outside diameters and are each provided with a thread. The threads engage the cover plate 6 and the handle 9 on the outside of the fuel assembly head along a diagonal of the fuel assembly head. The handle 9 has a central part 9a to be screwed to the coolant tube and two lateral cantilever arms 9b, 9c, which are supported on the top of the fuel assembly head and are firmly screwed to two diagonally opposed stay bolts 97 between these stay bolts and the aforementioned sheet-metal strips in the corner of the fuel assembly case WC. Each of the handle and the fuel assembly head are firmly screwed onto the threaded bolts 120 and 121 on the top of the fuel assembly head with a union nut 122, until they meet shoulders 123 and 124 on the stay bolts 120 and 121, as shown in FIG. 5. Due to the two stay bolts, the head part is firmly screwed to the coolant tube in such a manner as to be secure against rotation. Since the stay bolts 120 and 121 have different outside diameters, it is assured that the fuel assembly head will always be firmly screwed to the water tube WR in the same angular position with respect to the longitudinal axis of the water tube. As is seen in FIG. 8, the second end piece WRa on the lower end of the coolant tube WR, has a threaded bolt 130 on the top of the fuel assembly foot, which reaches through a leadthrough of the gridlike bottom plate 1, where it is firmly screwed with a nut until it meets. In a boiling water nuclear reactor, the nuclear reactor fuel assembly of FIG. 2 is vertically disposed, and a coolant that is formed of a two-phase mixture of water and steam flows through it in the fuel assembly case. Water enters the fuel assembly case 5 through the gridlike grate of the fuel assembly foot part having the bottom plate 1, into the fuel assembly case 5, and wet steam exits from the fuel assembly case through a gridlike grate of the fuel assembly head having the cover plate 6. Only water in the liquid phase is located in the coolant tube and effects increased reactivity in the reactor core of the boiling water reactor. In a further feature of the nuclear reactor fuel assembly, as is shown in FIGS. 4 and 6, the lengths of the stay bolts 120 and 121 protruding past the shoulders 123 and 124 on the upper end piece WRb of the coolant tube, are selected to be so large that the fuel assembly head and handle are not retained in contact with the shoulders 123, 124, but instead the fuel assembly head is displaceable counter to the spring force of the compression springs 95 seated on the fuel rods with respect to this end piece WRb. The compression springs 95, which are constructed as helical springs, have one end supported on the cover plate 6 of the fuel assembly head and another end supported on the fuel rods and therefore on the bottom plate 1 of the fuel assembly foot. The compression springs can thus resiliently intercept any displacement of the handle and of the fuel assembly head, for example from forces of gravity in the direction of the shoulders 123 and 124. The shoulders 123 and 124 are advantageously located between the fuel assembly head and the engagement point of the compression springs 95 on the fuel rods FR, so that these compression springs are not subject to blocking, if the fuel assembly head strikes the shoulders 123 and 124 of the stay bolts 120 and 121 acting as stop bodies. The union nuts 122 on the stay bolts 120 and 121 that are located on the upper surface of the fuel assembly head are also stop bodies on the upper end piece. These union nuts define an initial position of the fuel assembly head or cover plate 6. They are each guided in a respective sheath 131 located on the handle 9. The union nuts 122 each have a tang 133 acting as a locking device on the bearing surfaces thereof that are disposed on the handle 9. As is seen in FIG. 7, the tangs each lock into place in an annular groove 134 serving as a counterpart locking device, on the bearing surface of the handle 9 on the union nut 122. As a result, the locking of the union nuts 122 to the handle 9 can be released only if a weight, for instance, acts upon the handle (or the fuel assembly head) in the direction of the fuel assembly foot, and a separating motion between the union nuts 122 and the handle takes place in the longitudinal direction of the stay bolt and counter to the spring force of the compression springs 95. Advantageously, the stay bolts 120 and 121 are firmly screwed to the upper end piece of the coolant tube WR, so that they can be replaced without further action being required, for instance if their threads on the outside of the fuel assembly head 2 are damaged. Non-illustrated compression springs, which may be helical springs, for instance, and are supported on the fuel assembly head and on the shoulders 120 and 121, may also be mounted on the stay bolts 120 and 121. As FIG. 8 shows, the threaded bolt 130 of the end piece WRa on the lower end of the coolant tube may reach through the bottom plate 1 of the fuel assembly foot part and may be displaceable counter to the spring force of a compression spring 96, that is constructed as a helical spring, with respect to the fuel assembly foot. This compression spring 96 is mounted on the threaded bolt 130 of the lower end piece WRa and has one end supported on the end piece and therefore on the fuel assembly head and another end supported on the fuel assembly foot. The fuel assembly head and the handle 9 can then be firmly screwed until they meet the respective shoulders 123 and 124 of the stay bolts 120 and 121. The compression spring 96, which is subject to blocking and is disposed on the inside of the fuel assembly foot, serves on one end as a stop body for the end piece on the fuel assembly foot, and on the other end, a nut 135 which is seated on the stay bolt 130 and is screwed onto the fuel assembly foot from below serves as a stop body and thus defines an initial position for the fuel assembly foot. As FIG. 8 shows, the fuel assembly case WC that is firmly screwed to the fuel assembly head may be displaced on a jacket surface 136 of the fuel assembly foot, along with the fuel assembly head, in the longitudinal direction of the fuel assembly case. In the cover plate 6 shown in FIG. 9, the positions of the fuel rods can be shown as center points of meshes in a grid with square meshes. A central region of 3.times.3 meshes is intended to receive a square coolant tube ("water tube") and is covered by a connecting part 8 for a load-bearing or supporting connection between the cover plate and the water tube. Since it is advantageous for only some of the fuel rods to extend practically over the entire length between the bottom plate and the cover plate, while the other rods are shorter and carry an upper closure cap on the upper end, are already located in the vicinity of a spacer and are spaced apart from the cover plate, the water outlet in the cover plate is formed by outlet openings 7 that are located between the positions of long fuel rods, and by enlarged outlet openings 63 which are disposed in the projection or imaginary extension of the shorter fuel rods. Since screw connections between the cover plate and the fuel rods are to be avoided, the upper closure caps FRb of the longer fuel rods end in the form of smooth, unthreaded bolts, which are only loosely disposed in sleeves 61 at the corresponding positions on the cover plate. Accordingly, the cover plate is not strained by the weight of the fuel rods. Instead, it merely prevents an axial displacement of the fuel rods exceeding a predetermined play. The sleeves 61 that are disposed at the appropriate receiving positions of long fuel rods, are joined together by ribs 62. The cover plate can therefore be made so thin and permeable that the coolant can pass through it virtually unhindered. By suitable dimensioning of the inlet openings 3 in the bottom plate, a uniform flow through the fuel assembly can be achieved, and the pressure loss at the cover plate can be negligible as compared to the pressure loss of the coolant as it flows through the bottom plate and the fuel assembly. At least most of the enlarged outlet openings, that is most positions of the shorter fuel rods, are advantageously each located on the diagonal of the case cross section. Fuel rods that are adjacent to the case wall preferably belong to the group of the long fuel rods. It is particularly with a fuel assembly in which the support ribs of the spacers form 11 rows and 11 columns, that the shorter fuel rods are each disposed in the third row or the third column (counting from the case wall inward), as can be seen in FIG. 9. The cover plate is supported on the case wall and on its rounded corners by lateral bearing lugs 65 and reinforcements 66 of corner sleeves. The handle 9 may have markings 900, 901, which serve to identify the individual fuel assembly and to define right and left, in particular if the fuel fillings in the various fuel rods are different for individual fuel assemblies and their parts. A load-bearing or supporting connection between the handle 9 and the cover plate 6 is advantageously achieved if these two parts are constructed integrally, for instance as a cast workpiece. The head HD formed by the handle and the cover plate need merely be screwed through the connecting part 8 to the upper end piece WRb of the water tube, in order to achieve a load-bearing or supporting mechanical connection with the water tube. A part in the form of a collar 67 shown in FIG. 10 is formed onto the handle, and the upper end piece WRb of the water tube is constructed as a socket pin 92 that is passed through the cover plate and welded to the water tube. This socket pin carries a male thread 93 on its upper end that is passed through the collar, and a union nut 94 is screwed onto this thread from above. The cover plate is supported against the bottom plate by the helical spring 95 that is subject to compression and that advantageously is disposed on the upper end piece or pin 92 between the water tube and the cover plate. If the cover plate is therefore held down counter to the compressive force of the helical spring 95, then the union nut 94 can be screwed onto the male thread 93 of the socket pin 92 up to a desired end position. When the helical spring 95 is relieved, they then form the stop for the collar 67 and the cover plate 6. A securing cap which is also disposed between this collar 67 and the union nut 94, has an upper end that fits partway around the union nut 94 and secures it against falling out. If the helical spring 95 is relieved in the end position, then corresponding profiles of the collar 67 and therefore of the nut 94 mesh with one another and prevent the union nut 94 from being able to rotate relative to the socket pin 92. By using the fuel rod FR1 as an example, it is seen that its upper closure cap FRb is only loosely guided with a certain lateral play in the corresponding sleeve 61. In the illustrated embodiment of these closure caps, the fuel rods (together with the water tube) can also be moved longitudinally counter to the force of the helical spring 95, in order to compensate for an expansion of material such as can occur from radiation and heating during reactor operation. The union nut 94 then serves as a stop body on the upper end piece WRb of the coolant tube and fixes the maximal elongation of the spring 95 and therefore the maximum spacing between the fuel assembly head with the cover plate 6 and the foot part with the bottom part 1. In the longitudinal section of FIG. 10, a fuel rod FR2 is seen directly next to the long fuel rod FR1. The fuel rod FR2 is disposed in a row behind the fuel rod FR1, because the position located directly next to the rod FR1 is occupied by a short rod, that is not visible in FIG. 10. The upper closure caps FRb do not carry any screw thread. This is particularly true for the lower closure caps FRa as well, which are visible in FIG. 11 and which stand unscrewed on the bottom plate 1, as is shown in FIG. 11. This bottom plate is joined rigidly and in a load-bearing or supporting manner to the lower end piece WRa of the water tube, so that the bottom plate and the weight of the fuel rods standing on it is supported practically completely by the end piece of the water tube and is transmitted to the cover plate and the handle 9 through the releasable and force-locking connection of the upper water tube end piece WCb. The lower end piece WRa, with an end pin that is constructed as a tube piece 110, engages the bottom plate 1 and the foot part FT of the fuel assembly. It is screwed there and secured against rotation by a securing bolt 106, which engages a recess of the end piece WRa. The lower closure cap FRa of a long fuel rod FR1 can stand on the bottom plate or it can be loosely guided in appropriate receiving openings. At least for the short fuel rods, a plug connection is provided, for example a base 310 that is constructed as a bayonet mount, having an inside which is engaged by an appropriate adaptor on the lower closure cap of the short fuel rod and which can be locked by a quarter turn, like a plug connection. With the exception of the screw connection between the water tube and the bottom plate, there are accordingly no further screw connections on the foot part. The foot part itself includes the transition piece 2, which has a lower end with star-shaped brackets FTa and an opening 3a and forms a flow channel that is covered on its upper end by the bottom plate 1 with the water inlets 3, through which the coolant enters the interior of the fuel assembly. The lower edge WCa is supported and sealed off from the lateral upper edge of the foot part FT by a sealing spring 4. In the left-hand part of FIG. 11, which shows a side view of the foot part, a lower long side 402 of the sealing spring 4 can be seen. The lower long side 402 is feathered by slits 405, while another long side 401 is bent around an edge between the bottom plate and the transition piece. Since this sealing spring largely prevents an uncontrolled flow of coolant between the foot part and the fuel assembly case, the coolant stream can be split in a defined manner into partial streams flowing inside and outside the case WC, by means of suitable dimensioning of the inlet openings 3 and lateral outlet channels 211. In the present construction, the fuel assemblies are held in a skeleton having a load-bearing or supporting spine which is the water tube. The fuel rods are not part of the skeleton, and the bottom plate does not include any threaded bores for receiving the fuel assembly closure caps. The bottom plates there need merely form suitable stops, on which the closure caps are seated, but can otherwise be constructed freely. In particular, the inlet openings 3 may, for instance, be constructed as narrow slits or in some other way, so that they are not permeable or passable to relatively large foreign bodies. In that event, the bottom plate acts as a sieve at which broken-off pieces or other foreign bodies of certain dimensions are retained, so that they cannot travel into the interior of the fuel assembly and impede the coolant stream there. The water tube WR in FIG. 11 also has stop bodies 90, 91, each of which defines the axial position of one spacer SP. In order to mount these spacers, one stop body is secured to the water tube at a time, for instance by spot welding, beginning at one end of the water tube. Next, from the other, free end, the appropriate spacer is shoved up to the level of the already secured stop body and is retained there by then securing the other stop body to the water tube on the opposite side of the spacer. In this way, all of the spacers can be successively secured to the water tube, as long as at least the upper or lower end of the water tube is still accessible. Before the load-bearing or supporting skeleton is mounted in final form by screw connections of the water tube and the head part, the fuel rods are inserted into the meshes of the various spacers. The final installation of the load-bearing or supporting skeleton takes place then, by screwing the head part onto the water tube, with the fuel rods being introduced into their appropriate receiving positions on the cover plate from below. Then, the fuel assembly case need merely be slipped over this, the rods and the skeleton. As a result, the installation and dismantling of the fuel assembly and replacement of the fuel rods are reduced to a few simple manipulations. In particular, the fuel rods need not be screwed into the bottom plate and unscrewed from it. In FIG. 12, the bottom plate 1 is shown with the closure caps FRa of the long fuel rods standing on them and with the base 310 secured to the bottom plate. The profile of the base is engaged by a profiled extension of the lower closure cap of a shorter fuel rod, serving as an adaptor 305. The base 310 is retained in a bore 308 in the bottom plate. The shorter fuel rods also preferably extend over at least the lower third and, for instance, the lower half of the fuel assembly, so that all of the fuel rods extend through the meshes of the lower spacers. In FIG. 12, one of the spacers SP is shown in the upper part of the fuel assembly. The upper closure cap of a shorter fuel rod ends into its vicinity, while the longer fuel rod extend extends as far as the fuel assembly head. The fuel assembly case preferably has a square cross section and the thickness of its side walls is reduced, at least in some axial regions of the fuel assembly, as is shown in FIG. 13. It may be composed of a plurality of parts, and the side walls have weld seams 30, 31, which serve partly as reinforcements and partly to connect profiled parts. FIG. 14 shows the uppermost spacer, which is located under the cover plate of such a fuel assembly having shorter and longer fuel rods. The support ribs for supporting the fuel rods are constructed as annular sheaths 501, 502, 503, which are secured to one another and carry support knobs 504 and support springs 505 for supporting the fuel rods. Outer ribs 507 are supported by peripheral knobs 508 on the fuel assembly case and carry lugs 509 that face into the interior of the fuel assembly. In this way, a fuel assembly that is very stable mechanically is created. It is assembled from only a few parts in a simple manner and can be easily inspected and dismantled.