Fuel assembly for a nuclear reactor

Abstract: 1. Fuel assembly for a nuclear reactor with a plurality of parallel fuel rods (1), which are retained by means of spacers (2) arranged along the vertical extension of the fuel rods (1), with guide tubes (3) arranged between the fuel rods (1), the guide tubes (3) being fixed to the spacers (2), and with a top nozzle (4) and a bottom nozzle (5) between which the guide tubes (3) with the associated fuel rods (1) are fixed. According to the invention the guide tubes (3), at least at the spacers (2), are provided with external threads (7) adapted to engage with corresponding threads provided at the spacers (2) to fix these to the guide tubes (3).

Description: The invention relates to a fuel assembly for a nuclear reactor, preferably of the PWR-type, according to the precharacterising part of claim 1, 2 or 3. A fuel assembly of a known design comprises a plurality of fuel rods and guide tubes, the latter either accommodating control rods or serving as water tubes in certain special nuclear reactors. The fuel rods and guide tubes are maintained in predetermined mutual positions by means of spacer grids, hereinafter simply called spacers, which are positioned in a number of locations along the vertical extension of the fuel assembly. Top and bottom nozzles are arranged at the ends of the fuel assembly and are fixed to the guide tubes which extend somewhat above and below the ends of the fuel rods. In fuel assemblies of this type, the spacers are prevented, in operation, from moving along the guide tubes by means of a weld to the guide tube or a bulge in the guide tube at the respective spacer. A drawback with such an embodiment is that it is difficult to detach a guide tube from the fuel assembly for repair. The invention aims at developing a fuel assembly of the above-mentioned kind where the guide tubes and the spacers are easily detachable from one another. To achieve this aim the invention suggests a fuel assembly according to the introductory part of claim 1, which is characterized by either of the characterizing parts of the three independent claims 1 to 3. Further developments of the invention are characterized by the features of the dependent claims 4 to 9. According to the invention, the guide tubes, at least at the spacers, are provided with external threads in order to fix the spacers to the guide tubes. This enables the guide tubes to be threaded into and out of the spacers. This is further facilitated if the spacers are provides with threads, which are uniformly oriented in relation to the threads of the guide tubes, in the guide tube positions. By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a fuel assembly in side elevation, Figure 2 a cross section of the fuel assembly in Figure 1 at the location of a spacer, Figures 3-6 different ways of attaching the spacers to the threaded guide tubes, Figures 7-9 different embodiments of a spacer suitable to be fixed in the threads of the guide tubes, and Figures 10 and 11 a guide tube with longitudinal recesses or slots in the threads. In Figure 1, numeral 1 designates fuel rods which are retained into a fuel rod bundle by spacers 2. The guide tubes 3 for control rods or water are also fixed in the spacers. The ends of the guide tubes 3 extend somewhat beyond the ends of the fuel rods and are fixed to the top nozzle 4 and the bottom nozzle 5. A known attachment of a guide tube 3 to the top nozzle is shown encircled at 6. Figure 2 shows a cross section of the fuel assembly in Figure 1 at the location of a spacer 2 and with the guide tubes 3. Figures 3-5 show different types of possible attachments between spacer 2 and guide tube 3. In Figure 3 the guide tube 3 is threaded along its entire length with a slightly rising thread 7 with a depth of approximately 0,5 mm. The remaining part of the wall of the guide tube 3 has approximately the same thickness. In this embodiment the spacers 2 are provides with external sleeves 8, fixed to the spacers by welding and provided at their inside with threads matching the threads of the guide tubes 3. The outside diameter of the guide tube 3 is the same along their entire length, while the inside diameter of the guide tube 3 is somewhat reduced at the lower part thereof. The fixing to the top and bottom nozzles 4, 5 is performed by means of the threaded-on spacing sleeves 9, 10 and fixing sleeves 11, 12, respectively. The fixing sleeve 11 is completely open to allow the passage of a control rod (not shown), whereas the fixing sleeve 12 is only provided with a small drainage hole 13. Means for securing guide tubes and sleeves against rotation may be provided but are not shown. When assembling the fuel rod bundle, the spacers 2 are arranged in a fixture whereupon the guide tubes 3 are threaded into the spacers 2 from above. It is then important that the threads 14 of the spacers 2 for the different guide tubes 3 are arranged with uniform orientation to enable the threads 7 of the guide tubes 3 to engage uniformly in the corresponding threads 14 of the spacers; otherwise stresses will arise between spacers 2 and guide tube 3 during the threading operation. It would, of course, also be possible to provide the guide tube 3, at the lower part, with a threaded pin which precedes the guide tube 3 and takes up a thread in the sleeves 8 of the spacers 2, whereupon the threads 7 of the guide tube 3 engage with the thread 14 of the sleeve while the guide tube 3 continues to rotate. In this way, the guide tube 3 is automatically threaded to the spacer without any stresses in the bundle arising. When the guide tubes 3 have been threaded into the spacers 2, the spacing sleeves 9, 10 will be threaded onto the guide tubes 3, the fuel rods 1 will be inserted into the spacers 2, and the top and bottom nozzles 4, 5 will be fixed to the guide tubes 3 with the threaded fixing sleeves 11, 12. The spacers 2 are locked to the guide tubes 3 in a satisfactory manner and are unable to slide along these. In spite of this, a guide tube 3 can be easily detached by simply detaching the fixing sleeves 11, 12 and thereafter threading the guide tube 3 out of the bundle. In Figures 3-5 the whole guide tube 3 is threaded. The threads contribute to increased turbulence in the cooling water which flows along the guide tubes, and in this way the cooling of the fuel rods 1 in the fuel assembly is improved. In Figure 4, threads 14 are provided directly in the spacer plate, and the guide tube 3 is threaded directly to the bottom nozzle 5. At the bottom of the guide tube 3, a plug 15 with a drainage hole 13 is welded to the guide tube. At the top, the guide tube 3 is provided with a spacing sleeve 9 and is fixed to the top nozzle 4 by means of a conical fixing sleeve 16. In Figure 5, the spacer 2 has been provided with tabs 17 running in the threads 7 of the guide tube 3. In order to brake the free falling control rods in the event of a sudden reactor shut down, the lower part of the guide tubes 3 is provided with a smaller diameter. In similar manner as in Figure 3, the guide tube 3 is fixed to the bottom nozzle 5 by means of a spacing sleeve 10 and an open fixing sleeve 18. A plug 15 with a drainage hole 3 is inserted into the guide tube 3 at the bottom. At the top the guide tube 3 is fixed to the top nozzle 4 by means of a spacing sleeve 9 and an open fixing sleeve 19. Thus, in a device of this kind, no special thread has to be provided in the spacers 2, which considerably simplifies their manufacturing. Figure 6 shows a guide tube 3 which is threaded only in the area of the spacers 2 and a small portion below these. At the spacers 2, sleeves 8 with an internal thread 14 are attached. Because the thread 7 continues somewhat below the sleeve 8 of the spacer 2, the turbulence of the cooling water is increased and the cooling in the fuel assembly is improved. Figure 7 shows a partial section of a spacer 2 with tabs 17 for a guide tube. The spacer 2 for the guide tube 3 is unthreaded. Figure 8 shows a cell in a spacer 2. Directly in the plate wall of the cell, slots serving as threads 14 have been provided. Figure 9 shows how a guide tube 3 has been threaded into such a spacer cell. Figure 10 shows a section of a guide tube 3 with the threads 7. In the threads 7 a number of shallow slots 21 have been milled. Figure 11 shows the guide tube 3 in Figure 10 seen from the side. The slots may extend straight along the vertical extension of the guide tube 3 or they may extend in a spiral around the guide tube in order to further improve the agitation in the cooling water of the fuel assembly.

Claims: Fuel assembly for a nuclear reactor, comprising a plurality of parallel fuel rods (1), which are retained by means of spacers (2) arranged at a number of locations along the vertical extension of the fuel rods (1), guide tubes (3) arranged between the fuel rods (1), the guide tubes (3) being fixed to the spacers (2), and a top nozzle (4) and a bottom nozzle (5) between which the guide tubes (3) with the associated fuel rods (1) are fixed, characterized in that the guide tubes (3), at least at the locations of the spacers (2), are provided with external threads (7) and that the spacers in the corresponding positions are either provided with sleeves (8) fixed to the spacers, which are provided on their inside with matching threads to accomodate the threaded guide tubes, or are provided with such threads directly in the spacer plate. Fuel assembly for a nuclear reactor, comprising a plurality of parallel fuel rods (1), which are retained by means of spacers (2) arranged at a number of locations along the vertical extension of the fuel rods (1), guide tubes (3) arranged between the fuel rods (1), the guide tubes (3) being fixed to the spacers (2), and a top nozzle (4) and a bottom nozzle (5) between which the guide tubes (3) with the associated fuel rods (1) are fixed, characterized in that the guide tubes (3), at least at the locations of the spacers (2), are provided with external threads (7) and that the spacers are provided with tabs (17) in the guide tube positions, which tabs (17) are arranged to engage with the threads (7) of the guide tubes (3). Fuel assembly for a nuclear reactor, comprising a plurality of parallel fuel rods (1), which are retained by means of spacers (2) arranged at a number of locations along the vertical extension of the fuel rods (1), guide tubes (3) arranged between the fuel rods (1), the guide tubes (3) being fixed to the spacers (2), and a top nozzle (4) and a bottom nozzle (5) between which the guide tubes (3) with the associated fuel rods (1) are fixed, characterized in that the guide tubes (3), at least at the locations of the spacers (2), are provided with external threads (7) and that the walls of the spacers cell in the positions of the guide tubes are provided with slots penetrating the cell walls and adapted to accomadate portions the threads of the guide tubes. Fuel assembly according to any of the preceding claims, characterized in that threads (14) of the spacers are uniformly oriented in relation to the threads (7) of the guide tubes (3). Fuel assembly according to any of the preceding claims, characterized in that the guide tubes are provided at their ends with threads for fixing the guide tubes (3) to the top and bottom nozzles (4,5). Fuel assembly according to any of the preceding claims, characterized in that the threads (7) of the guide tubes (3) extend a certain distance below the spacer positions (2) in order to improve the turbulence in the cooling water flowing around the guide tubes (3). Fuel assembly according to any of claims 1 to 5, characterized in that threads (7) are arranged along the entire length of the guide tubes (3). Fuel assembly according to any of the preceding claims, characterized in that slots (21) extending axially along the guide tube (3) are provided in the threads. Fuel assembly according to claim 8, characterized in that the slots (21) are arranged in spirals around the guide tube (3).

Inventors: NYLUND OLOV, NYLUND, OLOV
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1991
Main classification: G21C3
IPC codes: G21C3