Improved boiling water reactor fuel assembly

Abstract: A fuel assembly for boiling water reactors includes, in addition to the usual outer flow channel which surrounds the fuel rods, an inner flow channel (6) having rectilinear sides disposed within the fuel assembly, extending vertically therethrough, and arranged to receive cooling water at or below the bottom of the assembly. The channel is of such size as to span several fuel rods in each direction. A closure (8) is provided in the channel within the critical heat flux zone of the assembly. At least one inner tube (10) extends through the closure. The channel is closed at its upper end (12), and the inner tube (10) is open at its upper end (14), which is located near the upper end of the channel. Egress openings (16) are provided in the walls of the channel just above the closure. These openings are so positioned relative to the fuel tubes that jets of water emitted from them do not impinge directly on the fuel tubes, but rather enter the flow spaces between them. Other egress openings are provided in the walls of the channel below the closure.

Description: The present invention relates to a fuel assembly for a boiling water reactor as defined in claim 1. European patent application EP-A-240 894 discloses a fuel assembly for boiling water nuclear reactors (BWR). It is characterized by a "water rod," i.e. a tube extending vertically through the fuel assembly parallel to the fuel rods. It is open at its lower end to receive cooling and moderating water, but is closed at its upper end. A plurality of holes are formed in the tube to permit egress of water. An inner tube, closed at its upper end and open at its lower end, is arranged concentrically in the water tube. This inner tube is provided with several holes near its upper end. This upper end is closed with a reduced portion of an end plug which also closes the upper end of the water tube. The annulus formed by the inner tube and the water tube is closed at the lower end of the former. Openings are provided in the water tube above its juncture with the inner tube. These holes are located in the critical heat flux zone of the assembly. There are also openings below this juncture, some of which are located at the lower end of the critical heat flux zone. In this critical heat flux zone dried vapor exists, and dissipating of heat from the fuel rods may become more difficult. There may also be other openings farther down the tube. In operation, water enters the lower end of the water tube and flows upwardly through it. This adds moderation to the assembly and improves its performance, and introduction of water into and just below the critical heat flux zone improves the cooling and permits operation of the assembly at higher power. The advantages of a flow of non-boiling water through portions of the assembly has been recognized for some time as desirable. The object of this invention is to obtain these advantages as well as to prevent damming up of water and gases in the inner tube at its upper end. SUMMARY OF THE INVENTION According to this invention, an inner flow channel having rectilinear sides is disposed within the fuel assembly, extending vertically therethrough and arranged to receive cooling water at or below the bottom of the assembly. The inner flow channel is of such a size as to span several fuel rods in each direction. A closure is provided in the channel within the critical heat flux zone of the assembly. At least one inner tube extends through the closure. The inner flow channel is closed at its upper end, and the inner tube is open at its upper end, which is located near the upper end of the channel. Egress openings are provided in the walls of the channel just above the closure. These openings are so positioned relative to the fuel tubes that the jets of water emitted from them do not impinge directly on the fuel tubes, but rather enter the flow spaces between them. In one embodiment, the inner flow channel is a flat sided, box-like structure, positioned centrally of the assembly. In another, it is of cruciform shape. Still other arrangements are possible, such as narrow rectangular boxes extending inwardly from the walls of the outer flow channel which forms the boundary of the assembly. BRIEF DESCRIPTION OF THE DRAWING In the drawing, Figure 1 is a cross section through the upper portion of the assembly taken on the line 1-1 of Figure 2. Figure 2 is a vertical section through the assembly taken on the line 2-2 of Figure 1. Figure 3 is a horizontal section through the upper portion of another embodiment of the invention taken on the line 3-3 of Figure 4. Figure 4 is a vertical section of the embodiment of Figure 3 taken on the line 4-4 of Figure 3. DETAILED DESCRIPTION In describing my invention, reference will be made to European patent application EP-A-0 240 894. A fuel assembly for a boiling water reactor of the type to which this invention applies is shown in Figure 1 of that European patent application. Like the assembly in that figure, the embodiment of this invention illustrated in Figures 1 and 2 includes an outer flow channel 2, and a plurality of parallel vertical fuel rods 4. Within this outer flow channel is an inner flow channel 6 which, as best shown in Figure 1, is cruciform in section, with its ends at approximately the mid points of the sides of outer flow channel 2. Like the water tubes of the above mentioned European patent application inner flow channel 6 will either be provided with openings for inlet of water slightly above the lower tieplate of the assembly, or it may extend below that tieplate, as shown in Figures 2 and 4 respectively of that European patent application. In the upper portion of the inner flow channel 6 is a closure 8, through which extends an inner tube 10. The upper end of the inner flow channel 6 is closed at 12 while the upper end of the inner tube 10 is open as shown at 14. The inner flow channel 6 is provided with intermediate egress openings 16, positioned below the closure 8, near the lower end of the critical heat flux zone of the assembly. It is also provided with upper egress openings 18, positioned above the closure 8 within the critical heat flux zone. The intermediate openings 16 will normally be positioned in the range of about 50 percent to 65 percent of the height of the assembly measured from the bottom, and the upper openings 18 will be distributed in the range of about 65 percent to 75 percent of the height. The two sets of openings may be spaced a distance apart, e.g. 10 percent of the height of the assembly, although openings uniformly distributed along the upper half of the assembly could also yield satisfactory results. In operation, cooling water will enter the inner flow channel 6, at or near the bottom of the assembly, and flow upwardly therethrough. The greater proportion of the water, e.g. about two-thirds, will exit through openings 16. The remainder, e.g. one-third, will enter the inner tube 10 and flow upwardly through it, downwardly within inner flow channel 6, and out through openings 18, where it will enter the critical heat flux zone. The openings 16 and 18 are so sized that the entire inner flow channel 6 is maintained full of water at all times, thereby increasing the moderation of the assembly at the same time the flow outwardly through openings 16 and 18 supplies coolant to the critical heat flux zone, where it is most needed. This has been found to materially increase the power that can be generated by an assembly containing a given amount of fuel. While I have shown only one inner tube 10, it is obvious that there could be others, of the same form, positioned in the arms of cruciform inner flow channel 6. Moreover, the central inner tube could be omitted and all the inner tubes located in the arms. In Figures 3 and 4 I have shown an alternate embodiment of my invention. In this embodiment the assembly is provided with a centrally positioned inner flow channel 17 of square cross section. This inner flow channel is provided with a closure 19, through which extends an inner tube 20. The inner flow channel 17 is closed at its upper end 22 while the inner tube 20 is open at its upper end 24. The inner flow channel 17 is provided with intermediate egress openings 21 and 26, and with upper openings 28, which correspond in their position and function to openings 16 and 18, respectively, in the embodiment of Figures 1 and 2. As shown in Figures 1 and 3, the openings 16, 18, 21, 26, and 28 are so positioned that they do not impinge directly on a fuel rod, but rather direct the jet streams into the spaces between the rods. This avoids inducing vibration in the fuel rods, which may result in fretting at their points of support.

Claims: A fuel assembly for a boiling water reactor comprising a group of vertical, parallel, elongated fuel rods (4) containing a fissionable material, and an outer flow channel (2) surrounding said fuel rods (4), said assembly having a critical heat flux zone in its upper portion; at least one inner flow channel (6; 17) preferably having substantially rectilinear sides spanning at least several fuel tubes (4), said inner flow channel (6; 17) being constructed and arranged to receive cooling water near the lower end of said assembly, said inner flow channel (6; 17) extending upwardly through said assembly parallel to said fuel rods (4) and closed at its upper end (12); a closure (8; 19) in the upper portion of said inner flow channel (6; 17); an inner tube (10; 20) extending and having an opening through said closure (8; 19); and having an open upper end (14; 24) adjacent to the top of said inner flow channel (6; 17); and upper openings (18; 28) in said inner flow channel (6; 17) above said closure (8; 19), and located within said critical heat flux zone whereby cooling water will flow upwardly through said inner flow channel (6; 17), upwardly through said inner tube (10; 20), then downwardly through said inner flow channel (6; 17) and outwardly through said upper openings (18; 28) into the critical heat flux zone. An assembly as defined and claim 1, and further including at least one intermediate egress opening (16; 21; 26) in said inner flow channel (6; 17) adjacent to the lower end of said critical flux zone. An assembly as defined in claim 2, wherein said intermediate egress opening (16; 21; 26) is located in the range of about 50 percent to about 65 percent of the height of the assembly measured from the bottom. An assembly as defined in claim 3, wherein said upper exit opening (18; 28) is located in the range of about 65 percent to about 75 percent of the height of the assembly measured from the bottom, said intermediate (16; 21; 26) and upper opening (18; 28) being spaced a distance apart. An assembly as defined in claim 1, wherein said inner flow channel (6) is cruciform in shape with its arms extending from side to side of said fuel assembly, at about the midpoint of the sides thereof. An assembly as defined in claim 5, wherein said inner tube (10) is located at the crossing point of the arms of said cruciform flow channel. An assembly as defined in claim 1, wherein said inner flow channel (17) is a substantially rectangular member centrally positioned in said fuel assembly. An assembly as defined in claim 1 or 2, wherein the openings in the inner flow channel (6; 17) are so positioned relative to the fuel rods (4) that the jets of water emitted from said openings enter the flow spaces between the fuel rods (4).

Inventors: PATTERSON JOHN F, PATTERSON, JOHN F.
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1988
Main classification: G21C3
IPC codes: G21C3