Patent Number: 053274725
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a boiling water reactor having a pressure vessel 231, in which a reactor core is disposed, that has vertically disposed nuclear reactor fuel assemblies 232, as shown in FIGS. 2-7, between which absorber assemblies 242 are disposed that are driven into a space between the fuel assemblies or retracted from the space in order to control the reactor. An outlet 233 for water vapor at the top of the pressure vessel 231 is connected to a steam turbine 234, which drives an electric generator 235. A condenser 236 associated with the steam turbine 234 is connected laterally through a feedwater pump 237 and a feedwater inlet 238 to the top of the pressure vessel 231. This feedwater pump 237 pumps condensed steam from the steam turbine 234 back into the pressure vessel 231 as feedwater. In older boiling water reactors, the pressure vessel 231 also has a coolant outlet 239 below the feedwater inlet 238, laterally of the reactor core having the nuclear fuel assemblies 232. The pressure vessel 231 also has a coolant inlet 241 below the reactor core. A coolant pump 240 connected to the coolant outlet 239 and to the coolant inlet 241 pumps water continuously out of the pressure vessel 231, through the coolant outlet 239 and back into the pressure vessel 231 through the coolant inlet 241, thus assuring a continuous coolant flow through the reactor core and thus longitudinally through the nuclear fuel assemblies 232, beginning at the bottom of the pressure vessel 231. In modern reactors, this pump 240 is disposed in the interior of the pressure vessel. One of the fuel assemblies 232 of FIG. 2 for a boiling water reactor of FIG. 1, has a fuel assembly top, which is not identified by a reference numeral, with a handle 2 on top of a square grid plate. Two or four stay bolts, that are likewise not shown in the drawing, are located on top of this square grid plate. An elongated sheet-metal fuel assembly case, box or jacket 3 made of a zirconium alloy and associated with the fuel assembly rests on these stay bolts with two or four non-illustrated crosswise strips, that are also made of sheet metal that is made of a zirconium alloy, which are mounted on the inside of two or four corners of the upper end the fuel assembly case 3. Each crosswise strip is screwed to the applicable stay bolt. The fuel assembly case 3 is square in cross section and open on both ends. The grid plate itself is provided with a number of flow openings in the longitudinal direction of the fuel assembly 232, through which the coolant flows in the reactor core of the boiling water reactor. This grid plate is at right angles to the longitudinal direction of the fuel assembly 232. The side walls of the fuel assembly case 3 close off the fuel assembly at the sides. The fuel assembly of FIG. 2 is also provided with a base 4, which also has a hidden and non-illustrated square grid plate. This square grid plate also has a number of coolant flow openings in the core of the boiling water reactor, extending longitudinally of the fuel assembly 232. The lower surface or underside of the grid plate of the fuel assembly base 4 is provided with a fitting device 5 that is open toward the grid plate and is inserted into a fitting opening on a so-called lower core grid plate located in the core of the boiling water reactor. On the upper end, the fuel assembly 232 is fixed in a mesh, opening or space of a so-called upper core grid. The fuel assembly 232 of FIG. 2 also has a row of nuclear fuel-filled fuel rods, which are constructed as retaining rods 9, for the top part and bottom part 4 of the fuel assembly 232. These retaining rods 9 are screwed into the grid plate of the fuel assembly base 4 and reach through the grid plate of the fuel assembly top, where they are screwed to the grid plate with a nut located on the top of the grid plate. Other fuel rods 10 filled with nuclear fuel are loosely inserted by their ends into openings in the grid plates of the top and the base 4 of the fuel assembly. Holding-down springs which are constructed as helical springs are mounted on their upper end. These springs are compression springs and each is supported at one end on the fuel rod 10 and at the other end on the lower surface or underside of the grid plate of the fuel assembly top. Finally, the fuel assembly 232 of FIG. 2 has a plurality of square gridlike spacers between the fuel assembly top and the fuel assembly base. The spacers are located in a cross section of the fuel assembly case 3 and are aligned with the square grid plates of the top part and the base part 4. One gridlike spacer 22 can be seen in FIG. 2. The other gridlike spacers are constructed identically to the spacer 22 but are concealed both by the side walls of the fuel assembly case 3 and by the grid plate of the top part and the grid plate of the base part 4 and therefore cannot be seen. The plan view of FIG. 3 is a portion of FIG. 2 showing the coolant outflow end of a gridlike spacer 22, which is made of a nickel-chromium-iron alloy. This coolant outflow side faces toward the top of the fuel assembly 232. The gridlike spacer 22 has two groups of flat, planar ribs 23 and 24, which are located in a cross section of the fuel assembly case 3 and thus on a cross section of the fuel assembly 232. The ribs 23 ("crosswise ribs") of one group and the ribs 24 ("lengthwise ribs") of the other group penetrate one another at right angles. The spacing between two ribs 23 of one group is equal to the spacing between two ribs 24 of the other group. Correspondingly, the ribs 23 and 24 form square meshes 25 of equal area, which are located at points where equidistant lengthwise rows that are parallel to one another ("lines Z") intersect with equidistant crosswise rows ("columns S") that are parallel to one another. One retaining rod 9 or one fuel rod 10 is guided through each square mesh 25. Non-illustrated springs and knobs are located inside the meshes 25 at the ribs 23 and 24, assuring a positive holding connection of the retaining rod 9 or the fuel rod 10 to the gridlike spacers. At an intersection 26 of one rib 23 and one rib 24, the rib 23 has a three-dimensionally curved vane or blade 27 or 28 on each respective side of the rib 24. Each vane tapers in the coolant flow direction. At an adjacent intersection 29, the rib 24 has one of these vanes 27 or 28 on each respective side of the rib 23. These vanes 27 and 28 increase gradually outwardly from the edge of the applicable rib facing toward the intersecting rib. The vanes mounted on the ribs 23 have a curvature about a direction parallel to these ribs 23, while the vanes mounted on the ribs 24 have a curvature about a direction parallel to the ribs 24. In order to form this curvature, two of the vanes 27 and 28 located at an intersection of the ribs 23 and 24 are three-dimensionally curved in different directions. The rib 24, which has the two vanes 27 and 28 of the rib 23 on either side of it at the intersection 26, also has the two vanes 27 and 28 at the intersection 27, which are constructed identically to the other vanes 27 and 28. These vanes 27 and 28 at the rib 24 are curved toward different meshes 25 from those toward which the vanes 27 and 28 on the rib 23 at the intersection 26 are curved. The arrow A indicates the direction in which the side view of a rib with the two vanes 27, 28 appears in FIG. 4. The same is true for the rib 23 with respect to the intersection 29 next to the intersection 26. The four retaining rods 9 or fuel rods 10 immediately surrounding one intersection 26 or 29 define flow subchannels or secondary channels 32, 33 having a center in which the intersection 26 or 29 is located. In the flow subchannels at the intersections of the ribs 23 and 24, coolant flowing through the gridlike spacer shown in FIG. 3 at right angles to the plane of the drawing, from below that plane to above it, is made to swirl, as is represented by arrows 30 and 31, around the center of the flow subchannels. Although there is practically no net flow in the horizontal direction between the various flow subchannels, the two swirling flows of adjacent flow subchannels mix together somewhat at their boundary surfaces, which promotes temperature equalization and reinforces the swirl produced in the adjacent flow subchannel. In the upper part of the fuel assembly, the coolant is in the form of a two-phase mixture of water and steam, and vanes attached there assure that the water is spun against the outer surfaces of the retaining rods 9 and the fuel rods 10, thus counteracting dryout. In the spacer of FIG. 5, dashed lines represent the position of the fuel rods 9 and 10 that define flow subchannels 32 and 33. Four vanes 101, 102, 103, 104 and 101', 102', 103', 104' are respectively provided in each of these flow subchannels. The vanes 101 . . . 104 are disposed rotationally symmetrically about the center line of the flow channel 32, or in other words the line of intersection of a lengthwise rib 110 with a crosswise rib 111, where the two ribs are also joined together by spot welds 105. Reference numerals 106 and 107 indicate knobs on the ribs, against which the fuel rods 9 and 10 are pressed by means of opposed springs 108, 109, in order to define the lateral spacing between the fuel rods. On either side of the spot weld 105, the lengthwise rib 110 carries the vanes 101, 103, which are disposed and bent rotationally symmetrically in the direction of an arrow D, while in the adjacent flow subchannel 33, the crosswise rib 112 intersecting the lengthwise rib 110 includes corresponding vanes 102' and 104', but they are disposed and bent rotationally symmetrically in the opposite direction indicated by an arrow D'. The arrows D and D' therefore indicate the direction of rotation of the swirl in the subchannel. The vanes 102, 104 and 101', 103', respectively, which are mounted in addition to those of FIG. 3, are subordinate to these rotational symmetries, which are each applicable to all of the vanes of one flow subchannel. In the gridlike spacer of FIGS. 6a and 6b the meshes 25 for the retaining rods 9 and the fuel rods 10 are likewise located at intersections of parallel lengthwise rows Z and parallel crosswise rows S that are orthogonal to those lines. The lengthwise rows Z are equidistant from one another, as are the crosswise rows S. The spacing between two crosswise rows is also equal to the spacing between two lengthwise rows. The meshes 25 are formed by hollow-cylindrical sheaths 70 of equal height, which have the same inside and outside cross section. Non-illustrated contact springs and contact knobs for a retaining rod 9 or a fuel rod 10 are located in the sheaths 70. The center of the inside cross section of each sheath 70 is disposed at an intersection of one line Z and one column S. One end of all of the sheaths 70 is located in a cross-sectional plane of the fuel assembly case 3, and the other end of all of the sheaths is located in a cross-sectional plane parallel to that cross-section plane. Adjacent sheaths touch one another along a jacket or mantel line, at which they are welded to one another. Each respective group of four sheaths 70, disposed at the intersections of two adjacent lines Z and two adjacent columns S, form a flow subchannel parallel to these sheaths 70, which is located in the center between these four sheaths 70. Two vanes 72 that are disposed opposite each other along a sheath diameter, are formed at an end of these sheaths 70 facing away from the coolant flow. For each sheath 70, the two vanes 72 are rotationally symmetrical with respect to a longitudinal sheath axis in the center of the sheath. The vanes taper in the coolant outflow direction. The vanes are also each curved three-dimensionally inward, forming a bulge, into one respective flow subchannel. The vanes 72 of the sheaths 70 touching one another are also oppositely rotationally symmetrical with respect to the longitudinal axis in the center of the applicable sheath 70. Two vanes 72 protrude into each flow subchannel and are rotationally symmetrical with respect to a central axis of this flow subchannel that is parallel to the sheaths 70. In this way, in one flow subchannel, the vanes 72 are oppositely rotationally symmetrical to the vanes in every other flow subchannel. Adjacent flow subchannels are located alternatingly, so that one is on an intermediate line UZ parallel to the lines Z and the next is on an intermediate column US parallel to the columns S, and so forth. A coolant flowing from below the plane of the drawing in FIG. 6 to above it, orthogonally to the plane of the drawing, is accordingly provided with a swirl, which is symbolized by respective arrows 73 and 74, in the flow subchannels. Each swirl in one subchannel has a reinforcing effect on the swirl in an adjacent subchannel, and water droplets contained in the coolant are spun outward against the outer surface of the retaining rods 9 or fuel rods 10, as in the case of the gridlike spacer of FIG. 3. The more vanes a subchannel has, and the more its cross section decreases, and the more the pressure loss in the vertical flow increases, yet it is still advantageous to provide four vanes 72, 75, 76, 77 in each flow conduit, having a rotationally symmetrical configuration and shape that produces a spacer as shown in FIG. 7. Once again, the arrow A indicates a view toward a sheath 70 shown in FIG. 8. It can also be seen that by providing indentations 78 in the sheaths, knobs are formed, against which the fuel rods 9, 10 are pressed by springs 79 that engage two adjacent sheaths. An outer rib 80 has contact knobs 81 that rest on side walls 82 of the fuel assembly case, it runs along an outer edge of the spacer and it has tabs 83. Like the vanes in the flow subchannels, the tabs 83 are located on the side of the spacer facing away from the flow, they taper in the flow direction, and are inclined relative to the case wall. Reference numeral 84 indicates the wall of a water tube, which in this case occupies the cross section of 3.times.3 grid meshes and is seated in the center of the fuel assembly. An inner rib 85 with corresponding tabs 86 defines the spacer relative to the water tube 84. FIG. 9 shows a cross section of a portion of a corresponding structure, which is disposed between the water tube 84 and the fuel assembly case having the side wall 82 and serves initially only as a support for the vanes in the flow subchannels. It is therefore initially a mixing grid, but by using suitable spring and knob combinations it can be expanded at any time to make a spacer. A controllable absorber element is indicated at reference numeral 90. Such absorber elements are only located outside the fuel assembly case and are therefore protected by the case walls from any possible horizontal flows of liquid/steam mixture. The spacer of FIGS. 10-12 is square and is likewise made of a nickel-chromium-iron alloy. Two flat, planar outer ribs 323 and 324 can be seen, which are disposed on edge and at right angles to one another and form a rounded portion at the corners of the spacer 22. The spacer 22 also has grid meshes 25, which are located like the squares of a checkerboard at positions disposed in dense lines and in columns at right angles to the lines. One non-illustrated nuclear-fuel-containing retaining rod or fuel rod of the fuel assembly 232 reaches through each of the grid meshes 25. The outer ribs 323 and 324 are at right angles to this fuel assembly, and these outer ribs 323 and 324 face flat toward it. Inside the outer ribs 323 and 324 of the spacer 22 are pairs of mutually aligned main sheaths 327 and 328, which have longitudinal axes that are parallel to one another and to the fuel rods in the spacer 22, like the squares of the same color in a checkerboard at the positions of the grid meshes 25, in lines and in columns at right angles to the lines, in each case leaving one intermediate position open between two occupied positions. The main sheaths 327 and 328 of all of the pairs of main sheaths of the spacer 22 have a cross section with a congruent outer contour, which is a regular octagon. On two sides of this octagon, which are parallel to the same outer rib 323 or 324, the main sheaths 327 and 328 are provided with connecting ribs 329 and 330, which extend over a direction that is parallel to the longitudinal axes of the pairs of the main sheath 327 and 328. At the main sheaths 327 and 328, these connecting ribs 329 and 330 are each formed on to the middle of the side of the regular octagon forming the outer contour of the cross section, and their width equals approximately one-third the length of the side of this rectangular octagon. Two connecting ribs 329 and 330 seated on parallel sides of this octagon are each curved in the same direction in the middle. In other words, one connecting rib 329 is curved outward with respect to the main sheath 327 and 328, and one connecting rib 330 is curved inward with respect to the main sheath 327 and 328. In the middle of the sides of the outer contour of their cross section forming a rectangular octagon, the main sheaths 327 and 328 also have respective rigid knobs 331 and 332 on the applicable main sheath wall. The rigid knob 331 faces inward, if the connecting rib 329 faces outward on the side of the outer contour with respect to the main sheath 327 or 328, and the rigid knob 332 faces outward with respect to the main sheaths 327 or 328 if the connecting rib 330 faces inward with respect to this main sheath 327 or 328. In the spacer, pairs of additional or spacer sheaths 333 and 334, which are aligned with one another, are disposed in the diagonal direction between the main sheaths 327 and 328 and have a cross section that is smaller than the cross section of the main sheaths 327 and 328. The spacer sheaths have a square outer contour, with a length on a side that is equal to the length of the side of the outer contour of the cross section of the main sheaths 327 and 328, forming a regular octagon. These spacer sheaths 333 and 334 are located between the respective main sheaths 327 and 328 and are each formed by two respective spacer sheath parts 433 and 434, each of which is formed on the outer edge of one main sheath 327 or 328, on the side of the outer contour between two sides with the connecting ribs 329 and 330 and the knobs 331 and 332, respectively. Each spacer sheath part 433 and 434 is half of one spacer sheath 333 or 334 of two adjacent main sheaths 327 and 328, respectively. These half spacer sheaths 333 are welded to one another at welding points or locations 439 and 440. The main sheaths 327, together with the spacer sheaths 333 formed onto them, form a first partial grid of the spacer 22, and the main sheaths 328, together with the spacer sheaths 334 welded onto them, form a second partial grid, parallel to the first partial grid. Each of the spacer sheaths 333 and 334 are seated on the outside of these partial grids and form additional sheaths in the flow subchannel that is formed in the respective center between four retaining rods 9 or fuel rods 10. These four retaining rods 9 or fuel rods 10 are each located in meshes 26 of the gridlike spacer, in two adjacent lines and two adjacent columns. As is shown only in FIG. 12 for the sake of clarity, one spacer sheath 333 on the coolant outflow side of the gridlike spacer of FIGS. 4 and 5 has two vanes 172 that curve three-dimensionally inward into the flow subchannel and taper toward one another in the coolant outflow direction. These vanes 172 are located on two opposed coolant outflow edges that are parallel to one another and thus at the coolant outflow ends of the main sheath 327. These vanes are rotationally symmetrical to a central axis of the flow subchannel extending through the intersection points of the diagonals of the cross sections of the spacer sheaths 333. Vanes on spacer sheaths that are immediately adjacent to sides of the main sheaths 327, in sublines and subcolumns defined by these sides in the spacer sheaths 333 shown in FIG. 6, carry vanes on the coolant outflow edges that in contrast are rotationally symmetrical to the vanes on the spacer sheath 333 of FIG. 11. Once again, it is advantageous to supplement each of the pairs of vanes shown in one flow subchannel with a further pair of vanes, that are adapted to the rotational direction in the applicable flow subchannel. The disadvantages entailed by the resultant increased flow resistance can be more than compensated for by the advantages of making the liquid/steam mixture turbulent.