Patent Number: 058621962
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a boiling water reactor fuel assembly 1 which comprises an elongated tubular container having rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends so as to form a continuous flow passage that the coolant of the reactor flows through. The fuel assembly 1 comprises a large number of equally elongated tubular fuel rods 3, arranged in parallel in a bundle, in which pellets 4 of a nuclear fuel are arranged. The fuel rods 3 are retained at the top by a top tie plate 5 and at the bottom by a bottom tie plate 6. The fuel rods 3 are kept spaced from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. FIG. 2 shows a pressurized-water reactor fuel assembly 1 which comprises a number of elongated tubular fuel rods 3 and control rod guide tubes 8 arranged in parallel. The fuel rods 3 contain pellets 4 of a nuclear fuel. The control rod guide tubes 8 are retained at the top by a top nozzle 5 and at the bottom by a bottom nozzle 6. The fuel rods 3 are kept spaced from each other by means of spacers 7. FIGS. 3a-b show, in perspective views, a spacer 7 with an orthogonal latticework. FIG. 3 shows a spacer 7 composed of sleeves 9. Each sleeve 9 is intended to position an elongated element extending therethrough, for example a fuel rod 3 or a control rod guide tube 8. FIG. 3b shows a spacer 7 composed of cells 9. Between the fuel rods 3 arranged in the cells 9 and the sleeves 9, respectively, channels 10, referred to as secondary channels, are arranged. FIGS. 3c-d show, in a top view and a side view, respectively, a latticework of sheet-metal strips 9a, 9b. The strips are arranged crosswise and standing on edge, and are surrounded by a frame 9c. FIGS. 4-13 illustrate several alternative embodiments of guide vane trees according to the present invention. FIGS. 4-8 show an example with two, three or four branches arranged at a trunk in the form of a substantially rectangular plate arranged parallel to the substantial coolant flow. FIGS. 9-13 show alternative embodiments of the guide vane tree. FIG. 4a shows a blank for a guide vane tree 11 with a first and a second folding line 12a, b. The guide vane tree also include a center line 13 where the angle of the folding lines 12a, b to the center line 13 is designated .mu..degree. and shown to be 45.degree.. A first tab 14a to the left of and above the first folding line 12a is intended to be folded 90.degree. around the first folding line 12a in a direction towards the viewer. A second tab 14b to the right of and above the second folding line 12b is intended to be folded 90.degree. around the second folding line 12b and in a direction away from the viewer. When the first and second tabs 14a, b are folded down, an appearance of the guide vane tree 11 from the side as shown in FIG. 4b is obtained. FIG. 4c shows a top view of the guide vane tree 11 arranged between four fuel rods 3. The tree 11 is attached to a structure comprised by the spacer 7. The structure consists of sheet-metal strips, arranged crosswise and standing on edge, which form cells through which the fuel rods 3 run. FIG. 5a shows a blank for a guide vane tree 11 with a first, a second and a third folding line 12a, b, c and a center line 13. The blank is provided with a slit 15a arranged between the second 12b and third 12c folding lines and the center line 13. A first tab 14a to the left of and above the first folding line 12a is intended to be folded 90.degree. around the first folding line 12a, in a direction towards the viewer. A second tab 14b to the right of and above the second folding line 12b is intended to be folded 90.degree. around the second folding line 12b, in a direction away from the viewer and 90.degree. around the center line 13, in a direction towards the viewer. A third tab 14c arranged to the right of and above the third folding line 12c is intended to be folded 90.degree. around the third folding line 12c and in a direction away from the viewer. When the first, second and third tabs 14a, b, c are folded to their final positions, an appearance of the guide vane tree 11 from the side as shown in FIG. 5b is obtained. FIG. 5b also shows a fourth and fifth folding line 16a, b on each side of the center line 13. On each outer side of the fourth 16a and fifth 16b folding lines, respectively, a first 17a and a second 17b attachment tab are folded around the fourth 16a and fifth 16b folding lines, respectively, in a direction towards the viewer. The angle for the folding is adapted to the available space. Further, the blank is folded around the center line 13 in a direction away from the viewer so as to form an angle .differential..degree. of 120.degree. in FIG. 5c, between the fourth 16a and fifth 16b folding lines. FIG. 5c shows the guide vane tree 11 in a view from above arranged between four fuel rods 3. The tree 11 is attached to a structure comprised by the spacer 7. The structure consists of sheet-metal strips, arranged crosswise and standing on edge, forming cells through which the fuel rods 3 are running. FIGS. 6a-c show a guide vane tree of a particularly advantageous design. FIG. 6a shows a blank for a guide vane tree 11 with a first, a second, a third, and a fourth folding line 12a-d and a center line 13. The blank includes with a first slit 15a arranged between the second 12b and third 12c folding lines and the center line 13. The blank also includes a second slit 15b arranged between the third 12c and fourth 12d folding lines and the center line 13. A first tab 14a to the left of and above the first folding line 12a is intended to be folded 90.degree. around the first folding line 12a, in a direction towards the viewer. A second tab 14b to the right of and above the second folding line 12b is intended to be folded 90.degree. around the second folding line 12b, in a direction away from the viewer and 90.degree. around the center line 13, in a direction away from the viewer. A third tab 14c is arranged to the right of and above the third folding line 12c and is intended to be folded 90.degree. around the third folding line 12c and in a direction away from the viewer and 90.degree. around the center line 13, in a direction towards the viewer. A fourth tab 14d arranged to the right of and above the fourth folding line 12d is intended to be folded 90.degree. around the fourth folding line 12d, in a direction away from the viewer. When the first, second, third, and fourth tabs 14a-d are folded to their final positions, an appearance of the guide vane tree 11 from the side as shown in FIG. 6b is obtained. FIG. 6c shows the guide vane tree 11 in a view from above arranged between four fuel rods 3 in a structure of sleeve spacers 9 of the type shown in FIG. 3a. FIG. 6b also shows a fifth 16a and a sixth 16b folding line on each side of the center line 13. FIG. 6d shows that, on the outer side of the fifth folding line 16a, a first attachment tab 17a is folded around the fifth folding line 16a in a direction away from the viewer. On the outer side of the sixth folding line 16b, a second attachment tab 17b is folded around the sixth folding line 16b in a direction towards the viewer. FIG. 6d shows the guide vane tree 11 in a view from above arranged between four fuel rods 3. The guide vane tree 11 is attached in a structure comprised by the spacer 7. The structure consists of sheet-metal strips, arranged crosswise and standing on edge, forming cells through which the fuel rods 3 run. The somewhat twisted attachment of the guide vane tree 11 at an angle .beta..degree., in FIG. 6d shown to be 20.degree., gives an optimum cooling of the surrounding fuel rods 3. The location of the guide vane tree 11, rotated through an angle of .beta..degree., may also be used for spacers 7 of the type indicated in FIG. 3a. FIGS. 7a-e show a guide vane tree 11 with four guide vanes 14a-d, as shown in FIG. 6, but where two of the guide vanes 14a, b are placed at the same level. FIG. 7a shows a blank for a guide vane tree 11 with a first, a second, a third, and a fourth folding line 12a-d as well as a center line 13. The blank includes with a first slit 15a arranged between the second 12b and third 12c folding lines and the center line 13. The blank also includes a second slit 15b arranged between the third 12c and fourth 12d folding lines and the center line 13. The first tab 14a to the left of and above the first folding line 12a is intended to be folded 90.degree. around the first folding line 12a, in a direction towards the viewer. The second tab 14b to the right of and above the second folding line 12b is intended to be folded 90.degree. around the second folding line 12b, in a direction away from the viewer and 90.degree. around the center line 13, in a direction away from the viewer. The third tab 14c is arranged to the right of and above the third folding line 12c and intended to be folded 90.degree. around the third folding line 12c and in a direction away from the viewer and 90.degree. around the center line 13, in a direction towards the viewer. The fourth tab 14d arranged to the right of and above the fourth folding line 12d is intended to be folded 90.degree. around the fourth folding line 12d, in a direction away from the viewer. When the first, second, third, and fourth tabs 14a-d are folded to their final positions, an appearance of the guide vane tree 11 from the side as shown in FIG. 7b is obtained. FIG. 7c shows the guide vane tree 11 in a view from above arranged between four fuel rods 3 in a structure of sleeves 9, also shown in FIG. 3a. FIGS. 8a-d show a side view of the attachment of a guide vane tree 11 of the same type as that shown in FIG. 6b. FIG. 8a shows an attachment where the guide vane tree 11 is intended to project outside the upper edge of the spacer 7, this upper edge being arranged at the reference numeral 18. FIG. 8b shows an attachment where the guide vane tree 11 is intended to be arranged fully immersed into the spacer 7. This is a particularly advantageous embodiment when the guide vane tree is fully integrated into the structure. The immersed location gives the tree good protection against external influence during, for example, mounting of fuel rods. FIG. 8c shows a side view of how the guide vane tree 11 may be attached on one side to the spacer 7. This one-sided attachment is also clear from FIG. 8d, showing a view from above. The guide vane tree 11 may, of course, also be attached on three or four sides, although this is not explicitly shown in any figure. FIG. 9a shows a side view of a guide vane tree 11, the trunk of which consists of a sheet-metal strip twisted into a helical spring 19. Four guide vanes 14 are attached to the outwardly facing surfaces of the helical spring 19, at different levels and with an angular pitch of 90.degree.. The guide vanes 14 consist of square plates folded 90.degree. around their respective diagonals and attached to the helical spring 19 by one plate edge 14e. FIG. 9b shows a top view of the guide vane tree 11 shown in FIG. 9a. FIG. 10a shows in a view from the side a guide vane tree 11, the trunk of which consists of a sheet-metal strip twisted into a helical spring 19, as in the embodiment shown in FIG. 9. In the surfaces of the helical spring 19, along three out of four sides, four square tabs are punched out at different levels and with a 90.degree. angular pitch. The non-punched side 20 is parallel to a conceived center line 13 through the helical spring 19. The square tabs are then folded 90.degree. around a line through the non-punched side 20 and 90.degree. around a diagonal through the square tab. FIG. 10b shows a top view of the guide vane tree 11 shown in FIG. 10a. FIG. 11a shows a guide vane tree 11, the trunk of which consists of a solid pin 21 to which are attached guide vanes 14 in the same way as in FIG. 9. FIG. 11b shows a top view of the guide vane tree 11 shown in FIG. 11a. FIG. 12a shows a guide vane tree 11, the trunk of which consists of a tube 22 to which are attached guide vanes 14 in the same way as in FIGS. 9 and 11. FIG. 12b shows a top view of the guide vane tree 11 shown in FIG. 12a. FIG. 13a shows a guide vane tree 11, the trunk of which consists of a tube 22 to which are attached guide vanes 14 in the same way as in FIG. 10. FIG. 13b shows a top view of the guide vane tree 11 shown in FIG. 13a. FIGS. 14a, b show a guide vane tree 11 corresponding to that shown in FIG. 6 but where the guide vanes 14a-d exhibit an arched shape. FIG. 14a shows a side view of the guide vane tree 11 with arched guide vanes 14a-d . FIG. 14b shows a top view of the guide vane tree 11 arranged between four fuel rods 3, each surrounded by a spacer cell 9. FIG. 15 is a sketch showing the principle of how the guide vane trees 11 direct the coolant flow towards the fuel rods 3, arranged around them, in part of a fuel assembly according to FIG. 1 or 2. All the guide vane trees 11 are adapted to direct the flow in a counterclockwise direction. FIG. 16 is a sketch showing the principle of how the guide vane trees 11 direct the coolant flow towards the fuel rods 3, arranged around them, in part of a fuel assembly. The guide vane trees 11 are alternately adapted to direct the flow in a counterclockwise and a clockwise direction, respectively. Common to the exemplified guide vane trees is that the coolant, flowing up from and below, is diverted in several stages. When the coolant reaches the lowermost tab, the coolant changes direction and is deflected away from the tree. The tab arranged immediately above is reached by coolant which has already, at least partially, been diverted with the aid of the change of the direction of the coolant caused by the tabs located below. By connecting in series tabs which are arranged axially displaced, the coolant is deflected upon passage of the guide vane tree. By this series connection of axially displaced tabs, an increased cooling capacity and a reduced pressure drop are obtained in comparison with tabs positioned in a single plane. In cases where the guide vane tree is arranged immersed in the spacer structure and the spacer structure consists of a sleeve structure corresponding to that shown in FIG. 3a, part of the upwardly flowing coolant is guided by the sleeve walls and towards the guide vanes. By arranging the guide vane tree inside the spacer, the effect of the spacer is increased. The shape of the guide vane tree may be varied in many ways with respect to the size, shape, number, deflection in relation to the trunk, and location of the vanes along the trunk. In all the figures, the deflection members 14 are folded 90.degree. around the folding lines 12. For optimum function, however, an arbitrary magnitude of this angle may be chosen. The same applies to the angle .mu. of the folding lines 12 to the center line 13. In all figures the .mu. is shown as 45.degree. but may be chosen arbitrarily to achieve an optimum function. In the embodiments according to FIGS. 9-13, it may be of interest in certain cases to arrange more than four axially spaced-apart deflection members 14 along the guide vane (19, 21. 22). It is possible to construct the guide vane trees of Inconel or of a zirconium alloy.