FUEL ASSEMBLY FOR A NUCLEAR BOILING WATER REACTOR

Abstract

A fuel assembly for a nuclear power boiling water reactor, including: a fuel channel defining a central fuel channel axis, fuel rods, each having a central fuel rod axis, at least 3 water channels for non-boiling water, each water channel having a central water channel axis and each water channel having a larger cross-sectional area than the cross-sectional area of (the average) fuel rod. The fuel rods comprise a first group of full length fuel rods and a second group of shorter fuel rods. The fuel assembly comprises at least 5 fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these at least 5 fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.

Claims

1-15. (canceled)

16. A fuel assembly for a nuclear power boiling water reactor, comprising: a fuel channel extending in and defining a length direction of the fuel assembly and defining a central fuel channel axis extending in said length direction; fuel rods positioned such that they are surrounded by said fuel channel, each fuel rod having a central fuel rod axis extending substantially in said length direction; and water channels positioned such that they are surrounded by said fuel channel, the water channels being configured and positioned for, during operation, allowing non-boiling water to flow through the water channels, each water channel having a central water channel axis extending substantially in said length direction, wherein said fuel rods comprise a first group of fuel rods and a second group of fuel rods, wherein each fuel rod in said first group is a so-called full length fuel rod which extends from a lower part of the fuel assembly to an upper part of the fuel assembly, wherein each fuel rod in said second group extends from said lower part of the fuel assembly and upwards, but does not reach as high up as said full length fuel rods, wherein said water channels of the fuel assembly comprise at least three and no more than three water channels, each of which has a cross-sectional area which is at least twice as large as a cross-sectional area of each one of said fuel rods, or, in case the fuel assembly having fuel rods of different cross-sectional areas, at least twice as large as the average cross-sectional area of the fuel rods, wherein said at least three and no more than three water channels are positioned such that there is no further water channel of the fuel assembly, the central water channel axis of which is closer to the central fuel channel axis than the central water channel axis of each of said at least three and no more than three water channels, and wherein the fuel assembly further comprises at least five fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these at least five fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels of the fuel assembly.

17. A fuel assembly according to claim 16, wherein there is no full length fuel rod, the central fuel rod axis of which is positioned closer to the central fuel channel axis than the central fuel rod axis of any of said at least five fuel rods.

18. A fuel assembly according to claim 16, comprising at least seven fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these at least seven fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.

19. A fuel assembly according to claim 18, wherein there is no full length fuel rod, the central fuel rod axis of which is positioned closer to the central fuel channel axis than the central fuel rod axis of any of said at least seven fuel rods.

20. A fuel assembly according to claim 16, wherein each of said at least five fuel rods has a length that is less than 0.50 times the length of said full length fuel rods.

21. A fuel assembly according to claim 18, wherein each of said at least seven fuel rods has a length that is less than 0.50 times the length of said full length fuel rods.

22. A fuel assembly according to claim 20, wherein there is no fuel rod which is such that it is longer than 0.50 times the length of said full length fuel rods and has a central fuel rod axis which is positioned closer to the central fuel channel axis than the central fuel rod axis of any of said at least five fuel rods.

23. A fuel assembly according to claim 21, wherein there is no fuel rod which is such that it is longer than 0.50 times the length of said full length fuel rods and has a central fuel rod axis which is positioned closer to the central fuel channel axis than the central fuel rod axis of any of said at least seven fuel rods

24. A fuel assembly according to claim 16, wherein the cross-sectional area of each one of said at least three and no more than three water channels is between 3.0 and 10.0 times the cross-sectional area of each one of said fuel rods.

25. A fuel assembly according to claim 24, wherein the cross-sectional area of each one of said at least three and no more than three water channels is between 4.0 and 8.0 times the cross-sectional area of each one of said fuel rods.

26. A fuel assembly according to claim 16 wherein the fuel assembly has fuel rods of different cross-sectional areas and wherein the cross-sectional area of each one of said at least three and no more than three water channels is between 3.0 and 10.0 times the average cross-sectional area of the fuel rods.

27. A fuel assembly according to claim 26 wherein the fuel assembly has fuel rods of different cross-sectional areas and wherein the cross-sectional area of each one of said at least three and no more than three water channels is between 4.0 and 8.0 times the average cross-sectional area of the fuel rods.

28. A fuel assembly according to claim 16, wherein each of said at least three and no more than three water channels has a circular cross-section, at least in the portion of the water channel that is located at the level of said at least five fuel rods or said at least seven fuel rods.

29. A fuel assembly according to claim 16, wherein the fuel assembly comprises no more than 19 fuel rods, each of which fulfils the following criterion: the distance between the central fuel rod axis and the central fuel channel axis is less than the distance between the central water channel axis of at least one of said at least three and no more than three water channels and the central fuel channel axis.

30. A fuel assembly according to claim 29, wherein the fuel assembly comprises no more than 16 fuel rods, each of which fulfils the following criterion: the distance between the central fuel rod axis and the central fuel channel axis is less than the distance between the central water channel axis of at least one of said at least three and no more than three water channels and the central fuel channel axis.

31. A fuel assembly according to claim 30, wherein the fuel assembly comprises no more than 14 fuel rods, each of which fulfils the following criterion: the distance between the central fuel rod axis and the central fuel channel axis is less than the distance between the central water channel axis of at least one of said at least three and no more than three water channels and the central fuel channel axis.

32. A fuel assembly according to claim 16, wherein the fuel assembly comprises a substantially regular pattern of fuel rod positions, wherein each one of said at least three and no more than three water channels is positioned such that it replaces four fuel rods in this substantially regular pattern.

33. A fuel assembly according to claim 16, wherein the fuel assembly comprises 65-160 fuel rods.

34. A fuel assembly according to claim 33, wherein the fuel assembly comprises 100-120 fuel rods.

35. A fuel assembly according to claim 34, wherein the fuel assembly comprises 105-113 fuel rods.

36. A fuel assembly according to claim 35, wherein the fuel assembly comprises 109 fuel rods.

37. A fuel assembly according to claim 16, wherein the fuel assembly comprises 2-8 fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said full length fuel rods.

38. A fuel assembly according to claim 37, wherein the fuel assembly comprises 4-6 fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said full length fuel rods.

39. A fuel assembly according to claim 16, wherein the fuel assembly comprises at least 70 full length fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said full length fuel rods.

40. A fuel assembly according to claim 39, wherein the fuel assembly comprises at least 80 full length fuel rods.

41. A fuel assembly according to claim 40, wherein the fuel assembly comprises at least 90 full length fuel rods.

42. A fuel assembly according to claim 16, further comprising: a lower tie plate, positioned below the fuel rods, wherein a lower end of each of said at least three and no more than three water channels is attached to said tie plate; an upper lifting device, positioned above the fuel rods, including a handle for gripping and lifting a bundle of fuel rods; a plurality of spacer grids for holding the fuel rods, at least most of the spacer grids being attached to said at least three and no more than three water channels; and attachment rods, attached at a lower end to the upper part of said at least three and no more than three water channels and at an upper end attached to said upper lifting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG. 1 shows schematically a side view of a fuel assembly according to an embodiment of the invention.

[0066] FIG. 2 shows schematically a cross-section of an embodiment of a fuel assembly according to the invention.

DETAILED DESCRIPTION

[0067] An embodiment of the invention will now be described with reference to FIG. 1 and FIG. 2.

[0068] FIG. 1 shows schematically a side view of a fuel assembly 4 according to an embodiment of the invention. The fuel assembly 4 comprises a number of fuel rods 10 and water channels 14. A lower tie plate 20 is arranged below the fuel rods 10. A lower end of the water channels 14 is attached to the tie plate 20. Above the fuel rods 10 an upper lifting device 22 is arranged. The upper lifting device 22 has a handle 24 for gripping and lifting a bundle of fuel rods 10.

[0069] The fuel rods 10 are held by a plurality of spacer grids 26. It should be noted that FIG. 1 schematically shows only an upper and lower part of the fuel assembly 4. According to an embodiment, the fuel assembly 4 comprises ten spacer grids 26. The fuel assembly 4 also comprises attachment rods 28, which at a lower end are attached to the upper part of the water channels 14 and which at an upper end are attached to the upper lifting device 22. All spacer grids 26, with one exception, are attached to the water channels 14. The upper spacer grid 26 is positioned at the level of the attachment rods 28. The whole bundle of fuel rods 10 is thus held together with the help of the water channels 14, lower tie plate 20, attachment rods 28, upper lifting device 22 and spacer grids 26. It is therefore possible to lift the whole bundle of fuel rods 10 by gripping and lifting at the handle 24.

[0070] With reference also to FIG. 2 the fuel assembly 4 will now be described in more detail. The fuel assembly 4 comprises a fuel channel 6 which surrounds the bundle of fuel rods 10. In FIG. 1 the fuel channel 6 has been removed in the viewing direction in order to make it possible to see the components arranged inside the fuel channel 6. The fuel channel 6 extends in a length direction L. The length direction L is normally, when the fuel assembly 4 is in use in a nuclear reactor, the vertical direction. The fuel channel 6 has a central fuel channel axis 8 in said length direction L.

[0071] In FIG. 2, all the small circles refer to fuel rods 10. Each fuel rod 10 has a central fuel rod axis 12 (shown only for one fuel rod 10), which extends substantially in the length direction L.

[0072] The larger circles in FIG. 2 show the water channels 14. The water channels 14 are configured and positioned for allowing non-boiling water to flow through the water channels 14, when the fuel assembly 4 is in use in a nuclear reactor. Each water channel 14 has a central water channel axis 16 (shown only for one water channel 14 in FIG. 2), which extends substantially in the length direction L.

[0073] The fuel assembly comprises a first group of full length fuel rods 10. The full length fuel rods are not marked in FIG. 2 (i.e. they are shown by empty circles). The full length fuel rods 10 extend from a lower part of the fuel assembly 4 to an upper part of the fuel assembly 4, preferably through all the spacer grids 26. It can be noted that in FIG. 1 only full length fuel rods 10 are shown.

[0074] The fuel assembly 4 also comprises a second group of fuel rods 10. The second group of fuel rods 10 extend from the lower part of the fuel assembly (like the full length fuel rods) but do not reach as high up as the full length fuel rods.

[0075] The fuel rods 10 in said second group can have different lengths. In the shown embodiment, some fuel rods 10 are marked with one stroke. These fuel rods have a length of about 9/10 of the length of the full length fuel rods. In the shown embodiment, there are ten such fuel rods. When placing these 9/10 fuel rods the most reactive positions next to non-boiling water inside the water channels and outside the fuel channel are avoided. This is to minimize the negative impacts of having 1/10 less uranium in these rods, while serving their purpose of reducing pressure drop near the assembly outlet.

[0076] The fuel rods 10 marked with two strokes (a cross) have a length of about ⅔ of the length of the full length fuel rods. In the shown embodiment there are four such fuel rods. These ⅔ fuel rods are positioned halfway between the corner rods in the outer rows and columns of the 11×11 fuel rod array. This is to reduce cold reactivity in the upper part of the fuel bundle which improves the shutdown margin late in the fuel cycle when the power distribution has moved towards the top.

[0077] The fuel rods 10 marked with three strokes (a star) have a length of about ⅓ of the length of the full length fuel rods. In the shown embodiment there are twelve such fuel rods.

[0078] As shown in FIG. 2, the fuel assembly 4 according to this embodiment has three water channels 14. Each water channel 14 has a cross-sectional area which is about 5.5 times the cross-sectional area of each one of the fuel rods 10 (or, in case the fuel assembly 4 would have fuel rods 10 of different cross-sectional areas, about 5.5 times the average cross-sectional area of the fuel rods 10).

[0079] In the shown embodiment, there are only three water channels 14, i.e. no further water channels.

[0080] As shown in FIG. 2, there are eight centrally located fuel rods 10 of the shortest kind. These central short fuel rods 10 are positioned such that for each of these fuel rods 10 it is the case that the distance between the fuel rod axis 12 and the central fuel channel axis 8 is shorter than the distance between any of the water channel axes 16 of the water channels 14 and the fuel channel axis 8.

[0081] It should be noted that FIG. 2 shows a schematic cross-section of the fuel assembly 4 in the lower part of the fuel assembly (where also all the shorter fuel rods 10 are present).

[0082] There is no longer fuel rod 10 (no ⅔ fuel rod or 9/10 fuel rod or full length fuel rod) which is positioned closer to the central fuel channel axis 8 than the central fuel rod axis 12 of any of the eight central short fuel rods 10. Above the eight short central fuel rods 10, there is thus an empty space for water in the fuel assembly 4.

[0083] In addition to the eight central short ⅓ fuel rods, there are a further four such short fuel rods 10 located in the corners of the fuel assembly 4.

[0084] Each of the water channels 14 has a circular cross-section, at least in the lower part of the fuel assembly 4 where the shorter central fuel rods 10 are arranged.

[0085] In addition to the mentioned eight central short fuel rods 10, the fuel assembly 4 comprises a further six fuel rods, each of which fulfils the following criterion. The distance between the central fuel rod axis 12 and the central fuel channel axis 8 is less than the distance between the central water channel axis 16 of at least one of the three water channels 14 and the central fuel channel axis 8. In the shown embodiment, there are 14 fuel rods 10 that fulfil the mentioned criterion. These fuel rods 10 are located inside the dashed lines in FIG. 2. Each water channel 14 is positioned next to at least some of the eight centrally located short fuel rods 10.

[0086] As can be seen in FIG. 2, the fuel assembly 4 comprises a substantially regular pattern of fuel rod positions. Each one of the water channels 14 is positioned such that it replaces four fuel rods 10 in this regular pattern.

[0087] In the shown embodiment, the fuel assembly 4 thus comprises 83 full length fuel rods 10, ten 9/10 length fuel rods, four ⅔ length fuel rods and twelve ⅓ length fuel rods.

[0088] The shown embodiment provides an advantageous fuel assembly 4 with which the above described objects and advantages of the invention are achieved.

[0089] The present invention is not limited to the examples described herein, but can be varied and modified within the scope of the following claims.