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 include a first group of full length fuel rods and a second group of shorter fuel rods. The fuel assembly comprises 3 or 4 fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these 3 or 4 fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.

Claims

1. 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, 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 full length fuel rod that 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 is a part length fuel rod that 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 the fuel assembly comprises 3 water channels, each of which has a cross-sectional area that is at least twice as large as the average cross-sectional area of the fuel rods, wherein the water channels are positioned with no further water channel having its central axis closer to the central fuel channel axis than the central water channel axis of each of the 3 water channels, wherein the fuel assembly comprises 3 or 4, but not more than 4, second group fuel rods positioned with their central fuel rod axes being closer to the central fuel channel axis than any of the water channel axes of the water channels, and wherein the fuel assembly comprises no more than 3 water channels.

2. A fuel assembly according to claim 1, wherein no full length fuel rod is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said 3 or 4 second group fuel rods.

3. A fuel assembly according to claim 1, comprising 4 second group fuel rods positioned with their respective central fuel rod axes closer to the central fuel channel axis than any of the water channel axes of the water channels.

4. A fuel assembly according to claim 1, comprising a set of 6-12 second group fuel rods, wherein said set includes said 3 or 4 second group fuel rods positioned closer to the central fuel channel axis than any of the water channel axes of the water channels, wherein each fuel rod in said set has a length which is less than 0.80 times the length of said full length fuel rods, and wherein the fuel rods in said set are grouped together such that each fuel rod in said set is positioned next to at least one other fuel rod belonging to said set.

5. A fuel assembly according to claim 1, wherein each of said 3 or 4 second group fuel rods positioned closer to the central fuel channel axis than any of the water channel axes of the water channels has a length that is less than 0.50 times the length of said full length fuel rods.

6. A fuel assembly according to claim 5, wherein no fuel rod longer than 0.50 times the length of said full length fuel rods is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said 3 or 4 second group fuel rods positioned closer to the central fuel channel axis than any of the water channel axes of the water channels.

7. A fuel assembly according to claim 1, wherein each one of said at least 3 water channels has a cross-sectional area which is between 3.0 and 10.0, times the average cross-sectional area of the fuel rods.

8. A fuel assembly according to claim 1, wherein each of said water channels has a circular cross-section, at least in the lower part of the fuel assembly where the second group fuel rods are arranged.

9. A fuel assembly according to claim 1, wherein the fuel assembly comprises no more than 12 fuel rods having a distance between the central fuel rod axis and the central fuel channel axis less than the distance between the central water channel axis of at least one of said water channels and the central fuel channel axis.

10. A fuel assembly according to claim 1, wherein the fuel assembly comprises a substantially regular pattern of fuel rod positions, wherein each one of said water channels is positioned such that it replaces 4 fuel rods in this substantially regular pattern.

11. A fuel assembly according to claim 1, wherein the fuel assembly comprises 65-160 fuel rods.

12. A fuel assembly according to claim 1, wherein the fuel assembly further comprises 2-10 additional second group fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said full length fuel rods.

13. A fuel assembly according to claim 1, wherein the fuel assembly comprises at least 70 full length fuel rods.

14. A fuel assembly according to claim 1, further comprising: a lower tie plate, positioned below the fuel rods, wherein a lower end of each of said at least 3 water channels is attached to said tie plate, an upper lifting device positioned above the fuel rods and 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 3 water channels, and attachment rods attached at their lower ends to the upper part of said water channels and at their upper ends to said upper lifting device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically a side view of a fuel assembly according to an embodiment of the invention.

(2) FIG. 2 shows schematically a cross-section of an embodiment of a fuel assembly according to the invention.

(3) FIG. 3 shows schematically the same cross-section as FIG. 2 of another embodiment of a fuel assembly according to the invention.

(4) FIG. 4 shows schematically the same cross-section as FIG. 2 of a further embodiment of a fuel assembly according to the invention.

(5) FIG. 5 shows schematically the same cross-section as FIG. 2 of a still further embodiment of a fuel assembly according to the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(6) A first embodiment of the invention will now be described with reference to FIG. 1 and FIG. 2.

(7) 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.

(8) 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.

(9) 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. Also, two water channels 14 are shown in FIG. 1, although they in a side view would be partly hidden behind fuel rods 10. 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.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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.

(15) The fuel rods 10 marked with two strokes (a cross) have a length of about of the length of the full length fuel rods. In the shown embodiment there are six such fuel rods. These fuel rods are positioned about halfway between the corner rods in the outer rows and columns of the 1111 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.

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

(17) 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).

(18) In the shown embodiment, there are only three water channels 14, i.e. no further water channels.

(19) As shown in FIG. 2, there are eight centrally located fuel rods 10 of the shortest kind, i.e. eight central short fuel rods, which are grouped together and form a set of fuel rods, such that each fuel rod in this set is positioned next to at least one other fuel rod belonging to this set. Of these eight central short fuel rods 10, four are positioned such that for each of these four 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. These four short central fuel rod are thus (with reference to FIG. 2) the central fuel rod positioned on the fuel channel axis 8, the fuel rod positioned just below the central fuel rod, the fuel rod positioned just to the right of the central fuel rod and the fuel rod positioned next to the central fuel rod, one column to the left and one row up.

(20) 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).

(21) There is no longer fuel rod 10 (no 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 four central short fuel rods 10 which fulfill the above definition. Above the four short central fuel rods 10, there is thus an empty space for water in the fuel assembly 4. In fact, there is an empty space for water above all the mentioned eight centrally located short fuel rods 10.

(22) In addition to the eight centrally located short fuel rods, there are a further four such short fuel rods 10 located in the corners of the fuel assembly 4.

(23) 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.

(24) In addition to the mentioned four defined central short fuel rods 10, the fuel assembly 4 comprises a further two fuel rods, each of which fulfills 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 thus six fuel rods 10 that fulfill 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 two of the four defined centrally located short fuel rods 10.

(25) 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.

(26) In the shown embodiment, the fuel assembly 4 thus comprises 81 full length fuel rods 10, ten 9/10 length fuel rods, six length fuel rods and twelve length fuel rods.

(27) Further embodiments of the present invention are shown in FIGS. 3, 4 and 5. In these figures, the same markings and the same reference numbers are used as in FIG. 2. It will therefore now only be described how these embodiments differ from the embodiment shown in FIG. 2.

(28) FIG. 3 shows an embodiment which differs from the embodiment in FIG. 2 in that the fuel rods 10 that have a length of about of the length of the full length fuel rods are not positioned next to the periphery of the fuel assembly 4, but instead are positioned further inside the fuel assembly 4. In the embodiment of FIG. 2, the full length fuel rods which are positioned in the outer rows and columns next to the fuel rods of length are highly moderated in the upper region (since there is a large space for water next to these full length fuel rods). This means that the local effect in the higher part of these full length fuel rods is quite high. With the embodiment of FIG. 3, this local high effect is avoided.

(29) FIG. 4 shows an embodiment which differs from the embodiment in FIG. 2 in that two of the centrally located eight short fuel rods 10 have been replaced by full length fuel rods. Furthermore, in the embodiment of FIG. 4 there are eight (instead of six as in FIG. 2) fuel rods of the length arranged in the outer rows and columns. Since there are only six central short fuel rods in the embodiment of FIG. 4, the flow velocity in the central upper region is reduced. Furthermore, with the arrangement of FIG. 4, there is a more even distribution of the nuclear fuel in the upper part of the fuel assembly as compared to the embodiment of FIG. 2.

(30) FIG. 5 shows an embodiment which differs from the embodiment in FIG. 2 in that two of the centrally located eight short fuel rods 10 have been replaced by length fuel rods. Furthermore, in the embodiment of FIG. 5 there are only four length fuel rods which are positioned in the outer rows and columns. Instead, there are two length fuel rods which are positioned next to the centrally located length fuel rods (one length fuel rod positioned in the third row from above and the fifth column from the left and one length fuel rod positioned in the fifth row from above and the third column from the left). In the embodiment of FIG. 5, there are thus six centrally located length fuel rods and also four length fuel rods, i.e. together 10 fuel rods shorter than 0.80 of the length of the full length fuel rods, which are grouped together such that they form a set such that each fuel rod in the set is positioned next to at least one other fuel rod belonging to the set. This embodiment has appeared to bring about a good compromise of the advantages described in connection with the previous embodiments.

(31) The shown embodiments provide advantageous fuel assemblies with which the above described objects and advantages of the invention are achieved.

(32) Many variations of the illustrated embodiments are possible within the scope of the present invention. For example, the number of fuel rods may vary and the number of fuel rods of the different lengths may vary. For example, there may be fewer, or more, or no at all, fuel rods which have the length 9/10.

(33) The present invention is thus not limited to the examples described herein, but can be varied and modified within the scope of the following claims.