Fuel assembly for a nuclear power boiling water reactor

Abstract

The present invention concerns a fuel assembly for a nuclear power boiling water reactor. The fuel assembly comprises fuel rods. At least 95% of the fuel rods comprise nuclear fuel material in the form of U enriched in 235U. At least 20% of the fuel rods belong to a first set of fuel rods. The fuel rods in this first set comprise both U enriched in 235U and Th. The first set comprises a first and a second subset of fuel rods. The ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset. The invention also concerns a nuclear power boiling water reactor and a manner of operating such a reactor.

Claims

1. A fuel assembly for a nuclear power boiling water reactor, comprising: a plurality of fuel rods, each comprising a cladding tube and nuclear fuel material positioned in the cladding tube; and a fuel channel surrounding said plurality of fuel rods; wherein each one of at least 95% of the plurality of fuel rods comprises nuclear fuel material in the form of Uranium (U) enriched in 235U; wherein each one of at least 20% of the plurality of fuel rods belongs to a first set of fuel rods; wherein each fuel rod in the first set of fuel rods comprises both U enriched in 235U and Thorium (Th); and wherein the first set of fuel rods comprises at least a first and a second subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset wherein each one of between 20% and 90% of the plurality of fuel rods belongs to a second set of fuel rods, wherein each fuel rod in the second set comprises U enriched in 235U, but does not comprise a substantial amount of Th or a burnable absorber, wherein the enrichment of 235U in the U in the second set is within the range 4.00-6.00%.

2. A fuel assembly according to claim 1, wherein said first set of fuel rods further comprises a third subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said third subset, is lower than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset.

3. A fuel assembly according to claim 2, wherein said first set of fuel rods further comprises a fourth subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said fourth subset, is lower than the ratio, with regard to weight, between Th and U, in each fuel rod of said third subset.

4. A fuel assembly according to claim 1, wherein, in all of the fuel rods of each subset, the enrichment of 235U in the U is within the range 4.00-6.00%.

5. A fuel assembly according to claim 1, wherein the fuel rods of said subsets have the same enrichment of 235U in the U.

6. A fuel assembly according to claim 1, wherein said subsets of fuel rods are positioned in the fuel assembly such that fuel rods with a higher ratio, with regard to weight, between Th and U are positioned where there will be more moderator when the fuel assembly is in use in the nuclear power boiling water reactor, than where the fuel rods of a subset with a lower ratio, with regard to weight, between Th and U are positioned.

7. A fuel assembly according to claim 1, wherein between 25% and 80% of the plurality of fuel rods belong to said first set of fuel rods.

8. A fuel assembly according to claim 1, wherein each one of between 3% and 20% of the fuel rods belongs to a third set of fuel rods, wherein each fuel rod in the third set comprises U enriched in 235U and at least one kind of burnable absorber.

9. A fuel assembly according to claim 1, wherein in each fuel rod of said second set, the enrichment of 235U in the U is substantially the same as the enrichment of 235U in the U in the fuel rods of said subsets.

10. A fuel assembly according to claim 8, wherein in each fuel rod of said second set, the enrichment of 235U in the U is substantially the same as the enrichment of 235U in the U in the fuel rods of said subsets.

11. A fuel assembly according to claim 8, wherein each one of between 2% and 10% of the plurality of fuel rods belongs to a fourth set of fuel rods, wherein each fuel rod in the fourth set comprises U enriched in 235U, but does not comprise a substantial amount of Th or a burnable absorber, wherein the enrichment of 235U in the U in the fourth set is less than 4.00%.

12. A fuel assembly according to claim 1, wherein the U in the nuclear fuel material in the plurality of fuel rods is present in the form of Uranium dioxide (UO.sub.2) and the Th is present in the form of Thorium dioxide (ThO.sub.2).

13. A fuel assembly according to claim 1, wherein the nuclear fuel material in the plurality of fuel rods is in the form of pellets arranged on top of each other such that they form a stack of nuclear fuel pellets in the cladding tubes, wherein the different fuel pellets with nuclear fuel material within one and the same fuel rod have the same composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically a nuclear power boiling water reactor of a nuclear energy plant.

(2) FIG. 2 shows schematically a fuel assembly for a boiling water reactor.

(3) FIG. 3 shows schematically a cross-section of an embodiment of a fuel assembly according to the invention.

(4) FIG. 4 shows schematically a fuel rod.

DETAILED DESCRIPTION

(5) An embodiment of the invention will now be described with reference to the figures.

(6) FIG. 1 thus shows schematically a nuclear energy plant with a nuclear power boiling water reactor (BWR). The plant comprises a reactor vessel 10. In the core in the reactor vessel 10 a large number of fuel assemblies 12 are arranged. In the reactor vessel 10 there are also control rods 13, which can be inserted between the fuel assemblies 12 in order to control the operation of the nuclear reactor. Steam from the reactor vessel 10 is conducted via a conduit 14 to a turbine 15. With the help of the turbine 15 electric energy is generated by an electric generator 16. Steam from the turbine 15 is led to a steam condenser 17. The steam condenser 17 is cooled by cold water which is transported in a conduit 18. Water from the condenser 17 is fed back to the reactor vessel 10 via a conduit 19.

(7) FIG. 2 shows schematically a fuel assembly 12 in accordance with an embodiment of the present invention for a BWR. The fuel assembly may be about 4 m long and defines a length direction L. FIG. 2 shows an upper and a lower part of the fuel assembly. The fuel assembly 12 has a connecting member 20 at the bottom of the fuel assembly. At the top of the fuel assembly 12, there is a handle 22. The fuel assembly 12 comprises a number of fuel rods 23. A fuel channel 25 surrounds the fuel rods 23. According to this embodiment, the fuel channel 25 has a generally square shape (see also FIG. 3) and therefore has four corners. One corner is marked 24 in FIG. 2. It should be noted that in FIG. 2 part of the fuel channel 25 is removed in order to show the fuel rods 23 inside the fuel channel 25.

(8) When the fuel assembly 12 is in use in a nuclear power BWR, water enters into the fuel channel 25 at the bottom via the connecting member 20 and the produced steam exits at the top of the fuel channel 25.

(9) According to an embodiment of the present invention, each fuel rod 23 comprises a cladding tube 52 (see FIG. 4) and nuclear fuel material in the form of pellets 50 arranged such that they form a stack in the cladding tube 52. The nuclear fuel pellets 50 comprise nuclear fuel material in the form of UO.sub.2. The U is enriched in .sup.235U.

(10) FIG. 3 shows schematically a cross-section of a fuel assembly 12 according to an embodiment of the present invention.

(11) All the small circles in FIG. 3 are fuel rods 23 (see also FIG. 2). The fuel rods are surrounded by the above described fuel channel 25. The fuel assembly also comprises two water channels 28. During use, non-boiling water flows through these water channels 28. The water in the water channels 28 functions as a neutron moderator.

(12) When the fuel assembly 12 is used in a nuclear power BWR, two of the sides of the fuel assembly 12 (the side to the left in FIG. 3 and the side at the top of FIG. 3) will be positioned next to a space where control rod blades may be inserted. The corner 24 points to the cross-center of a cruciform control rod. Since the design of such control rods are known to a person skilled in the art, the control rod will not be described in more detail here.

(13) The fuel rods marked 41, 32, 26 and 9 in FIG. 3 together constitute a first set of fuel rods. The nuclear fuel material in each fuel rod in this first set comprises, in addition to the mentioned UO.sub.2, also ThO.sub.2.

(14) The first set of fuel rods 41, 32, 26, 9 comprises a first, second, third and fourth subset of fuel rods. The ratios, with regard to weight, between Th and U in the fuel rods of the different subsets differ from each other.

(15) In the shown embodiment, the first subset has six fuel rods marked 41. The ratio between Th and U in each fuel rod in this subset is such that the relation Th/(Th+U), with regard to weight, is about 0.41.

(16) The fuel rods of the second subset are marked 32. There are seven such fuel rods in the shown embodiment. The ratio between Th and U in each fuel rod in this second subset is such that the relation Th/(Th+U), with regard to weight, is about 0.32.

(17) The fuel rods of the third subset are marked 26. There are eighteen such fuel rods in the shown embodiment. The ratio between Th and U in each fuel rod in this third subset is such that the relation Th/(Th+U), with regard to weight, is about 0.26.

(18) The fuel rods in the fourth subset are marked 9. There are fourteen such fuel rods in the shown embodiment. The ratio between Th and U in each fuel rod in the fourth subset is such that the relation Th/(Th+U), with regard to weight, is about 0.09.

(19) The fuel rods that are not marked in FIG. 3 (empty small circles) constitute a second set of fuel rods. These fuel rods comprise nuclear fuel material in the form of UO.sub.2 but do not comprise any Th and not any burnable absorber. In to the shown embodiment there are thirty-two such fuel rods.

(20) For each of the fuel rods described so far (i.e. the fuel rods marked 41, 32, 26, 9 and the empty small circles) the enrichment of .sup.235U in the U is 4.95%.

(21) In FIG. 3 nine fuel rods are marked Gd. These fuel rods constitute a third set of fuel rods. Each fuel rod of this type comprises, in addition to the UO.sub.2, also a burnable absorber. According to this embodiment the nuclear fuel material of each of these fuel rods contains about 3.5%, with regard to weight, Gd.sub.2O.sub.3 as a burnable absorber. The enrichment of .sup.235U may be somewhat lower in these fuel rods than in the fuel rods of said first set and said second set.

(22) The fuel rods marked with a cross in FIG. 3 constitute a fourth set of fuel rods. The nuclear fuel material in each fuel rod in this set of fuel rods comprises U enriched in .sup.235U, but does not comprise any Th and not any burnable absorber. Furthermore, the enrichment of .sup.235U in the U in this fourth set of fuel rods is less than 3.00%. According to this embodiment, five such fuel rods 29 have the enrichment 2.08% and one fuel rod 30 has the enrichment 1.88%. The fuel rod 30 is positioned in the corner 24 which is intended to be positioned next to the cross-center of a cruciform control rod.

(23) It should be noted that concerning the mentioned subsets of fuel rods 41, 32, 26, 9, the fuel rods with a higher ratio, with regard to weight, between Th and U are positioned generally where there will be more moderator (more water) when the fuel assembly is in use in the nuclear power BWR than where fuel rods of a subset with a lower such ratio are positioned. It can be noted that in a fuel assembly for a BWR, there will generally be more moderator close to the corners of the fuel assembly.

(24) In addition to the materials described above in this embodiment, the nuclear fuel material in fuel rods may contain small amounts of other additives, as known to a person skilled in the art.

(25) In each fuel rod 13, the nuclear fuel material is arranged in the form of pellets 50 in the manner described in connection with FIG. 4. The different nuclear fuel pellets 50 within one and the same fuel rod 23 have the same composition.

(26) FIG. 1 also illustrates an embodiment of a nuclear power boiling water reactor according to the present invention, when the core of the reactor, inside the reactor vessel 10, has been loaded with a plurality of nuclear fuel assemblies 12 in accordance with the above described embodiment.

(27) According to the present invention, a manner of operating a nuclear power boiling water reactor of a nuclear energy plant comprises:

(28) arranging a plurality of nuclear fuel assemblies 12 according to the above described embodiment in the core of the nuclear reactor, and

(29) operating the nuclear reactor such that energy is produced, for example with the help of an electric generator 16 as shown in FIG. 1.

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