Fuel assembly and reactor core

Abstract

To provide a fuel assembly and a reactor core capable of achieving high enrichment and setting a void reactivity coefficient to a negative value even under a condition where neutron leakage and a control rod are not considered.SOLUTION: The fuel assembly 10 has a water rod, a full length rod fuel rod 1, a partial length fuel rod 2 having a fuel effective length shorter than that of the full length rod fuel rod 1, a channel box 16, a fuel spacer 15, an upper tie plate 18, and a lower tie plate 17, and includes plutonium in at least one of the full length rod fuel rod 1 and the partial length fuel rod 2, and has the partial length fuel rod 2 whose center axis is included in the region A.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel assembly comprising a plurality of fuel rods containing nuclear fuel material arranged in a rectangular cylindrical channel box, and to a reactor core in which the fuel assembly is loaded. [Background technology]

[0002] The core of a boiling water reactor is loaded with multiple fuel assemblies. Each fuel assembly includes multiple fuel rods, each containing multiple fuel pellets containing nuclear fuel material (e.g., uranium oxide), an upper tie plate (upper fuel support member) that supports the upper ends of the fuel rods, a lower tie plate (lower fuel support member) that supports the lower ends of the fuel rods, multiple fuel spacers that maintain the spacing between the fuel rods, and a rectangular tubular channel box with a square cross section. The channel box has its upper end attached to the upper tie plate and extends toward the lower tie plate, surrounding the multiple fuel rods bundled together by multiple fuel spacers arranged axially. Some of the fuel rods may be shorter than the other fuel rods (partial-length fuel rods).

[0003] Each fuel rod has a cladding tube, the lower end of which is sealed with a lower end plug, and the upper end of which is sealed with an upper end plug. Furthermore, each fuel rod contains a plurality of fuel pellets containing nuclear fuel material. A gas plenum is located within the cladding tube above the fuel pellet-filled region.

[0004] To control the reactor power, multiple control rods are inserted into the reactor core, specifically, between the multiple fuel assemblies loaded in the core. Some of the fuel rods in the fuel assemblies are filled with fuel pellets containing burnable poisons, such as gadolinium (Gd)-containing gadolinium (Gd2O3). The control rods and burnable poisons absorb excess neutrons generated by the nuclear fission of the nuclear fuel material. As the burnable poisons absorb neutrons, they transform into substances that are less resistant to neutron absorption. Therefore, the burnable poisons contained in new fuel assemblies (fuel assemblies with a burnup of 0 GWd / t) loaded into the reactor core disappear after a certain period of reactor operation has elapsed since the new fuel assemblies were loaded into the core. The reactivity of fuel assemblies from which the burnable poisons have disappeared decreases monotonically as the nuclear fuel material burns up. The reactor core is loaded with a plurality of fuel assemblies that have undergone different numbers of operating cycles, and therefore the core as a whole maintains a critical state throughout the reactor's operating period.

[0005] Spent nuclear fuel contained in spent fuel assemblies removed from nuclear reactors is reprocessed. The resulting mixed oxide fuel (MOX fuel), containing plutonium (Pu) and uranium, is used as nuclear fuel material. Fuel assemblies containing fabricated MOX fuel are called MOX fuel assemblies. Burnable poisons are also used in these MOX fuel assemblies. However, because the average neutron energy of a reactor core loaded with MOX fuel assemblies is higher (the neutron energy spectrum becomes harder), the neutron absorption effect of burnable poisons is reduced. For this reason, in MOX fuel assemblies, burnable poisons are generally added to the uranium fuel in fuel rods filled with uranium fuel that contains no Pu and has a low uranium content, thereby enhancing the neutron absorption effect of the burnable poisons. This phenomenon occurs both in fuel assemblies that use only uranium fuel and in those that use highly enriched nuclear fuel materials.

[0006] Furthermore, because plutonium (Pu) is recovered from spent nuclear fuel through reprocessing, unlike uranium fuel, it does not require an enrichment process to increase plutonium enrichment. In other words, since high-enrichment MOX fuel assemblies can be produced at relatively low cost, achieving high burnup using high-enrichment MOX fuel assemblies is effective in reducing costs. Because the neutron energy spectrum of high-enrichment MOX fuel assemblies is hardened, the difference in energy spectrum between fuel rods located on the periphery of the cross section of the fuel assembly and facing the water gaps between the fuel assemblies becomes large compared to fuel rods located in the center of the cross section. Note that, in this specification, "plutonium enrichment" refers to the content of fissile plutonium.

[0007] In boiling water reactors, bubbles (voids) form in the fuel assemblies during operation due to the boiling of the moderator. Generally, in boiling water reactors, the void fraction increases the higher you go in the core. The moderator density inside the voids is lower than that of the liquid phase. Therefore, when the volume fraction of voids in the moderator (void fraction) changes, the degree of moderation of neutrons in the fuel assembly changes, and the reactivity changes. The degree of change in reactivity in response to a change in void fraction is called the void reactivity coefficient. When the power of the fuel assembly increases and the void fraction within the fuel assembly rises, it is generally desirable for the void reactivity coefficient to be a negative value so that a negative feedback effect on the power occurs. In addition, the reactivity coefficient when the reactor is shut down, the moderator temperature in the reactor drops, and the moderator density increases is called the hot-cold swing.

[0008] Patent Document 1 proposes a technology for improving the reactivity coefficient. Patent Document 1 describes a fuel assembly in which part-length fuel rods and water rods are arranged in the center of the fuel assembly, packed together so that the water region in the cross section of the fuel assembly is at least a certain area. By placing a large water region in the fuel assembly, when the moderator density increases, the neutron energy spectrum becomes softer (over-moderated) than the region suitable for nuclear fission, reducing hot-cold swing. This makes it possible to increase the subcriticality when the reactor is shut down. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-090328 [Patent Document 2] International Publication No. 2006 / 068144 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-117771 Summary of the Invention [Problem to be solved by the invention]

[0010] In highly enriched MOX fuel assemblies with a harder neutron spectrum, the void reactivity coefficient at high void fractions shifts to the positive side. However, Patent Document 1 does not consider the need to make the void reactivity coefficient negative for highly enriched MOX fuel, as is the case with current fuel.

[0011] Therefore, the present invention provides a fuel assembly and a reactor core that can achieve high enrichment and make the void reactivity coefficient a negative value even under conditions that do not take neutron leakage or control rods into consideration. [Means for solving the problem]

[0012] In order to solve the above problems, the fuel assembly of the present invention is a fuel assembly having, excluding water rods, full-length fuel rods, part-length fuel rods having an effective fuel length shorter than that of the full-length fuel rods, a channel box, fuel spacers, an upper tie plate, and a lower tie plate, wherein at least one of the full-length fuel rods and the part-length fuel rods contains plutonium, and the fuel assembly has a region A where the distance to the water gap is 0.4L or more, where L is the shortest distance from the central axis of the fuel assembly to the water gap, and the fuel assembly has a part-length fuel rod whose central axis is included within region A.

[0013] Furthermore, the core according to the present invention is a core loaded with a plurality of fuel assemblies, each of which has, excluding water rods, full-length fuel rods, part-length fuel rods having an effective fuel length shorter than that of the full-length fuel rods, a channel box, a fuel spacers, an upper tie plate, and a lower tie plate, and is characterized in that at least one of the full-length fuel rods and the part-length fuel rods contains plutonium, and the core has a region A where the distance to the water gap is 0.4L or more, where L is the shortest distance from the central axis of the fuel assembly to the water gap, and the core is provided with part-length fuel rods whose central axis is included within region A. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a fuel assembly and a reactor core that can achieve high enrichment and make the void reactivity coefficient a negative value even under conditions that do not take neutron leakage or control rods into consideration. Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a MOX fuel assembly applied to an advanced boiling water reactor nuclear power plant according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a vertical cross-sectional view of the MOX fuel assembly shown in FIG. [Figure 3] 2 is a longitudinal sectional view of a nuclear reactor of an advanced boiling water nuclear power plant in which the MOX fuel assemblies shown in FIG. 1 are loaded into the reactor core. [Figure 4] FIG. 1 is an explanatory diagram showing the contribution of plutonium to the void reactivity coefficient for each fuel rod in a MOX fuel assembly with a high void fraction, in a quarter cross section of the MOX fuel assembly. [Figure 5] 1 is a graph showing the relationship between the distance of a fuel rod from the water gap and the contribution of plutonium in the fuel rod at that position to the void reactivity coefficient. [Figure 6] This is a graph showing the relationship between the moderator area in region A at the center of the fuel assembly and the change in void reactivity coefficient when one part-length fuel rod is added. [Figure 7] FIG. 10 is a cross-sectional view of a MOX fuel assembly applied to an advanced boiling water reactor nuclear power plant according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] The inventors have conducted various studies to realize a MOX fuel assembly that can mitigate the effect of shifting the void reactivity coefficient to the positive side due to high enrichment, and can make the void reactivity coefficient a negative value even under conditions that do not take neutron leakage or control rods into consideration. The results of this study and an overview of the newly discovered MOX fuel assembly are described below.

[0017] In a boiling water reactor (BWR), the neutron energy spectrum within a fuel assembly depends on the position of the fuel rod within the fuel assembly. In a BWR, a water gap is formed outside the channel box of a MOX fuel assembly, and saturated water exists in this water gap, which softens the neutron energy spectrum within the MOX fuel assembly. The neutron energy spectrum of fuel rods located at the corners of the MOX fuel assembly is the softest, and the neutron energy spectrum of fuel rods located closer to the central region of the fuel assembly, such as other fuel rods adjacent to the corner fuel rods and other fuel rods adjacent to these fuel rods, becomes harder. Furthermore, when a MOX fuel assembly is highly enriched, the neutron energy spectrum of the fuel rods located in the central region of the fuel assembly becomes harder.

[0018] As mentioned above, in a BWR, the void fraction increases the closer you go to the top of the core. When the void fraction is high, the moderator density decreases, making it difficult for neutrons to be moderated, and the neutron energy spectrum becomes harder. As a result, the neutron energy spectrum becomes harder, especially in the upper and central regions of the fuel assemblies. In fuel rods with a neutron energy spectrum that is harder than a certain level, the positive contribution to the void reactivity coefficient is large due to the change in the plutonium reaction rate when the void fraction increases. The inventors focused on this characteristic. Figure 4 shows the plutonium contribution to the void reactivity coefficient for each fuel rod in a high-void-fraction MOX fuel assembly. Figure 4 is an explanatory diagram showing the plutonium contribution to the void reactivity coefficient for each fuel rod in a high-void-fraction MOX fuel assembly in a quarter cross-section of the MOX fuel assembly. In Figure 4, the cross-section of the MOX fuel assembly is shown on the right, the fuel rod identifiers for the quarter region on the control rod side of the cross-section are shown in the upper left, and the plutonium contribution to the void reactivity coefficient for each fuel rod with a high void fraction shown in the upper left is shown in the lower left. As described above, it can be seen that fuel rods with a hard energy spectrum located in the central region of the fuel assembly contribute positively to the void reactivity coefficient.

[0019] When the void fraction within a fuel assembly changes, the power output of the fuel rods within the fuel assembly changes. The extent to which the power increases when the void fraction increases is affected by the hardness of the fuel rod's energy spectrum, due to the characteristics of plutonium, as described above. Figure 5 shows the distance from the fuel rod's central axis to the water gap, an indicator that has a significant impact on the fuel rod's energy spectrum, and the rate at which the reactivity of the fuel rod changes (the contribution to the void reactivity coefficient) when the void fraction changes from 70% to 40%. Figure 5 is a graph of the relationship between the distance of the fuel rod from the water gap and the contribution of plutonium in the fuel rod at that position to the void reactivity coefficient. The horizontal axis represents the relative distance from the fuel assembly central axis to the water gap, a real number ranging from 0 to 1, representing the distance from a given part-length fuel rod to the water gap. When the shortest distance from the fuel assembly central axis to the water gap is defined as L, fuel rods in the region where the distance to the water gap is 0.4L or greater have a particularly large positive contribution to the void reactivity coefficient. This region is referred to as Region A. By using short partial-length fuel rods in this region A, it is possible to remove fuel rods that contribute positively to the void fraction in the upper part of the fuel assembly where the void fraction is high, and to shift the void reactivity coefficient to the negative side.

[0020] When part-length fuel rods are placed in Region A and the moderator area in Region A expands above the fuel assembly, the neutron energy spectrum in Region A becomes softer. Therefore, increasing the number of part-length fuel rods beyond a certain number reduces the positive contribution of full-length fuel rods to the void reactivity coefficient in Region A. Therefore, even if full-length fuel rods are replaced with part-length fuel rods and the number of part-length fuel rods is increased, the void reactivity coefficient does not shift negatively. Figure 6 shows the relationship between the moderator area in Region A and the change in the void reactivity coefficient when one full-length fuel rod is replaced with a part-length fuel rod under constant Pu enrichment conditions. The horizontal axis represents the moderator area in Region A relative to the cross-sectional area of ​​the fuel assembly, ranging from 15% to 40%. When the cross-sectional area of ​​the fuel assembly is S, the negative effect of the part-length fuel rod placement on the void reactivity coefficient is significant within the range of 0.3S. An embodiment of the present invention that reflects the above-described study results will be described below with reference to the drawings. In the following, an advanced boiling water reactor (ABWR) will be described as an example of a boiling water reactor, but the present invention is not limited to this. For example, the present invention can be similarly applied to other reactors, such as a conventional boiling water reactor (BWR) that is equipped with a recirculation pump and circulates coolant water as a moderator by passing it outside the reactor pressure vessel and then flowing it back into a downcomer inside the reactor pressure vessel, an economical simplified boiling water reactor (ESBWR) that uses a natural circulation system for coolant water using a chimney, thereby eliminating the need for a recirculation pump in a BWR or an internal pump in an ABWR, or a resource-renewable boiling water reactor (RBWR). [Example]

[0021] A MOX fuel assembly according to a first embodiment, which is a preferred embodiment of the present invention and is applied to an advanced boiling water reactor (ABWR) plant, will be described with reference to FIGS. 1, 2 and 3. FIG. Before describing the MOX fuel assembly according to this embodiment, a schematic structure of a nuclear reactor in an advanced boiling water reactor (ABWR plant) to which this MOX fuel assembly is applied will be described with reference to FIG. 3 . A nuclear reactor 20 in an ABWR plant has a reactor pressure vessel 21, in which a core 22 loaded with a plurality of fuel assemblies 10 (see FIGS. 1 and 2 ) is disposed. Within the reactor pressure vessel 21, a cylindrical core shroud 23 surrounds the core 22, and a steam-water separator 24 disposed above the core 22 is installed at the upper end of the core shroud 23. Furthermore, a steam dryer 25 is installed within the reactor pressure vessel 21 above the steam-water separator 24. An annular downcomer 27 is formed between the outer surface of the core shroud 23 and the inner surface of the reactor pressure vessel 21. An internal pump 26 disposed in a downcomer 27 extends downward through the bottom of the reactor pressure vessel 21 and is attached to the bottom of the reactor pressure vessel 21. A feedwater pipe 28 and a main steam pipe 29 are connected to the reactor pressure vessel 21.

[0022] A lower plenum 31 is formed below the core 22 within the reactor pressure vessel 21. A plurality of control rod guide tubes (not shown) are arranged in the lower plenum 31. A plurality of control rods 30 (see FIG. 1), each with a cross-shaped cross section, that control the nuclear reaction of the fuel assemblies 10 are arranged separately in each control rod guide tube. A plurality of control rod drive mechanism housings (not shown) are attached to the bottom of the reactor pressure vessel 21 and extend downward from the bottom. A control rod drive mechanism (not shown) is arranged in each control rod drive mechanism housing and connected to the control rod 30.

[0023] As shown in FIG. 2, the multiple fuel assemblies 10 loaded in the reactor core 22 each include multiple fuel rods 11, a lower tie plate 17 (lower fuel support member), an upper tie plate (upper fuel support member) 18, multiple fuel spacers 15 arranged in the axial direction, and a channel box 16. The fuel assembly 10 is a MOX fuel assembly containing MOX fuel. Each fuel rod 11 has a cladding tube (not shown), the lower end of which is sealed with a lower end plug (not shown), and the upper end of which is sealed with an upper end plug (not shown). The cladding tube is filled with a plurality of fuel pellets (not shown) containing nuclear fuel material. A gas plenum (not shown) is formed within the cladding tube above the nuclear fuel material filling area where the fuel pellets are filled.

[0024] The channel boxes 16 are provided at two of the four corners of the upper tie plate 18 and are placed on two posts (not shown) that extend upward from the top surface of the upper tie plate 18. The upper end of the channel box 16 is attached to one of the posts facing the control rod 30 by a channel fastener (not shown). In this manner, the channel box 16 is attached to the upper tie plate 18. The channel box 16 extends from the upper tie plate 18 toward the lower tie plate 17. The channel box 16 surrounds each side of the upper tie plate 18 and the lower tie plate 17.

[0025] In a core 22 loaded with a plurality of fuel assemblies 10, one control rod 30 is substantially arranged for every four adjacent fuel assemblies 10. The upper ends of the four adjacent fuel assemblies 10 loaded in the core 22 are inserted into squares formed in an upper grid plate (not shown) that is disposed at the upper end of the core 22 and removably attached to the core shroud 23, and are supported by the upper grid plate. The lower ends of the fuel assemblies 10 are supported by fuel support fittings attached to a core support plate (not shown) that is disposed at the lower end of the core 22 and attached to the core shroud 23. In each of the four adjacent fuel assemblies 10, one of the four corners present in the cross section of the fuel assembly 10, i.e., the corner where a channel fastener (not shown) is disposed, faces the control rod 30.

[0026] A channel fastener (not shown) is arranged at one of the four corners of each fuel assembly 10 loaded in the core 22, facing the control rod 30. The channel fasteners (not shown) arranged at the corners of four adjacent fuel assemblies 10 contact each other to maintain a predetermined distance between the adjacent fuel assemblies 10 so that a control rod 30 can be inserted between the four fuel assemblies 10 (see paragraphs 0003 and 0005 and FIG. 2 of Patent Document 2, and paragraphs 0013, 9, and 10 of Patent Document 3). Each of the four fuel assemblies 10 is pressed against the upper grid plate by the channel fastener (not shown). One control rod 30 is inserted between four adjacent fuel assemblies 10. Reactor power is controlled by inserting and withdrawing the control rod 30 between the adjacent fuel assemblies 10. When the control rod 30 is inserted between the four fuel assemblies 10, the two blades of the control rod 30 face the outer surfaces of two side wall portions extending in two perpendicular directions from one corner of the channel box 16 facing the control rod 30, i.e., one corner of the channel box 16 (the corner on the channel fastener side) where the channel fastener (not shown) is located, as shown in Figure 1. A handle 19 (FIG. 2) is provided on the top surface of the upper tie plate 18. The handle 19 is grasped by a refueling machine (not shown) when transferring the fuel assembly 10 between the reactor core 22 and a fuel storage pool (not shown).

[0027] Cooling water in the downcomer 27 is discharged from the driven internal pump 26 and supplied to the reactor core 22 via the lower plenum 31. Within the reactor core 22, the cooling water is guided into the fuel assemblies 10. That is, the cooling water flows into the inside of the channel box 16 through the lower tie plate 17 and rises through the cooling water passages formed between the fuel rods 11. While rising through these cooling water passages, the cooling water is heated by the heat generated by the nuclear fission of the fissile material (fissile plutonium or fissile uranium) contained in the nuclear fuel material present in the fuel rods 11. Because part of the cooling water turns into steam, the cooling water becomes a gas-liquid two-phase flow containing water and steam.

[0028] This two-phase gas-liquid flow is discharged from the top of the fuel assembly 10, i.e., from the core 22, and flows into the steam separator 24. The two-phase gas-liquid flow is separated into water and steam in the steam separator 24. The separated water is discharged from the steam separator 24 into the downcomer 27, descends through the downcomer 27 as cooling water, and is pressurized by the internal pump 26. The separated steam is guided from the steam separator 24 to the steam dryer 25, where moisture is removed. The steam discharged from the steam dryer 25 after moisture removal is guided to a steam turbine (not shown) through a main steam pipe 29 and drives the steam turbine. A generator (not shown) connected to the steam turbine rotates, generating electricity. The steam discharged from the steam turbine is condensed into water in a condenser (not shown). This condensed water is supplied as feedwater into the reactor pressure vessel 21 through a feedwater pipe 28.

[0029] As shown in FIG. 1 , the fuel assembly 10 loaded in the core 22 has a plurality of fuel rods 11 arranged in a channel box 16 in a square lattice of 10 rows and 10 columns in the cross section of the fuel assembly 10. As shown in FIG. 1 , the fuel assembly 10 according to this embodiment does not have water rods. There are 100 fuel rods 11 in the cross section of the fuel assembly 10. The plurality of fuel rods 11 include full-length fuel rods 1 and part-length fuel rods 2. The part-length fuel rods 2 are shorter in length than the full-length fuel rods 1. In other words, the active fuel length of the part-length fuel rods 2 is shorter than the active fuel length of the full-length fuel rods 1.

[0030] In this embodiment, 24 part-length fuel rods 2 are arranged in the above-mentioned region A in the center of the fuel assembly 10, and the void reactivity coefficient at a high void fraction is shifted to the negative side compared to a cross section of the same Pu enrichment but without the part-length fuel rods 2. For example, under the condition that the average Pu enrichment of the cross section is 11.7%, the void reactivity coefficient when the void fraction changes from 40% to 70% is 7.78 × 10 compared to when the part-length fuel rods 2 are not arranged. ―4 In this embodiment, 24 part-length fuel rods 2 are arranged in the above-mentioned region A in the center of the fuel assembly 10, and the moderator area in region A is 30% of the cross-sectional area of ​​the fuel assembly 10.

[0031] As described above, according to this embodiment, it is possible to provide a fuel assembly and a reactor core that can achieve high enrichment and make the void reactivity coefficient a negative value even under conditions that do not take neutron leakage or control rods into consideration. Furthermore, according to this embodiment, the void reactivity coefficient is increased by 7.78×10 compared to the case where the part-length fuel rods 2 are not arranged. ―4 It can be shifted to the negative side by Δk / k / %VOID. [Example]

[0032] 7 is a cross-sectional view of a MOX fuel assembly applied to an advanced boiling water nuclear power plant according to a second embodiment of the present invention. In this embodiment, a plurality of fuel rods 11 are arranged in a channel box 16 in a square lattice of 12 rows and 12 columns in the cross-section of the fuel assembly 10A, and differ from the first embodiment in that 44 part-length fuel rods 2 are arranged in the above-mentioned region A in the center of the fuel assembly 10A. The same components as those in the first embodiment are designated by the same reference numerals, and redundant explanations will be omitted below.

[0033] As shown in Fig. 7, a fuel assembly 10A according to this embodiment, which is applied to an advanced boiling water reactor (ABWR) plant, is a MOX fuel assembly, and a plurality of fuel rods 11 are arranged in a channel box 16 in a square lattice of 12 rows and 12 columns in a cross section of the fuel assembly 10A. The plurality of fuel rods 11 include full-length fuel rods 1 and part-length fuel rods 2. As in the first embodiment, the part-length fuel rods 2 are shorter in length than the full-length fuel rods 1.

[0034] In this embodiment, 44 part-length fuel rods 2 are arranged in the above-mentioned region A in the center of the fuel assembly 10A, and the moderator area in region A is 26% of the cross section of the fuel assembly. In this embodiment, too, the void reactivity coefficient at high void fraction is shifted to the negative side compared to a cross section of the same Pu enrichment where part-length fuel rods 2 are not arranged. For example, under the condition that the average Pu enrichment of the cross section is 11.7%, the void reactivity coefficient when the void fraction changes from 40% to 70% is shifted to the negative side by 12.7 × 10 compared to when part-length fuel rods 2 are not arranged. ―4 It can be shifted to the negative side by Δk / k / %VOID.

[0035] According to this embodiment, in addition to the effect of the first embodiment, the void reactivity coefficient can be further shifted to the negative side.

[0036] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. [Explanation of symbols]

[0037] 1…Full length fuel rod 2…part length fuel rod 10,10A…Fuel assembly 11...Fuel rod 15...Fuel spacer 16...Channel Box 17...Lower tie plate 18...Upper tie plate 19...Handle 20…Nuclear reactor 21...Reactor pressure vessel 22...Reactor core 23... Core shroud 24...Steam water separator 25...Steam dryer 26...Internal pump 27...Downcoma 28...Water supply pipe 29...Main steam pipe 30...Control rod 31...Lower plenum

Claims

1. A fuel assembly including, excluding water rods, full-length fuel rods, part-length fuel rods having an effective fuel length shorter than that of the full-length fuel rods, a channel box, a fuel spacer, an upper tie plate, and a lower tie plate, A fuel assembly comprising a part-length fuel rod containing plutonium in at least one of the full-length fuel rods and the part-length fuel rods, the part-length fuel rod having a region A in which the distance to the water gap is 0.4L or more, where L is the shortest distance from the central axis of the fuel assembly to the water gap, and the part-length fuel rod having the central axis within region A.

2. 2. The fuel assembly of claim 1, 2. A fuel assembly comprising: full-length fuel rods and part-length fuel rods arranged in 10 rows and 10 columns in a cross section of the fuel assembly.

3. 2. The fuel assembly of claim 1, 12. A fuel assembly, comprising: full-length fuel rods and part-length fuel rods arranged in 12 rows and 12 columns in a cross section of the fuel assembly.

4. 2. The fuel assembly of claim 1, A fuel assembly characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

5. 3. The fuel assembly of claim 2, A fuel assembly characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

6. 4. The fuel assembly of claim 3, A fuel assembly characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

7. A reactor core loaded with a plurality of fuel assemblies, The fuel assembly includes, excluding water rods, full-length fuel rods, part-length fuel rods having an effective fuel length shorter than that of the full-length fuel rods, a channel box, a fuel spacer, an upper tie plate, and a lower tie plate, A reactor core characterized in that at least one of the full-length fuel rods and the part-length fuel rods contains plutonium, and has a region A where the distance to the water gap is 0.4L or more, where L is the shortest distance from the central axis of the fuel assembly to the water gap, and the core is provided with part-length fuel rods whose central axes are within region A.

8. 8. The reactor core of claim 7, A reactor core characterized in that the full-length fuel rods and part-length fuel rods are arranged in 10 rows and 10 columns in a cross section of the fuel assembly.

9. 8. The reactor core of claim 7, A reactor core characterized in that the full-length fuel rods and part-length fuel rods are arranged in 12 rows and 12 columns in a cross section of the fuel assembly.

10. 8. The reactor core of claim 7, A reactor core characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

11. 9. The reactor core of claim 8, A reactor core characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

12. 10. The reactor core of claim 9, A reactor core characterized in that the area of ​​the moderator in the region A is within 30% of the area of ​​the cross section of the fuel assembly.

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