JP3318210B2 - MOX fuel assembly and core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a MOX fuel assembly having a mixed oxide (MOX) fuel rod of uranium and plutonium loaded in a core of a boiling water reactor.

[0002]

2. Description of the Related Art Generally, in a core of a boiling water reactor, the reactor is shut down after a predetermined period (one cycle) of operation, and a part of loaded fuel is taken out and replaced with new fuel. Is done. Usually, the number of fuels to be exchanged is about 1/4 to 1/3 of the whole core. Once loaded, the fuel stays in the furnace for a period of 3-4 cycles before being removed from the core.

[0003] A core in which the number of fuels exchanged in each operation cycle is constant is called an equilibrium core, and fuels having different burnups stay in the furnace in accordance with the stay period. On the other hand, in the initially loaded core composed only of new fuel, after operation is performed for the first cycle, a part of the fuel is taken out and replaced with new fuel. The fuel taken out after the first cycle has lower burnup and lower energy generation than the fuel in the equilibrium core. For this reason, the initially loaded core generally employs a configuration including a plurality of fuels with different uranium enrichments depending on the length of stay in the furnace, thereby improving fuel economy.

[0004] As a prior art relating to such an initially loaded core, there is, for example, Japanese Patent Application Laid-Open No. 5-249270. The publication discloses a high enrichment fuel with an average enrichment of 3.4% by weight of fuel assemblies, a medium enrichment fuel of 2.3% by weight, and a 1.1% by weight of a fuel assembly.
A low-enriched fuel core is described. It also describes that fuel assemblies with lower average enrichment are taken out of the core in order, and fuel assemblies with higher average enrichment stay longer in the furnace.

On the other hand, in Japan, it is planned to use MOX fuel for light water reactors. As shown in FIG. 2, the neutron absorption cross section of plutonium is large in the thermal neutron region and the resonance absorption region. As shown in the figure, plutonium contained in MOX fuel is easier to absorb neutrons than uranium. In the case of using MOX fuel, a design has conventionally been made in consideration of this feature. For MOX fuel, it is effective to increase the burnup by increasing the plutonium enrichment in order to improve fuel economy. in this case,
The difference between the neutron absorption characteristics of the MOX fuel and the uranium fuel described above increases.

In particular, when a plurality of fuel assemblies having different average enrichments as described above are loaded in the initially loaded core, the difference in nuclear properties between the low-enriched uranium fuel and the MOX fuel increases. When fuel assemblies having a large difference in nuclear properties are adjacent to each other, neutron exchange occurs because the neutron spectra of adjacent fuel assemblies are different. For this reason, improvement of the thermal margin (maximum linear power density, minimum critical power ratio) at the beginning of the operation cycle has been a challenge.

Prior art techniques for improving this thermal margin include increasing the number of water rods or types of plutonium enrichment to reduce local output peaking. However, the former reduces fuel loading and the latter increases manufacturing costs.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an MO
An object of the present invention is to provide a MOX fuel assembly and a core that can improve the thermal margin even when the X fuel assembly and the uranium fuel assembly are mixed in the core.

[0009]

[0010]

[0011]

DISCLOSURE OF THE INVENTION The MOX fuel assembly of the present invention
The body has an average uranium enrichment U of the burnable poison-filled region of the burnable poisoned fuel rod and an average fissile plutonium enrichment E of the MOX fuel rod of U / E ≧ 1.3. Is configured to satisfy the relationship of

[0012]

In order to increase the thermal margin in the fuel assembly design, (1) reducing the output peaking (local output peaking) of the fuel rod of interest, and (2)
It is effective to reduce the output peaking of the fuel rods around the fuel rod of interest.

The operation of the present invention will be described below with reference to FIGS. 3 to 5 based on the results of analysis by the present inventors.

In a core in which fuel assemblies having different nuclear characteristics coexist, if the fuel rod of interest is located at the outermost periphery of the fuel assembly, the local power peaking is determined by the distribution of enrichment in the fuel assembly and adjacent fuel rods. It is determined by the type of the fuel assembly. Assuming the case where it is adjacent to the low-enrichment fuel assembly loaded in the control cell in the first loading core,
Local power peaking and B / A ratio of MOX fuel assembly (A: average fissionable plutonium enrichment of MOX fuel rod in outermost layer region, B: average fissile plutonium enrichment of MOX fuel rod in innermost region) FIG. 3 shows an analysis example in which the relationship is obtained. From the figure, it is effective to set B / A to 2.2 or more in order to make the local output peaking at the early fuel life (BOL) 1.1 or less at the end of fuel life (EOL). By setting the local output peaking ratio to 1.1 or less, it is possible to improve the thermal margin through combustion.

FIG. 4 shows the outermost layer region (facing low enrichment) when the enrichment distribution is uniform in the MOX fuel assembly adjacent to the low enrichment fuel assembly loaded in the control cell. 7 is an example of analysis of local output peaking of FIG.
The corner fuel rod with the highest output has output peaking twice or more the average of the outermost layer region. Therefore,
In order to flatten the local power peaking in the outermost layer region, the C / A ratio (A: the average fissionable plutonium enrichment of the MOX fuel rods in the outermost region, C: the fissile average of the fuel rods in the corner region) It is effective to set the plutonium enrichment to 0.5 or less. This corresponds to enlarging the enrichment distribution difference in the MOX fuel assembly, and the D / C ratio (D: the largest fissile plutonium enrichment of the MOX fuel rods of the fuel assembly, C : Average fissile plutonium enrichment in the corner fuel rods) is 4.5 or more.

In the MOX fuel assembly, a burnable poison fuel rod filled with burnable poison and uranium is disposed to control the excess reactivity. For burnable poisons to reduce local power peaking after combustion, burnable poison rods and MO
It is necessary to make the output of the X fuel rod equal. In the spectrum of the light water reactor, the MOX fuel has a fission cross section of 30% or more of the heat group as compared with the uranium fuel having the same fissile material weight ratio. Therefore, in order to equalize the output of the burnable poison rod and the MOX fuel rod, the average uranium enrichment of the burnable poison fuel rod in the region having the burnable poison is determined by the average fissile plutonium enrichment of the MOX fuel rod. It is effective to set 1.3 or more.

The MOX fuel assembly is loaded into the control cell 1 in the initially loaded core.
5 is shown in FIG. The region (X) adjacent to only one wing of the control rod when the MOX fuel assembly is divided into four equal parts by a cross parallel to the control rod 3 is most strongly affected by the adjacent low enrichment fuel. In order to improve the thermal margin of this area without breaking the symmetry of the enrichment distribution, it is effective to reduce the local output peaking of the second layer area adjacent to the outermost layer area (X area). . Specifically, it is effective that the number of the burnable poison fuel rods arranged in the second layer is (region adjacent to only one wing of the control rod)> (region other than the one on the left).

In the above description, the case where the number of grids of fuel rods is 8 × 8 in the related art is taken up. However, the number of grids such as 9 × 9, 10 × 10, etc., which is more suitable for higher burnup, is taken. The same applies to a fuel assembly with increased.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) A first embodiment of the present invention
The fuel assembly of the OX core uses the fuel assembly shown in FIG. The fuel assembly 4 includes fuel rods arranged in an 8 × 8 grid, a single large-diameter water rod 5 at the center, and a channel box 6 surrounding these fuel rods and the water rod. The numbers shown on the fuel rods indicate the differences in the enrichment levels and are as follows. The fissionable Pu enrichment of the MOX fuel rods 7, 8, 9, 10 is 5.4 wt%, 2.8 wt%, 2.1 wt%, respectively.
%, 1.1 wt%, and 12 of the 60 fuel rods
This is a burnable poison fuel rod 11 having a uranium enrichment of 4.5 wt%. Therefore, B of this aggregate is 5.4 and A is 2.36.
And B / A is 2.29. C is 1.
1, D is 5.4, C / A is 0.47, D / C is 4.
It is 9.

As a result, even if the present embodiment is disposed adjacent to a low-enrichment fuel assembly with a uranium enrichment of about 1.5 wt% loaded in the control cell, local output peaking at the beginning of an operation cycle can be reduced, and Thermal margin can be secured throughout the cycle.

(Embodiment 2) The second embodiment of the present invention
The fuel assembly of the OX core uses the fuel assembly shown in FIG. The fuel assembly 14 is composed of fuel rods arranged in an 8 × 8 grid, a single large-diameter water rod 15 at the center, and a channel box 16 surrounding these fuel rods and water rods as in the first embodiment. It is configured. The numbers shown on the fuel rods indicate the differences in the enrichment levels and are as follows. MOX fuel rods 17, 18, 1
The fissionable Pu enrichment of 9,20 is 5.5 wt.
%, 3.0 wt%, 2.1 wt%, and 1.2 wt%, and 12 of the 60 fuel rods have uranium enrichment of 4.9 wt%.
The burnable poison fuel rod 21 of FIG. Therefore, B of this aggregate is 5.5, A is 2.23, and B / A is 2.40.
It has become. C is 1.2, D is 5.5, and C
/ A is 0.5 and D / C is 4.6. U is 4.9, E is 3.6, and U / E is 1.36.

In this embodiment, as in the first embodiment, a thermal margin can be secured throughout the operation cycle. In this embodiment, as a result of increasing the uranium enrichment of the burnable poison fuel by 0.5 wt% from the embodiment 1, even if the enrichment of fissionable Pu other than the outermost periphery of the fuel assembly is 5.5 wt%, the fuel life is increased. Local output peaking at the last stage can be made equivalent to that of the first embodiment. As a result, the local output peaking at the beginning of the fuel life can be further reduced by 2% compared to the first embodiment.

(Embodiment 3) The third embodiment of the present invention
The fuel assembly shown in FIG. 7 is used as the fuel assembly of the OX core. The fuel assembly 24 includes fuel rods arranged in an 8 × 8 lattice, a single large-diameter water rod 25 at the center, and a channel box 26 surrounding these fuel rods and water rods, as in the first embodiment. It is configured. The numbers shown on the fuel rods indicate the differences in the enrichment levels and are as follows. MOX fuel rods 27, 28, 2
The fissionable Pu enrichment of 9,30 is 5.4 wt.
%, 2.8 wt%, 2.0 wt% and 1.1 wt%, and 12 of the 60 fuel rods have uranium enrichment of 4.9 wt%.
The burnable poison fuel rod 21 of FIG. Therefore, B of this aggregate is 5.4, A is 2.33, and B / A is 2.32.
It has become. C is 1.1, D is 5.4, and C
/ A is 0.47 and D / C is 4.9. Also, U
Is 4.9, E is 3.6, and U / E is 1.36.

In this embodiment, as in the first embodiment, a thermal margin can be secured throughout the operation cycle. further,
In the present embodiment, when the MOX fuel assembly is divided into four equal parts by a cross parallel to the control rod 3, the burnable poison fuel rod arranged in the second layer area of the area (X) adjacent to only one wing of the control rod The number is two, and one in the other area satisfies the relationship of (area adjacent to only one wing of the control rod)> (area other than the one on the left). Region X where local output peaking becomes severe
The output of the outermost fuel rod can be kept low. As a result, the minimum limit power ratio at the beginning of the fuel life can be further increased by 2% compared to the first embodiment.

(Embodiment 4) FIG. 8 shows a fourth embodiment of the present invention.
Shown in The fuel assembly 34 includes fuel rods arranged in a 9 × 9 lattice and two large-diameter water rods 35 at the center.
And a channel box 36 surrounding these fuel rods and water rods. By determining the enrichment distribution in the same manner as in the first embodiment, the same effect as in the first embodiment can be obtained. In this case, since the average linear output density is reduced by using a 9 × 9 grid, an effect of further improving the linear output density can be obtained.

FIG. 9 shows another fuel assembly configuration of the present invention. The fuel assembly 44 is composed of fuel rods arranged in a 9 × 9 grid, a single square tubular water rod 45 at the center, and a channel box 46 surrounding these fuel rods and the water rod. . By making the enrichment degree distribution similar to that of the first embodiment, the same effect as that of the first embodiment can be obtained. In this embodiment, the effect of improving the void coefficient can be obtained by increasing the water rod region.

(Embodiment 5) By loading at least one or more of the MOX fuel assemblies shown in Embodiments 1 to 4 in the core, the effect of improving the thermal margin can be obtained. But,
The MOX fuel in the region inside the outermost periphery having a high output in the reactor core, particularly in the region adjacent to the control cell composed of the low-enriched uranium fuel assembly, was removed from the MOX fuel assembly shown in the above Examples 1 to 4. By improving the body, the improvement effect can be increased.

[0030]

According to the present invention, even when the MOX fuel assembly and the uranium fuel assembly are mixed in the core, the thermal margin can be improved.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an 8 × 8 type fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a comparison of neutron absorption cross sections.

FIG. 3 is a diagram showing a relationship between local output peaking and a B / A ratio.

FIG. 4 is a diagram showing local output peaking in the outermost peripheral region.

FIG. 5 is a view showing a fuel assembly arrangement in a reactor core.

FIG. 6 is a sectional view showing an 8 × 8 fuel assembly according to a second embodiment of the present invention.

FIG. 7 is a sectional view showing an 8 × 8 type fuel assembly according to a third embodiment of the present invention.

FIG. 8 is a sectional view showing a 9 × 9 fuel assembly according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view showing a modification of the fourth embodiment.

[Explanation of symbols]

1 ... control cell, 2 ... low enrichment fuel assembly, 3 ...
Control rods, 4, 14, 24, 34, 44 ... fuel assemblies,
5,15,25,35… Thick water rod, 6,1
6, 26, 36, 46 ... channel box, 7-1
0,17-20,27-30 ... MOX fuel rods 11,12
1, 31: burnable poison fuel rod, 45: square tube type water rod.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hidemitsu Shimada 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi, Ltd. Power & Electric Equipment Development Division (72) Inventor Kunikazu Kando, Hitachi City, Ibaraki Prefecture Hitachi 1-1, Hitachi, Ltd. (72) Inventor Sadayuki Izutsu 3-2-1 Sachimachi, Hitachi, Ibaraki Pref. Hitachi Engineering Co., Ltd. (72) Inventor Satoshi Fujita, Sachiyuki Hitachi, Ibaraki Hitachi Engineering Co., Ltd. 3-2-1 Machi-cho (72) Inventor Junichi Koyama 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. 7-2-1, Omika-cho, Hitachi City Hitachi, Ltd. Power and Electricity Development Division (56) References JP JP-A-10-2980 (JP, A) JP-A-9-304566 (JP, A) JP-A-8-86894 (JP, A) JP-A-7-270563 (JP, A) JP-A-7-244181 (JP) JP-A-6-347578 (JP, A) JP-A-5-134074 (JP, A) JP-A-4-236393 (JP, A) JP-A-4-204291 (JP, A) 4-9796 (JP, A) JP-A-3-279891 (JP, A) JP-A-3-128482 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G21C 3/328 G21C 5/00

Claims (8)

(57) [Claims] 1. A MOX fuel assembly in which a plurality of fuel rods including a MOX fuel rod and a burnable poison fuel rod filled with burnable poison and uranium are arranged in a square lattice, wherein the burnable poison fuel is The average uranium enrichment U of the burnable poison-filled area of the rod and the average fissile plutonium enrichment E of the MOX fuel rod satisfy the relationship of U / E ≧ 1.3. A MOX fuel assembly, comprising: 2. A fuel cell system according to claim 1, wherein the fuel assembly is divided into an outermost layer region in which an outermost fuel rod is loaded and an inner layer region inside the outermost region in a cross section perpendicular to the axial direction. The average fissile plutonium enrichment A of the MOX fuel rods in the outermost layer region and the average fissile plutonium enrichment B of the MOX fuel rods in the innermost region satisfy the relationship of B / A ≧ 2.2. A MOX fuel assembly characterized by having a configuration as described above. 3. Oite to claim 2, mean and fissile plutonium enrichment C of the fuel rods of the outermost corners, is said fissile plutonium enrichment A, C / A ≦ 0.5 Characterized by satisfying the following relationship:
Fuel assembly. 4. Oite to claim 2 or 3, and a maximum fissile plutonium enrichment D of the MOX fuel rods, a fissile plutonium enrichment C average fuel rods of the outermost corners Characterized by satisfying the relationship of D / C ≧ 4.5. 5. Oite to any one of claims 1 to 4, further comprising a burnable poison and uranium filled burnable poison rods, it is loaded in a reactor core having a control rod of cruciform M
An OX fuel assembly, wherein the fuel assembly is divided into four equal parts by a cross parallel to the control rod, and the four regions are adjacent to only one wing of the control rod.
When divided into a region and a second region other than the region, the number of burnable poison fuel rods loaded in the second layer from the outermost layer in the first region is smaller than that in the second region. A MOX fuel assembly characterized by a large number. 6. Oite to any one of claims 1 to 5, wherein M
A MOX fuel assembly, wherein the uranium base material of the OX fuel rod is depleted uranium. 7. A MOX fuel assembly, characterized in that Oite to any one of claims 1 to 6, wherein the burnable poison is gadolinia. 8. A plurality of fuel assemblies, and control rod to be inserted between the fuel assemblies, in a nuclear reactor core that includes a furnace meter Sokan to any one of claims 1 to 7 MOX described
A nuclear reactor core, wherein at least one fuel assembly is loaded.