JPH05164869A - Fuel assembly - Google Patents

Abstract

PURPOSE:To average the output distribution in an assembly by arranging fuels whose average pellet density in the area is larger, on the area where the ratio of hydrogen to atomic number is larger in the operation. CONSTITUTION:A fuel assembly 1 of a square form is composed of a channel box 2, 28 fuel rods 3, 32 fuel rods 4, and one water rod 5. The volume ratio of water to fuel is about 3.2, and a soft system of neutrons is formed. The fuel pellet densities of the fuel rods 3 and 4 are about 10.75 and about 10.28g/cm<3> respectively, and the average density is about 10.5g/cm<3>. Both the fissionable plutonium enrichments of the fuel rods 3 and 4 are about 3.5w/o, and they are obtained by enriching a natural uranium. Since the fuels of a larger pellet density are used for the fuels at the periphery near the gap water where the output is liable to be increased, and the neutron spectrum there is hardened so as to reduce the output in such a way, the output peaking is reduced about 0.5 compared with the fuels in the uniform pellet density, and the output distribution is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly used in a boiling water reactor, and more particularly to flattening the power distribution in a fuel assembly having a mixed oxide fuel of uranium-plutonium (MOX fuel). Fuel assembly.

[0002]

2. Description of the Related Art Generally, in a boiling water reactor, a plurality of fuel rods are arranged in a channel box, and non-boiling gap water exists outside the channel box. Therefore, in the vicinity of this gap water, a good state of neutron moderation locally occurs, and the neutron spectrum becomes soft.

On the other hand, fissile plutonium such as ²³⁹
Pu has a fission cross section in the thermal energy region (1 eV or less) that is more than twice as large as that of ²³⁵ U. Moreover, ²³⁹ Pu
As shown in FIG. 2, the ratio of the fission cross section in the thermal energy region to the fission cross section in the higher energy region is larger than that of ²³⁵ U shown in FIG. Therefore,
When plutonium fuel is used in the existing thermal neutron reactor, the fuel output in the peripheral portion near the gap water tends to be larger than when uranium fuel is used.

As a conventional means for solving this problem and realizing flattening of the output distribution, there is, for example, JP-A-60-147685. This is to make the fissile plutonium enrichment of the fuel in the peripheral portion where the output peak is likely to occur lower than that of the fuels in other regions and to flatten the output distribution.

[0005]

However, the above-mentioned prior art uses the fuel having a relatively high fissionable plutonium enrichment in the central portion of the fuel assembly having a hard neutron spectrum. Occurs. That is, in the central part, difficult to proceed the combustion because the neutron spectrum is hard in the combustion end, fissile materials have remained more than the peripheral portion, and by conversion of the parent substance (e.g., ²³⁸ U), the fissile material A large amount (for example, ²³⁹ Pu) is generated. As described above, since more fissile material exists in the central portion than in the peripheral portion, the output has a distribution having a peak in the central portion at the end of combustion. Further, a large amount of fissile material remains unburned in the central portion, which is not preferable from the viewpoint of fuel economy.

An object of the present invention is to flatten the power distribution in the fuel assembly without deteriorating the fuel economy.

[0007]

The above object is achieved by using a fuel having a higher pellet density in the peripheral portion where the neutron spectrum is soft and the output is high in the early stage of combustion, as compared with the fuel in other regions. ..

[0008]

The function of the present invention will be described below.

Generally, when the ratio of the number of hydrogen atoms to the number of fuel atoms increases, the ratio of hydrogen atoms per fuel atom increases, so that neutrons are likely to be slowed down, resulting in a soft neutron spectrum. As shown in FIG. 2 and FIG. 3, the fissile nuclide has a large fission cross section in the thermal energy region. Therefore, in general, when the neutron spectrum becomes soft, the infinite multiplication factor of neutrons at the initial stage of combustion increases.

On the other hand, when a fuel having a low pellet density is used in a fuel assembly of the same shape, the amount of the fuel substance is reduced, so that the ratio of hydrogen to fuel atoms is increased. As a result, the neutron spectrum becomes soft and the infinite neutron multiplication factor in the early stage of combustion increases.

As a concrete example, FIG. 4 shows the relationship between the pellet density and infinite neutron multiplication factor in plutonium fuel. Note that the infinite neutron multiplication factor is pellet density 10.5.
The difference is shown with reference to the case of g / cc (conventional fuel). Also, the fissile plutonium enrichment is
It is 3.5 w / o. In the early stage of combustion, even with the same fissile plutonium enrichment, the infinite neutron multiplication factor increases by about 1% Δk when the pellet density decreases by about 10%. However, at the average take-out burnup of 30 GWd / t, the difference is reduced to about 0.3% Δk.

Therefore, by using a fuel having a higher pellet density than other regions in the peripheral portion having a soft neutron spectrum and close to the gap water, the output distribution can be flattened through combustion. In other words, the output of the peripheral portion near the gap water, which tends to increase in the early stage of combustion and decrease in the latter stage of combustion, is lowered in the early stage of combustion without lowering the fissile plutonium enrichment in that region, and , It becomes possible to raise it at the end of combustion. Moreover, since the fissile plutonium enrichment of the fuel in the central portion is not relatively high, the fuel economy does not deteriorate as described above.

[0013]

EXAMPLES The fuel assembly of the present invention will be described below with reference to examples.

FIG. 1 shows a first embodiment of the fuel assembly according to the present invention. In the present embodiment, the fuel assembly 1 has a quadrangular shape, and includes the channel box 2, 28 fuel rods 3, 32 fuel rods 4 and one water rod 5.
It consists of The water-to-fuel volume ratio of this fuel assembly is about 3.2, which is a soft system of neutron spectrum. Fuel rod 3 has a fuel pellet density of 10.75 g / c
c plutonium fuel loaded, fuel rod 4,
It was loaded with plutonium fuel having a fuel pellet density of 10.28 g / cc. The average fuel pellet density of the assembly is 10.5 g / cc as in the conventional fuel. The fuel rods 3 and 4 each have a fissile plutonium enrichment of 3.5 w / o and are enriched in natural uranium.

In the present embodiment, the fuel having a higher pellet density than other regions is used as the fuel in the peripheral portion which is close to the gap water and tends to have a high output, thereby making the neutron spectrum hard and reducing the output. is doing. as a result,
Compared to a fuel with a uniform pellet density, the output peaking in the fuel assembly is reduced by about 0.5%, which has the effect of improving the output distribution. Moreover, since the fissile plutonium enrichment in the central portion is not increased, fuel economy is not impaired.

FIG. 5 is a diagram showing a second embodiment of the fuel assembly according to the present invention. The fuel assembly of this embodiment is composed of forty fuel rods 3 and twenty fuel rods 6. The fuel rod 6 is loaded with plutonium fuel having a fuel pellet density of 10.0 g / cc. The average fuel pellet density of the assembly is 10.5 g / cc. The fuel rod 6 has a fissile plutonium enrichment of 3.5 w / o,
Enriched with natural uranium.

By the way, since the water in the water rod does not boil like the gap water, a good state of neutron moderation locally occurs around the water rod. In consideration of this point, in this example, fuel having a large pellet density was used around the gap water and the water rod. As a result, the output peaking in the fuel assembly is reduced by about 0.8% as compared with the fuel having a uniform pellet density, which has the effect of improving the output distribution. Moreover, since the fissile plutonium enrichment in the central portion is not increased, fuel economy is not impaired.

FIG. 6 is a diagram showing a third embodiment of the fuel assembly according to the present invention. The fuel assembly of this embodiment is composed of four fuel rods 7, twenty four fuel rods 8, twelve fuel rods 9 and twenty fuel rods 10. Fuel rod 7 was loaded with uranium fuel having a pellet density of 10.5 g / cc and enrichment of 2.0 w / o. Fuel rod 8 had a pellet density of 10.75 g / cc and a fissile plutonium enrichment. Fuel rod 9 loaded with 2.0 w / o plutonium fuel, fuel rod 9 loaded with plutonium fuel having a pellet density of 10.75 g / cc and a fissile plutonium enrichment of 4.5 w / o, fuel rod 10 Has a pellet density of 10.
Fissile plutonium enrichment of 4.5w at 05g / cc
/ O plutonium fuel was loaded. The average fuel pellet density of the assembly is 10.5 g / cc. Plutonium is enriched in natural uranium.

In this embodiment, in order to reduce the output peaking, the fissionable plutonium enrichment of the fuel in the peripheral portion near the gap water is lowered, and uranium fuel is used especially in the corner portion where the output peak is likely to occur. .. As a result, the output distribution in the fuel assembly can be further flattened, in addition to the effect of using the fuel having a high pellet density in the peripheral portion and the portion adjacent to the water rod.

[0020]

EFFECTS OF THE INVENTION According to the present invention, a fuel having a higher pellet density than that of fuel in other regions is used in the soft portion of the neutron spectrum, in the peripheral portion close to the gap water, or in the portion close to the water rod. The output distribution within the fuel assembly can be flattened without deteriorating the fuel economy.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of a fuel assembly of the present invention.

FIG. 2 is an explanatory diagram showing the relationship between neutron energy and fission cross section of ²³⁹ Pu.

FIG. 3 is an explanatory diagram showing a relationship between neutron energy and a ²³⁵ U fission cross section.

FIG. 4 is an explanatory diagram showing the relationship between the pellet density of plutonium fuel and infinite neutron multiplication factor.

FIG. 5 is a cross-sectional view showing a second embodiment of the fuel assembly of the present invention.

FIG. 6 is a sectional view showing a third embodiment of the fuel assembly of the present invention.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Channel box, 3, 4 ... Fuel rod loaded with plutonium fuel, 5 ... Water rod.

Claims (3)

[Claims] 1. Plutonium, which is loaded into the core of a nuclear reactor using light water as a coolant and moderator and reprocessed from spent fuel, is loaded as all or part of the fuel, and from multiple regions. In this fuel assembly, a fuel having a high area average pellet density is arranged in an area having a high hydrogen-to-fuel atom number ratio during operation. 2. The fuel assembly according to claim 1, wherein when the fuel in the fuel assembly is divided into a first layer fuel and another fuel from the outside, the average fuel pellet density is increased outside. .. 3. The fuel according to claim 1, wherein when the fuel in the fuel assembly is divided into gap water outside the channel box or fuel adjacent to the water rod and other fuel, the average fuel pellet density is outside the channel box. The fuel assembly constructed so that the gap water or the fuel adjacent to the water rod becomes larger.