JPS60222791A - Nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a light water nuclear reactor, and in particular, by providing a distribution of the amount of nuclear fuel (amount of uranium) in the radial direction of the reactor, it is possible to save uranium and improve the mechanical efficiency of the fuel. This article relates to a nuclear reactor suitable for ensuring soundness.

[Background of the invention]

Conventionally, in a nuclear reactor of a boiling water nuclear power plant, an equal amount of uranium is present in the fuel assembly.

If the amount of uranium present in all the fuel assemblies loaded in the reactor is the same, the macroscopic cross-sectional area is constant within the reactor, so neutrons can be distributed anywhere in the reactor.
It is slowed down in the same way, absorbed in the same way, and undergoes nuclear fission in the same way. Therefore, if a chain reaction of nuclear fission is started from neutrons existing in a certain place in the reactor, the descendants created by the chain reaction will not be unevenly distributed in a specific place in the reactor, but will be distributed as uniformly as possible within the reactor. Try to spread it in a distributed way.

Normally, in a nuclear reactor, there is no reflector at the outermost periphery of the reactor, so the neutron flux is generally large at the center of the reactor core and small at the periphery of the core, resulting in an in-reactor neutron flux distribution as shown in Figure 1. .

The output power P of the fuel assembly in the nuclear reactor is given by the following equation.

P=Σφ1 ・σ ・N ・・・・・・・・・ (1)
I where φ,; neutron flux σ of neutron energy group i in the fuel assembly; microscopic nucleus of neutron energy group i as fission cross section N; atomic density i of fissile material in the fuel assembly; neutron energy group Therefore, from the relationship in equation (1) above, the power distribution within the reactor will be similar to the neutron flux distribution shown in Figure 1 if the amount of uranium in all fuel assemblies within the reactor is the same. , which is large in the center of the furnace and becomes smaller toward the periphery.

Burnup is defined as the amount of heat generated per unit amount of uranium. Therefore, if the amount of uranium in each fuel assembly in the reactor is the same, the burnup distribution in the reactor will be similar to the power distribution in the reactor, with high burnup in the center and low in the periphery of the core. It becomes a degree distribution.

From the perspective of effective use of uranium, burnup must be uniform in all fuel assemblies within the reactor.

In other words, it has been found that it is effective to have a uniform burnup distribution within the furnace. To achieve this, it is sufficient to increase the amount of uranium in areas with high output and decrease the amount of uranium in areas with low output.

However, it was previously believed that increasing the amount of uranium in the central region of the reactor core, which has high output, would reduce the thermal margin.

On the other hand, in recent years, techniques for improving thermal margin have been developed. (Japanese Patent Laid-Open No. 53-40188) Thermal margin of the fuel assembly within the nuclear reactor is ensured by suppressing the maximum output within the reactor to a certain value or less.

The maximum power in the reactor is calculated by multiplying the product of the following three peaking coefficients by the average power of the fuel assembly in the reactor. 3
The two peaking coefficients are: (,) Radial output peaking coefficient Ratio between the maximum output of the fuel assembly in the reactor and the reactor average output (b) Axial output peaking coefficient Maximum output in the vertical direction of the fuel assembly and vertical Ratio to directional average power (C) Local power peaking coefficient is the ratio between the maximum power of the fuel rods in the fuel assembly and the average power of the fuel rods in the fuel assembly.

Recently developed technology divides the fuel assembly vertically into two regions from approximately the center, distributes different enrichments, and increases the infinite multiplication factor in the upper region compared to the lower region, which is particularly effective for boiling water. This makes it possible to suppress the increase in axial power peaking caused by the vertical distribution of steam voids, which is unique to type nuclear reactors.

It has been proven that the effect of flattening the axial power distribution is excellent, and it has become widely adopted.

However, as a result of recent fuel technology development, countermeasures for PC engineering (fuel-cladding action) such as barrier fuels have been developed, and the flattening of the output distribution as before is no longer necessary, and the horns and thermal margins have been reduced. Of these, the linear power density can be increased within a range where the health of the fuel can be maintained. Such cores require a new core design that takes advantage of the characteristics of light water reactors.

[Purpose of the invention]

The purpose of the present invention is to reduce the amount of uranium in the fuel assemblies loaded in the radial direction from the center of the reactor core to the periphery by utilizing the characteristics of light water reactors. The object of the present invention is to provide a nuclear reactor suitable for improving uranium conservation and ensuring fuel integrity by making the directional burnup distribution uniform.

[Summary of the invention]

The principle feature of the present invention is that by arranging fuel assemblies with a large amount of uranium in areas of high output in the nuclear reactor and fuel assemblies with a small amount of uranium in areas of low output in the reactor, The purpose is to ensure the mechanical integrity of the fuel by equalizing the burnup distribution within the tank, making effective use of the loaded uranium, and lowering the maximum burnup.

In the present invention, as a method for obtaining the above effect, the amount of uranium in the fuel assembly in the reactor is adjusted by inserting rods that do not contain uranium into the fuel assembly and by changing the number of rods.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail by way of examples.

This example shows an example of a nuclear reactor in which the interior of the nuclear reactor core is divided into three regions and fuel assemblies employing water rods as rods that do not contain uranium are loaded.

FIG. 2 shows radial region division in the nuclear reactor of this embodiment. In FIG. 2, 1 is a fuel assembly placed in area 1, 2 is a fuel assembly placed in area 2, and 3 is a fuel assembly placed in area 3. In this example, region 1 is located at the center of the reactor core,
Regions 2 and 3 are located closer to the core.

FIG. 3 is a cross-sectional view of the fuel assemblies arranged in region l. 4 is a control rod, 5 is a channel box, 6 is a uranium fuel rod, and 7 is a water rod. In FIG. 3, the number of water rods in the fuel assembly arranged in region l is one.

FIG. 4 is an internal sectional view of the fuel assembly arranged in area 2. In FIG. 4, the number of water rods in the fuel assembly arranged in region 2 is four.

FIG. 5 is a cross-sectional view of the fuel assembly arranged in region 3. In FIG. 5, the number of water rods in the fuel assembly arranged in region 3 is 16. Also, Figure 3,
The fuel assembly shown in Figures 4 and 5 has 8 rows and 8 fuel rods.
8x8 fuel arranged in a grid of columns.

In addition, fuel assemblies 1 and 2 are arranged in each of the three areas.
．． The average enrichment of 3 is almost the same.

Therefore, if the average amount of uranium per fuel rod in the fuel assembly in this example is A, the amount of uranium per fuel assembly located in region 1 is MUI, and the amount of uranium per fuel assembly located in region 2 is The amount of uranium per fuel assembly MU2 and the weight of uranium MU3 per fuel assembly placed in area 3 are as follows: MU,1=62・A MU2=60・A MU3=48・A Therefore, MUI, MU2. The following relationship exists in MU3.

MU 1 > MU 2 > MU 3 ...... (2
) Adjusting the amount of uranium by the number of water rods in a fuel assembly is not possible with the current manufacturing technology for manufacturing fuel assemblies because conventional fuel assemblies have one or two water rods. It is considered to be advantageous also because no special manufacturing technology is required.

As in this example, the infinite multiplication factor of a fuel assembly in which the amount of uranium is adjusted by the number of water rods in the fuel assembly is:
If the average concentration is the same, the deceleration effect may be optimized depending on the number of water rods and may increase slightly, but within the range of the number of water rods in this example, it is approximately the same. .

The core of current boiling water reactors is cylindrical and there is no reflector around the core, so neutrons leak more around the core, and the neutron flux distribution is high at the center and low around the core. The distribution will be as shown in Figure 1.

In this embodiment as well, the output distribution is as shown in FIG.

From the viewpoint of uranium conservation, it is known that it is effective to burn the loaded uranium without waste, that is, to make the burnup distribution in the radial direction within the reactor core uniform.

The burnup here is the amount of heat generated per unit amount of uranium, expressed in units of MWd/l. (
(See formula below) The amount of heat generated corresponds to the power value (MW) within the core, and is large in the central region of the core and small in the periphery of the core.

The burnup distribution in this example is as follows.

The average output in area 1 is PL (2-1, 2), the average output in area 2 is P2 (=1.0), and the average output in area 3 is P3 (70, 5), as described above. Using the amount of uranium per fuel assembly arranged in each region, the average burnup for each region is: Average burnup in region 1 (E 1) E 1 = P 1/M
Average burnup in U 1 region 2 (E 2) E 2 = P
2/MU Average burnup in region 3 (E 3) E 3
= P, 3/MU 3.

Here, Pi>P2>P3 MUI>MU2>MU3 Since it is, ElriE2riE3 becomes.

As described above, it can be seen that according to the present invention, the burnup distribution within the nuclear reactor can be made uniform, and it is effective in saving uranium.

Furthermore, in order to ensure the mechanical integrity of the fuel, it is effective to make the burnup distribution in the furnace as uniform as possible and to lower the maximum burnup of the fuel taken out from the furnace. In the present invention, since the burnup distribution in the furnace is flattened compared to the conventional one, the maximum burnup of the extracted fuel is lowered, which is effective in ensuring the mechanical soundness of the fuel.

As a modification of the present invention, a method may be considered in which the amount of uranium is adjusted by changing the effective length of the fuel.

〔Effect of the invention〕

According to the present invention, it is possible to make the burnup distribution within the nuclear reactor uniform. By making the burnup distribution uniform within the reactor, uranium can be used more effectively and uranium savings can be realized, and the maximum burnup is lowered, which is effective in ensuring the mechanical integrity of the fuel.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the radial neutron flux distribution inside the reactor, Fig. 2 is a diagram showing the radial region division within the reactor (second quadrant), and Fig. 3 is a diagram showing the neutron flux distribution in the radial direction inside the reactor. FIG. 4 is an explanatory diagram of a fuel assembly placed in area 2 in an embodiment of the present invention, and FIG. 5 is an explanatory diagram of a fuel assembly placed in area 3 in an embodiment of the present invention. It is an explanatory diagram. 1.2.3... Fuel assembly, 4... Control rod, 5...
・Cha $ I Sec. 2 Prison $ 3 Sec. 4 - 5th Ward

Claims (1)

[Claims] 1. In a nuclear reactor consisting of a large number of fuel assemblies with equal external dimensions, the amount of nuclear fuel present in the fuel focus decreases in the radial direction from the center of the reactor to the periphery. A nuclear reactor characterized in that multiple types of fuel assemblies with different amounts of nuclear fuel present in the assemblies are arranged in the reactor. 2. In claim 1, rods that do not contain fissile material are arranged in the fuel assembly, and the amount of nuclear fuel is varied for each fuel assembly depending on the number of rods that do not contain fissile material, A fuel assembly is arranged in which the number of rods that do not contain fissile material increases in the radial direction from the center of the reactor to the periphery, and the amount of nuclear fuel in the fuel assembly decreases in the radial direction from the center of the reactor to the periphery. A nuclear reactor characterized by: 3. In claim 2, water rods (with an outer diameter that is the same or almost the same as the outer diameter of the rods that contain fissile material in the fuel assembly) are used as rods that do not contain fissile material in the fuel assembly. A nuclear reactor characterized by allowing light water to flow inside the cladding tube. 4. In claim 3, the reactor core is divided into two or more regions in the radial direction from the center of the reactor to the periphery,
Fuel assemblies with the same number of water rods are arranged in each region, and as you move toward the periphery of the reactor, the number of water rods in the fuel assemblies belonging to the region increases, so that fuel assemblies are arranged in the radial direction of the reactor. A nuclear reactor characterized by reducing the amount of nuclear fuel in the body.