JPS5826292A - Fuel assembly - 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 The present invention relates to a fuel assembly incorporating a plurality of fuel rods for a nuclear reactor.

Fuel rods for power reactors, etc. that use fissile material as fuel are
For ease of handling and control of the reactor, it is common to incorporate a plurality of fuel rods into an integrated fuel assembly and load it into the reactor. FIG. 1 shows an example of such a fuel assembly.

Shown in Figure 2. In the tooth, the fuel assembly 10 has a fuel rod 14 in the channel box 12 as shown in FIG.
It is structured in a grid of 8 rows and 8 columns. A fuel rod spacer 16' (f-) is interposed between each fuel rod 14, and the lower end of the fuel rod 14 is inserted into the lower tie plate 18, and the -L end is compressed between the upper type L/-t 20. It is in contact with the interposed extension spring 22 and is prevented from moving within the channel box 12. Also, on the upper side of the channel box 12,
Guide vanes 23 are provided to facilitate loading into the reactor core, and a retaining portion 24 for suspending the fuel assembly 10 is provided at the upper end of the channel box 12.

The maximum in-core power of a nuclear reactor operated with a large number of fuel assemblies as described above loaded is determined by multiplying the product of the following three peakings by the average power of the in-reactor fuel assemblies. The three peakings are: the first is the radial power peaking, which is the ratio between the maximum power of the fuel assembly in the reactor and the average reactor power, and the second is the radial power peaking.

The axial ratio peaking is the ratio between the vertical maximum output of the fuel assembly and the vertical average output, and the third is the local power peaking, which is the ratio between the maximum output of the fuel rod in the fuel assembly and the average output in the vertical direction. This is the ratio to the fuel rod average output. In addition, the output P of each fuel rod in the fuel assembly is determined by the thermal neutron flux at the fuel rod position being φ, the fission cross section of the fissile material being σf1, and the fissile material atoms in the fuel rod (hereinafter referred to as fuel atoms) being If the concentration is 'tN, then it is given by P=φ・σ, -N.

Furthermore, in order to burn fuel efficiently and extend the combustion period, it is necessary to increase the so-called infinite multiplication factor of the fuel assembly. In order to increase this infinite multiplication factor, it is more effective to increase the density (concentration) of fuel atoms in regions where thermal neutron flux is large, and to decrease the density of fuel atoms in regions where thermal neutron flux is small. It is known. Therefore, in a boiling water reactor, the thermal neutron flux is large at the periphery of the fuel assembly and small at the center, due to the inconsistency of the neutron moderator and the neutron absorption effect of the fuel rods themselves. In a fuel assembly for a boiling water reactor, it is desired that the density of fuel atoms at the periphery of the fuel assembly be greater than that at the center. Increasing the density of fuel atoms at the periphery of the fuel assembly compared to the center means that the output power of the fuel rods at the periphery of the fuel assembly is increased by a large amount, and that of the center is This will reduce the power output of the fuel rods and increase the local power peaking.

However, in a boiling water reactor, the axial power peaking increases due to the effect of voids generated in the water in the fuel assembly, so it is necessary to reduce the local power peaking. That is, when a boiling water reactor is operated, the water in the fuel assembly boils as the temperature inside the reactor rises by -L, creating voids, which collect at the top of the fuel assembly and generate neutrons. On the other hand, in the lower part of the fuel assembly, neutrons are decelerated by water and become thermal neutrons suitable for nuclear fission reactions, increasing the nuclear fission reaction. Output beaking increases. Therefore, as mentioned above, the reactor output is determined by multiplying the product of the three peakings by the average output of the fuel assembly in the reactor, so as the axial output peaking increases,
It is necessary to reduce local power peaking. For this reason, conventionally, in fuel assemblies for boiling water reactors, in order to minimize local power peaking, the density of fuel atoms in each fuel rod is different, and the center of the fuel assembly has a concentrated concentration of fuel atoms. Fuel rods with a high concentration of fuel atoms were arranged, and fuel rods with a low enrichment of fuel atoms were arranged at the periphery of the fuel assembly. An example of this is shown in FIG. In the figure, 10 is a fuel assembly, 12 is a channel box that accommodates fuel rods, and 26 is a control rod inserted into the reactor.

28 is the highest enrichment fuel rod with the highest concentration of fuel atoms, 30 is the high enrichment fuel rod with the second highest enrichment, and 32 is an intermediate high enrichment fuel rod with a lower concentration than the high enrichment fuel rod 30. 34 is an intermediate low enrichment fuel rod with a lower enrichment, 36 is a low enrichment fuel rod with a lower enrichment than the intermediate low enrichment fuel rod 34, and 38 is a lowest enrichment fuel rod with the lowest concentration of fuel atoms. It's a fuel rod. Further, 40 is a fuel rod containing a so-called burnable poison such as gadolinium Qd that absorbs neutrons, and 42 is a fuel rod containing a burnable poison that decelerates neutrons and initiates a nuclear fission reaction in the center of the fuel assembly 10. It is a water rod housed to increase the

On the other hand, in recent years, a technology has been developed in which the fuel rod is divided into two regions approximately at the center in the longitudinal direction, and the concentration of substances contained in the fuel rod is varied in each region. Then, the concentration of fuel atoms in the upper part of the fuel assembly is made higher than in the lower part, the infinite multiplication factor in the upper part of the fuel assembly is made large, and the infinite multiplication factor in the lower part of the fuel assembly is made small.
It became possible to homogenize the nuclear fission reaction in the longitudinal direction of the fuel assembly, and to suppress the increase in turbulence caused by steam voids characteristic of boiling water reactors (Japanese Patent Laid-Open No. 53-40188). Although the thermal margin within the boiling water reactor has been increased by this technique of suppressing the increase in axial power peaking, the concentration of fuel atoms at the center and the periphery of the fuel assembly remains the same as before. Therefore, the effect of the infinite multiplication factor of the entire fuel assembly, ie, increasing the infinite multiplication factor, has not changed significantly from the conventional method.

The present invention has been made in consideration of the above facts, and aims to increase the concentration of fuel atoms at the periphery of the fuel assembly to lengthen the entire combustion period without changing the concentration of fuel atoms in the entire fuel assembly. The purpose is to provide a fuel assembly that can.

The present invention provides a fuel assembly in which a plurality of nuclear reactor fuel rods that use fissile material as fuel are assembled and integrated in parallel.
The average concentration of nuclear fuel material in the fuel rods at the periphery of the fuel assembly is made larger than the average concentration of nuclear fuel material in the fuel rods at the center of the fuel assembly, and the proportion of the material contained in the fuel rods is increased. By changing the lengthwise direction, the infinite multiplication factor in the upper part of the fuel assembly is made larger than the infinite multiplication factor in the lower part of the fuel assembly, and the infinite multiplication factor of the entire fuel assembly is increased, thereby increasing the combustion of the fuel assembly. It is structured so that the period is long.

A preferred embodiment of the fuel assembly according to the present invention will be described in detail with reference to the accompanying drawings. Incidentally, members corresponding to those shown in the conventional example are given the same reference numerals and explanations thereof will be omitted.

FIG. 4 is a layout diagram of fuel rods in an embodiment of the fuel assembly according to the present invention, and FIG. 5 is a diagram showing the concentration of fuel atoms in the fuel rods of FIG. 4. Note that the concentration of fuel atoms in the fuel rod (!I degree of contraction) is determined by the maximum enrichment, high enrichment, and
The concentration decreases in the following order: intermediate high enrichment, intermediate low enrichment, low enrichment, and minimum enrichment. In FIGS. 4 and 5, the fuel assembly 10 has two water rods 42 installed in its center, and 46 fuel rods arranged in a grid of 8 rows and 8 columns. 2 from the outside of the fuel assembly 10.
The highest enrichment fuel rods 44, two of which are arranged at the center of each side of the second layer, are 3.7% by weight (hereinafter referred to as WLO).
The fuel atoms (referred to as ) are almost uniformly distributed throughout the fuel rod. Furthermore, the four high enrichment fuel rods 46 arranged at the center of each side of the outermost layer of the fuel assembly 10 have an average concentration of fuel atoms in the fuel rods of 3.5W10. The upper half has a concentration of 3.7 W/O1 and the lower half has a concentration of 3.3W10. The intermediate high enrichment fuel rods 48 arranged on both sides of the high enrichment fuel rod 46 have an average concentration of fuel atoms of 3.1 W10, with an upper half of 3.3 W/O1 and a lower half of 2.9 W. /O. Further, the intermediate low enrichment fuel rods 50 arranged at each corner of the second layer from the outside of the fuel assembly 10 have fuel atoms of 2.9W10 uniformly distributed therein. The low enrichment fuel rods 52 located at each of the outermost corners have an average concentration of fuel atoms of 2.4W10, with the upper half having 2.6W10 and the lower half having 2.2W10. Fourteen minimum enrichment fuel rods 54 are arranged at the center of the fuel assembly 10, and fuel atoms having a concentration of 2.2W10 are uniformly distributed in the minimum enrichment fuel rods 54. In addition, outside the fuel assembly 10! In FIG. 4 of the second layer from the side, 56 arranged at the upper left and lower right and 58 arranged at the upper right and lower left are fuel rods containing burnable poison (Gd), respectively. The fuel rod 56 containing burnable poison is 2.9W1
0 fuel atoms and 4.5W10 burnable poison are uniformly distributed, and the fuel 58 containing burnable poison is 2.9W10.
of fuel atoms are uniformly distributed, and the upper half is 3.5W
10 burnable poisons and the lower half contains 4.5W10 burnable poisons.

Fuel assembly lO in which fuel rods are assembled in the above arrangement
In this case, the concentration of fuel atoms is higher at the top than at the bottom, and the average concentration of fuel atoms is higher at the periphery than at the center. Therefore, the infinite multiplication factor in the longitudinal direction of the fuel assembly 10 is made larger in the upper part than in the lower part, so that the nuclear fission reaction is made uniform, and the axial output r-king is reduced. On the other hand, as mentioned above, by increasing the concentration of fuel atoms at the periphery of the fuel assembly 1o, where there are many thermal neutrons suitable for nuclear fission reactions, the decrease in the infinite multiplication factor at the center of the fuel assembly 10 is greater than that at the center of the fuel assembly 10. The infinite multiplication factor at the peripheral portion can be increased, and the infinite multiplication factor of the entire fuel assembly 1o can be increased.

The relationship between the infinite multiplication factor and the fuel burnup when a nuclear reactor is operated using the fuel assembly of the above embodiment and the conventional fuel assembly in which the concentration of fuel atoms in the entire fuel assembly is equal is shown in the sixth diagram.
As shown in the figure. In FIG. 6, the vertical axis is the infinite multiplication factor, and the horizontal axis is the burnup, which is expressed in MWd/l (megawatt day/ton). In addition, A is the measured value of the upper part of the fuel assembly of this example, B is the measured value of the lower part of the fuel assembly of this example, C is the measured value of the upper part of the conventional fuel assembly, and D is the measured value of the conventional fuel assembly. This is the measurement value at the bottom. As shown in the figure, in the fuel assembly of the example, the infinite multiplication factor increases by 0.01 in the portion where the burnup is large (in the latter half of the fuel life) compared to the conventional fuel assembly. As a result, even if the concentration of fuel atoms in the entire fuel assembly is the same as the conventional one, the fuel assembly of this example has a larger infinite multiplication factor than the conventional one in the latter half of the fuel life, so the combustion period can be lengthened. Can be done. That is, even if the same amount of fuel is used, more energy is extracted, and fuel economy can be greatly improved.

In the above embodiment, the axial output peaking of the fuel assembly 1o is reduced by changing the concentration of fuel atoms. It can be carried out. Further, in the above embodiment, a fuel assembly was described in which the fuel rods were assembled in a lattice shape with t-8 rows and 8 columns, but the fuel assembly is not limited to the lattice shape with 8 rows and 8 columns. Arrangement 1 may be arranged in a complete circle.

As explained above, according to the present invention, the concentration of the content in the fuel rods is changed to make the infinite multiplication factor in the upper part of the fuel assembly larger than the infinite multiplication factor in the lower part. By increasing the concentration of atoms from the center, the combustion period of the fuel can be extended.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the fuel assembly, and Figure 2 is from ■-■ in Figure 1.
3 is a diagram showing the arrangement of fuel rods in a conventional fuel assembly, FIG. 4 is a diagram showing the arrangement of fuel rods in a fuel assembly according to an embodiment of the present invention, and FIG. FIG. 6 is a diagram showing the concentration of fuel atoms in the fuel rods, and FIG. 6 is a diagram showing the relationship between the burnup of the fuel assembly and the infinite multiplication factor. 10... Fuel assembly, 28.44... Highest enrichment fuel rod, 30.46... High enrichment fuel rod, 32.48.
...Intermediate high enrichment fuel rod, 34,50...Intermediate low enrichment fuel rod, 36.52...Low enrichment fuel rod, 38゜5
4...Minimum enrichment fuel rod, 40,56.58...Acceptable 1st phantom 3121st 1000th

Claims (1)

[Claims] 1. In a fuel assembly in which a plurality of fuel rods for a nuclear reactor using fissile material as fuel are assembled and integrated in parallel, the average concentration of nuclear fuel material in the fuel rods at the periphery of the fuel assembly is calculated as the fuel assembly. By increasing the average concentration of nuclear fuel material in the fuel rods in the center of the body and by varying the proportion of the material contained in the fuel rods in the longitudinal direction, the infinite multiplication factor in the upper part of the fuel assembly can be increased as described above. A fuel assembly characterized by having a larger infinite multiplication factor than the lower part of the fuel assembly.