JPH07111469B2 - Fuel assembly - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a fuel assembly used in a boiling water reactor.

[Technical background of the invention and its problems]

Generally, a fuel assembly for a boiling water reactor is formed as shown in FIG.

That is, in the fuel assembly 1, a large number of fuel rods 2, 2 ... Are arranged in a lattice in a long rectangular channel box 6, and the upper ends thereof are the upper tie plate 3 and the lower ends thereof are the lower type. The rate 4 and the intermediate portion are supported by the spacer 5.

As shown in FIG. 4, the fuel assembly 1 is loaded adjacent to the control rod 8 in the furnace. In addition, the fuel rods 2 arranged in an 8 × 8 square lattice have the enrichment levels 11, 12, 13, 14, 15 in the figure (the same symbols indicate the same enrichment).
Are arranged differently. Further, the fuel assembly 1
In place of the fuel rod in which the fissile material is enclosed, two water rods 7 through which a moderator flows are arranged in the central portion of.

The output P of each fuel rod 2 of the fuel assembly 1 is given by the following equation.

P = φ ・ σ ・ N where φ: thermal neutron flux at the position of fuel rod 2 σ: fission cross section of fissionable material N: atomic density of fissionable material of fuel rod 2.

In a boiling water reactor, the thermal neutron flux distribution becomes large at the outermost peripheral portion of the fuel assembly 1 and conversely becomes small at the central portion due to the non-uniformity of the moderator density distribution, the effect of neutron absorption by the fuel, etc. There is. Therefore, when all the fuel rods 2 in the fuel assembly 1 have the same enrichment (atomic density of fissionable material), the output of the fuel rods 2 at the outer peripheral portion of the fuel assembly 1 is large and Since it becomes smaller, the relative output distribution in the fuel assembly 1 becomes larger at the outer peripheral portion.

On the other hand, in order to improve the fuel efficiency of the fuel assembly 1, it is necessary to improve its infinite multiplication factor. This infinite multiplication factor is defined as follows. That is, in general, when fissionable material undergoes fission, one neutron hits and several neutrons are emitted by the fission, but the number of emitted neutrons in the infinite system The divided value is called infinite multiplication factor.

Conventionally, in order to improve the infinite multiplication factor of the fuel assembly, the enrichment distribution in the fuel assembly is determined so that the relative output distribution in the fuel assembly is large in the outer peripheral portion, and the outer peripheral portion is more preferable than the central portion. Concentration is high. In the fuel assembly having such enrichment distribution, the infinite multiplication factor from the early stage to the middle stage of combustion is improved as compared with the fuel in which the enrichment in the central portion is higher than the enrichment in the outer peripheral portion.

However, in boiling water reactors, control rods are used to suppress excess reactivity at the beginning of the operating cycle, and burnable poison is mixed in the fuel rods. In other words, the surplus neutrons generated by nuclear fission are absorbed by the control rods and burnable poisons, so even if the infinite multiplication factor is improved from the early stage of combustion to the middle stage as in the past, effective use of uranium , Less effective.

By the way, as for the fissile material that contributes to the infinite multiplication factor, U-235 occupies most of the initial stage of combustion, but the proportion of Pu-239 increases as the combustion progresses. Considering this point,
In order to effectively use uranium, that is, improve combustion efficiency, surplus neutrons are fissionable parent substances (eg U-238)
It has been desired to increase the amount of fissionable material (Pu-239) produced by absorbing as much as possible in order to improve the infinite doubling rate from the middle stage to the end stage of combustion.

[Object of the Invention]

The present invention has been made in view of the above-mentioned demand, and an object thereof is to provide a fuel assembly having excellent combustion efficiency.

[Outline of Invention]

The present invention, in a fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged in a lattice, a water rod through which a moderator flows is arranged in the central portion, and the inside is 3 of the 1st area of the 1st lap from the outermost circumference, the 2nd area of the 2nd lap, and the 3rd area inside the 1st, 2nd areas
And the average enrichment of the nuclear fuel material of the fuel rods in each region is higher in the third region than in the first region and in the second region.
The fuel assembly is characterized in that the region is higher than the third region.

With this configuration, fuel rods with high enrichment can be placed in the region where the thermal neutron flux is relatively low, and fuel rods with low enrichment can be placed in the region where the thermal neutron flux is relatively high. From the middle to mid-term, increase the amount of fissile material produced by neutron spectrum hardening in the highly concentrated region,
Improving the infinite multiplication factor from the middle to the end of combustion,
It is possible to improve the fuel economy by increasing the take-out burnup of the fuel assembly at the same infinite multiplication factor.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. It should be noted that the same components as those of the conventional one described above are denoted by the same reference numerals in the drawings, and description thereof is omitted.

FIG. 1 shows a first embodiment of the present invention, in which four water rods 7 are arranged at the center of the fuel assembly 1A. Further, the inside of the fuel assembly 1A is divided into three regions, a first region on the first lap from the outermost periphery, a second region on the second lap, and a third region inside these first and second regions. . That is, in FIG. 1, the region of the fuel rod 2 outside the broken line A is the first region, the region of the fuel rod 2 indicated by A between the broken line A and the broken line B is the second region, and the region inside the broken line B is the fuel. The area of the rod 2 is the third area. Then, the average enrichment of the nuclear fuel material of the fuel rods 2 in each of the regions is the second region, the third region, the first region.
₂ > ₃ > ₁ is established so that the areas become smaller in the order of the areas, that is, when the average enrichment of the first area is ₁ , the average enrichment of the second area is ₂ , and the average enrichment of the third area is _3. It is located in.

Table 1 shows the embodiment of the present invention described with reference to FIG.
The average enrichment of the nuclear fuel material in the fuel rod 2 in the first region, the second region, and the third region of the conventional example of FIG.
It shows a comparison of the average enrichment of the entire fuel assembly.

As is clear from the table, in the conventional example, the average enrichment of the first region is arranged to be lower than the average enrichment of the other regions, but in the present embodiment, the average enrichment of each region is , The second region, the third region, and the first region are arranged in this order, and the average enrichment of the fuel assembly is the same as in the conventional example and the present embodiment.

FIG. 2 illustrates the burnup change of this fuel assembly 1, 1A at infinite multiplication factor. The vertical axis shows infinite multiplication factor and the horizontal axis shows burnup, respectively, and the fuel assembly shown in Table 1 is shown. It shows a comparison between the conventional example (broken line C) and the present example (solid line D) when the average enrichment of 1,1A is the same. Here, the increase of the infinite multiplication factor in the initial stage of fuel is due to the combustion of gadolinia which is a burnable poison in the fuel rod 2, and the decrease of the infinite multiplication factor after that is due to the combustion of uranium in the fuel rod 2. It is a thing.

According to the present embodiment, the difference between the first region, the second region, and the third region of the first circumference from the outermost circumference of the fuel assembly 1A is made large. Compared with the case where the average enrichment of the fuel rods 2 is higher than that of the fuel assembly 1, the fissionable material (Pu-23
As the accumulation of 9) progresses, the infinite multiplication factor increases from the middle stage of combustion to the final stage. In addition, 4 in the center of the fuel assembly 1A
The number of water rods is arranged, and the spectrum of the central portion of the fuel assembly is softened as compared with the case where two conventional water rods are arranged, which is useful for combustion of the accumulated Pu-239.

In this embodiment, four water rods are arranged at the center of the fuel assembly 1A, but the same effect can be obtained by arranging one large diameter water rod instead of these four water rods. Is.

〔The invention's effect〕

As described above, according to the combustion assembly of the present invention, since the infinite multiplication factor can be improved, the fuel extraction burnup is increased, and more energy can be extracted, and the combustion of fuel The excellent effect of increasing efficiency can be achieved.

[Brief description of drawings]

FIG. 1 is a fuel rod enrichment distribution diagram showing a first embodiment of a fuel assembly according to the present invention, and FIG. 2 is an infinite multiplication factor of a combustion assembly comparing the embodiment of FIG. 1 with a conventional example. FIG. 3 is a partial cutaway perspective view of a general fuel assembly, and FIG. 4 is a fuel rod enrichment distribution map showing a conventional fuel assembly. 1,1A ... Fuel assembly, 2 ... Fuel rod, 3 ... Upper tie plate, 4 ... Lower tie plate, 5 ... Spacer, 6 ... Channel box, 7 ... Water rod, 8 ... Control Rod, 11 …… Highest enrichment fuel rod, 12 …… High enrichment fuel rod, 13 …… Low enrichment fuel rod, 14 …… Lowest enrichment fuel rod, 15 …… Gadolinia fuel rod.

Claims (3)

[Claims] 1. A fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged in a grid pattern, wherein a water rod through which a moderator flows is arranged in the central portion, and the inside of the fuel rod is 3 of the first area on the first lap from the outermost circumference, the second area on the second lap, and the third area inside the first and second areas.
And the average enrichment of the nuclear fuel material of the fuel rods in each region is higher in the third region than in the first region and in the second region.
A fuel assembly, wherein the region is higher than the third region. 2. The fuel assembly according to claim 1, wherein the water rods that circulate through the moderator are composed of four water rods having the same outer diameter as a normal fuel rod. 3. The fuel assembly according to claim 1, wherein the water rod that circulates the moderator is composed of one water rod having a diameter larger than that of an ordinary fuel rod.