JPS6219792A - Fuel aggregate - 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 [Technical Field of the Invention] The present invention relates to a fuel assembly used in a boiling water nuclear reactor.

[Technical background of the invention and its problems]

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

That is, the fuel assembly 1 has a large number of fuel rods 2, 2... arranged in a lattice shape in a rectangular tube-shaped long channel box 6, and the upper ends of the fuel rods are connected to an upper tie plate 3.
, the lower end is supported by a lower tie plate 4, and the middle part is supported by a spacer 5.

As shown in FIG. 4, this fuel assembly 1 is loaded adjacent to the control rod 8 in the reactor.

In addition, the fuel rods 2 arranged in an 8×8 square lattice have an enrichment level of 11, 12, 13, 14, 15 (in the figure).
The same symbols indicate the same enrichment).

It is located. Furthermore, in place of the fuel rods containing fissile material, two water rods 7, through which a moderator flows, are arranged in the center of the fuel assembly.

By the way, 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 σ at the position of the fuel rod 2: fission cross section of the fissile material N: atomic density of the fissile material of the fuel rod 2.

In a boiling water reactor, the thermal neutron flux distribution is large at the outermost part of the fuel assembly 1, and conversely becomes small at the center, due to the non-uniformity of the moderator density distribution and the neutron absorption effect of the fuel. ing. Therefore, if all the fuel rods 2 in the fuel assembly 1 have the same enrichment (atomic density of fissile material), the output of the fuel rods 2 on the outer periphery of the fuel assembly 1 will be large according to the above equation. Since it becomes smaller at the center, the relative power distribution within the fuel assembly 1 becomes larger at the outer periphery.

On the other hand, in order to improve the combustion efficiency of the fuel assembly 1, it is necessary to improve its infinite multiplication factor.

This infinite multiplication factor is defined as follows.

In other words, in general, when a fissile material undergoes fission, several neutrons are emitted by fission when one neutron hits it, but when considering an infinite system, the number of emitted neutrons can be calculated by the number of incoming neutrons. The divided value is called the infinite multiplication factor.

Conventionally, in order to improve the infinite multiplication factor of the fuel assembly, the enrichment distribution within the fuel assembly was determined so that the relative power distribution within the fuel assembly was larger at the outer periphery, and the concentration distribution was higher at the outer periphery than at the center. It is highly concentrated. In a fuel assembly with such an enrichment distribution, the infinite multiplication factor from the early stage to the middle stage of combustion is improved compared to a fuel whose enrichment level in the central part is higher than that in the outer peripheral part.

However, boiling water reactors use control rods to suppress excess reactivity at the beginning of the operating cycle, and also contain pernable poison in the fuel rods. In other words, the surplus neutrons generated by nuclear fission are absorbed by the control rods and purnable poisons, so even if the infinite multiplication factor is improved from the early to middle stages of combustion as in the past, it will not be possible to effectively utilize uranium. , less effective.

By the way, LJ-235 accounts for most of the fissile material contributing to the infinite multiplication factor at the initial stage of combustion, but as combustion progresses, the proportion of PIJ-239 increases.

Considering this point, in order to effectively utilize uranium, that is, improve combustion efficiency, it is necessary to absorb as much of the excess neutrons into fission parent material (for example, U-238, etc.) as possible to increase the amount of fissile material (Pu-239) produced. It was desired to increase the infinite multiplication rate from the middle to the end of combustion.

[Purpose of the invention]

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

[Summary of the invention]

The present invention replaces fissile material with a moderator and places an encapsulated water rod in the center, and also places highly enriched fuel rods in an area where the thermal neutron flux is relatively low. By arranging fuel rods with low enrichment in the region of high enrichment, from the early to middle stages of combustion, the amount of neutron absorption into the fissile parent material increases due to neutron spectral hardening in the region of high enrichment, resulting in the production of fissile material. A fuel characterized by increasing the amount of fuel, increasing the infinite multiplication factor from the middle stage to the end stage of combustion, and increasing the take-out burnup of the fuel assembly at the same infinite multiplication factor, thereby improving fuel economy. It is in the collective.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIGS. 1 and 2. Incidentally, the same components as those of the prior art described above are designated by the same reference numerals in the drawings, and their explanations will be omitted.

FIG. 1 shows a first embodiment of the present invention, in which four water rods 7 are arranged at the center of a fuel assembly 1A. In addition, inside the fuel assembly 1A, there are a first area on the first turn from the outermost circumference, a second area on the second turn, and these first areas.
It is divided into the third @IQ (D 3 m region) inside the second region. That is, in FIG. 1, the region of the fuel rod 2 outside the broken line A is the first region, and the broken line The area of the fuel rod 2 between the broken lines B is the second area, and the area of the fuel rods 2 inside the broken line B is the third area.Then, the average enrichment of the nuclear fuel material of the fuel rods 2 in each area is The second area, the third area, and the first area become smaller in this order, that is,
If the average enrichment of the first region is 101, the average enrichment of the second region is 102, and the average enrichment of the third region is 103, then 102>103
> is arranged so that 1 holds true.

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

The margins are arranged as shown in Table 1 below, but in this example, the average enrichment of each region is arranged in the order of the second region, the third region, and the first region, and the fuel assembly The average concentration of is the same in both the conventional example and the present example.

Figure 2 explains the burnup change of the infinite multiplication factor of fuel assemblies 1 and 1A, with the infinite multiplication factor on the vertical axis and the burnup on the horizontal axis, and the fuel assemblies shown in Table 1. 1,1
A comparison is shown between the conventional example (broken line C) and the present example (solid line D) when the average concentration of A is the same. here,
The increase in the infinite multiplication factor at the beginning of combustion is due to the combustion of katolinia, a burnable poison, in the fuel rod 2, and the subsequent decrease in the infinite multiplication factor is due to the combustion of uranium in the fuel rod 2. .

According to this embodiment, since the difference between the first region, the second region, and the third region of the first round from the outermost periphery of the fuel assembly 1A is increased, the difference between the outermost periphery of the conventional fuel assembly 1 and Compared to the case where the average enrichment of the fuel rods 2 is higher than the average enrichment of the fuel assembly 1, fissile material (Pu-
As the accumulation of 239> progresses, the infinite multiplication factor increases from the middle stage to the final stage of combustion. In addition, four water rods are placed in the center of the fuel assembly 1A, and the spectrum at the center of the fuel assembly is softened compared to the conventional case where two water rods are placed. Pu-2
It is useful for burning 39.

In this embodiment, four water rods are arranged in the center of the fuel assembly 1A, but the same effect can be obtained even if one large diameter water rod is arranged instead of these four water rods. It is.

〔Effect of the invention〕

As explained above, according to the fuel assembly of the present invention, since the infinite multiplication factor can be improved, the fuel extraction burnup increases, it becomes possible to extract more energy, and the fuel combustion Qin can have an excellent effect of increasing efficiency.

[Brief explanation of the drawing]

Fig. 1 is a fuel rod enrichment distribution diagram showing a first embodiment of the fuel assembly according to the present invention, and Fig. 2 is an infinite multiplier of a fuel assembly comparing the embodiment shown in Fig. 1 with a conventional example. FIG. 3 is a fuel rod enrichment distribution diagram showing an aggregate. 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 degree fuel rod, 14...lowest enrichment fuel rod, 15...fuel rod containing gadolinia. Agent Patent Attorney Noriyuki Chika Yudo Hirofumi MitsumataFigure 1Figure 2Figure 3Figure 4

Claims (1)

[Claims] 1. In a fuel assembly for a boiling water reactor in which a plurality of fuel rods are arranged in a lattice pattern, a water rod through which a large amount of moderator flows is arranged in the center, and The inside is divided into the first area on the first turn from the outermost circumference of the fuel rod, and the second area on the second turn from the outermost circumference of the fuel rod.
and a third region inside the first and second regions, and the average enrichment of nuclear fuel material in 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 characterized in that the area is higher than the third area. 2. The fuel assembly according to claim 1, wherein the water rod containing a large amount of moderator is constituted by four water rods having an outer diameter equal to the outer diameter of a normal fuel rod. 3. The fuel assembly according to claim 1, wherein the water rod enclosing a large amount of moderator is constituted by one water rod having a diameter larger than that of a normal fuel rod.