JPH06160567A - Fuel assembly - Google Patents

Abstract

(57) [Summary] [Purpose] The average axial enrichment in the fuel assembly is gradually reduced from the upper region to the lower region, or the combustible poison concentration is gradually increased to increase the average core axis during power operation. A fuel assembly that flattens the directional power distribution is provided. [Structure] In a fuel assembly 1 composed of a plurality of fuel rods, the average enrichment of the effective fuel portion is approximately 1.0 wt% or more 1.5
wt% or less and the axial direction is the upper region 5, the central region 6 and the lower region 7, and the average enrichment of the upper region 5 is at least the central region 6 and the average enrichment of the central region 6 is the lower region. It is characterized by being 7 or more.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a fuel assembly for a boiling water reactor.

[0002]

2. Description of the Related Art Generally, in a boiling water reactor, the coolant flowing from the lower part of the core is boiled into a two-phase flow, and the vapor volume ratio is about 70% at the core outlet. Since the coolant also functions as a moderator for neutrons, the moderator effect is large in the lower part of the core, and the nuclear fission reaction of U-235 due to thermal neutrons tends to proceed.

Further, the neutron moderating effect is small in the upper core, and the fission reaction of uranium U-235 due to thermal neutrons is slower than that in the lower core, but U-238 absorbs fast neutrons and fissile plutonium (Pu) The reaction that produces

For this reason, the output distribution at the time of output operation has a lower peak in the initial use of the fuel assembly, but at the end of use of the fuel assembly, the reactivity of the upper part of the fuel assembly is relatively higher than that of the lower part. Even though the cooling effect of the coolant is small in the upper part of the core, the power distribution during power operation has an upper peak.

Therefore, in the conventional fuel assembly, which has a fuel enrichment distribution in three regions in the axial direction, the average enrichment in the upper portion is set to be equal to or less than the average enrichment in the central portion. Normally, since the fuel assemblies are used for 3 or 4 cycles, the cores have different burnups, and therefore fuel assemblies having different axial power distributions are mixed.

That is, the core is composed of the new fuel having the lower peak output distribution during the power operation, the third and fourth cycle fuels having the upper peak, and the second cycle fuel having the intermediate power distribution. .

The average axial power distribution in the core is determined by the mutual influence of the axial power distributions of the fuel assemblies, and in a core in which fuels having various axial power distributions are mixed, new fuel and The individual axial power distributions of the 3rd and 4th cycle fuels are flattened by the interaction with the adjacent fuels and have no extreme peaks, and the core average axial power distribution during the power operation becomes almost flat.

In designing the fuel assembly, the axial enrichment distribution and the axial burnable poison content distribution are:
The core average axial power distribution during power operation is determined so as not to make an extreme peak.

[0009]

When one cycle operation period assumed at the time of designing a fuel assembly and an operation significantly different from the number of fuel assembly replacements are performed, the core during power operation is A large deviation occurs in the average axial power distribution.

When the operation period of the cycle immediately preceding the cycle is shortened due to a change in the operation plan or the like,
Since the burnup of fuel at the 2nd, 3rd, and 4th cycles in the cycle is reduced, the core average axial power distribution during power operation is in a lower peak state.

Also, for some reason, when the amount of fuel in the 3rd and 4th cycles decreases and it is necessary to increase the number of loaded new fuel assemblies, the core average axial power distribution during power operation shows a lower peak. Become.

When the lower peak of the core average axial power distribution during such a power operation becomes extremely large, it is conceivable that troubles such as not satisfying the thermal limit value of the fuel may occur.

The object of the present invention is to reduce the average enrichment in the axial direction of the fuel assembly in order from the upper region to the lower region, or to increase the concentration of combustible poisons in sequence to increase the fuel concentration during power operation. It is to provide a fuel assembly that flattens the core average axial power distribution.

[0014]

[Means for Solving the Problems] In a fuel assembly loaded in the core of a reactor composed of a plurality of fuel rods, the average enrichment of the effective fuel portion of the fuel assembly is approximately 1.0.
wt% or more and 1.5 wt% or less, and the axial direction is the upper region, the central region, and the lower region, and the average enrichment of the upper region is at least the central region and the average enrichment of the central region is at the lower region. It is characterized in that it is more than the area.

Further, the burnable poison is contained in one or more of the upper area, the central area and the lower area in the axial direction. The content of the burnable poison is such that the content of the burnable poison in the upper area is at the center. It is characterized in that the content rate of the burnable poison in the central area is less than the lower area.

Further, the present invention is characterized in that natural uranium is provided on at least one of the upper and lower ends of the fuel assembly, and the other part is axially formed into three regions of an upper region, a central region and a lower region.

[0017]

By loading the fuel assembly of the present invention, the power distribution in the core during power operation can be set to the upper peak with a lower degree of the lower peak. Therefore, it is possible to flatten the core average axial power distribution during power operation. Further, by using the axial distribution of the content ratio of the burnable poison in combination with the axial distribution of the uranium enrichment, the reactivity distribution in the axial direction can be the upper peak.

Further, if the average enrichment of the whole is approximately 1.0 wt% or more and 1.5 wt% or less, the whole fuel assembly has a low reactivity, so the reactivity distribution has an upper peak, but this fuel assembly The reactivity of the upper region is lower than that of the conventional fuel assembly, and a sufficient reactor shutdown margin in which the upper reactivity is important is obtained.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. The axial fuel and poison distribution chart of FIG. 1 shows the first embodiment, and FIG. 2 is a cross-sectional view of the fuel assembly. The fuel assembly 1 is formed by arranging 62 fuel rods 2 and 2 water rods 3 in a square lattice shape, and surrounding them by a channel box 4.

The distribution of fuel and poisons is as shown in FIG.
The upper region 5 has an average enrichment of 1.5 wt% and contains no gadolinia. Next, the average enrichment of the central region 6 is 1.4%, which is lower than that of the upper region 5, and the number of fuel rods containing gadolinia, which is a burnable poison, is one.
It is 6.0 wt%.

Furthermore, the average enrichment in the lower region 7 is 1.2 wt%, which is lower than that in the central region 6, the number of fuel rods with gadolinia is one, and the gadolinia concentration is 6.0 wt% as in the central region 6. There is. In addition, the upper and lower ends 8, 9
Is equipped with natural uranium.

The axial fuel and poison distribution charts of FIGS. 3 to 5 respectively show other embodiments of the present invention. In the second embodiment, as shown in FIG. 3, the average enrichment of the upper region 5 is 1.5 wt%, and the average enrichment of the central region 6 is
The average enrichment in the lower region 7 is 1.4 wt% and 1.3 wt%, and the enrichment is set to be lower from the upper region 5 to the lower region 7. Neither of them contained gadolinia, which is a burnable poison.

In the third embodiment, as shown in FIG. 4, the average fuel enrichment in the upper region 5, the central region 6 and the lower region 7 is 1.3 wt%, but the fuel rod with gadolinia is in the center. One is provided in the partial region 6 and two are provided in the lower region 7, and the gadolinia concentration in the central region 6 is 3.0 wt%.
Therefore, the lower region 7 is set as high as 6.0 wt%.

In the fourth embodiment, as shown in FIG. 5, the average enrichment in the upper region 5 is 1.5 wt%, the number of fuel rods containing gadolinia, which is a burnable poison, is one, and the gadolinia concentration is
It is 1.0 wt%. Next, the average concentration of the central area 6 is 1.4w
t%, lower than the upper region 5, and 1 fuel rod with gadolinia
In the book, this gadolinia concentration is 3.0 wt%, which is higher than the upper region 5.

The average enrichment in the lower region 7 is 1.3.
wt% lower than the central region 6 and fuel rod 1 with gadolinia
Although it is a book, this gadolinia concentration is set to 6.0 wt% which is higher than that of the central region 6. Natural uranium is provided at the lower end portion 9. Since the respective fuel and poison concentration distributions in the upper region 5, the central region 6 and the lower region 7 are different as described above, the axial output distributions are also different.

The characteristic comparison curve diagram of FIG. 6 shows the combustion change of the infinite multiplication factor of the fuel in the first embodiment with the average enrichment of 3.3.
It is shown in comparison with the wt% fuel at the 3.5th cycle. The solid line a is the fuel assembly of the present invention, and the dotted line b is the average enrichment of 3.3 wt% 3.
5th cycle fuel is shown. From Fig. 6, it can be seen that the average enrichment of 1.0 to 1.5 wt% is appropriate for simulating the fuel in the third and fourth cycles.

The characteristic comparison curve diagram of FIG. 7 shows an example in which the core average axial power distribution during power operation is corrected by using the fuel assembly according to the first embodiment of the present invention. In the case where the dotted line d is constituted only by the conventional fuel assembly, the output distribution has an extremely downward peak. However, as shown by the solid line c, when the fuel assembly of the first embodiment is loaded as a part of the new fuel, the relative output of the upper part becomes large, and the peak of the lower part becomes small instead, and the output becomes smaller. The distribution is flattened.

The following claims and embodiments are as follows. (1) “Claim 2, in a fuel assembly loaded in a core of a reactor composed of a plurality of fuel rods, the average enrichment of the effective fuel portion of the fuel assembly is about 1.0 wt% or more and 1.5% or more.
The content of the burnable poison in the upper region is not more than wt% and the axial direction is made into three regions of an upper region, a central region and a lower region, and at least one region of these three regions contains a burnable poison. A fuel assembly characterized in that the content ratio is less than or equal to the central region and the content ratio of the burnable poison in the central region is less than or equal to the lower region. "

(2) "A natural uranium portion is provided on at least one of the upper and lower end portions of the fuel assembly, and the other portion is axially formed into three regions of an upper region, a central region and a lower region. The fuel assembly according to claim 1 or 2.

[0030]

As described above, according to the present invention, the core average axial power distribution during power operation does not have an extreme lower peak and is easily flattened to sufficiently secure the thermal limit value in the core. It has the effect of improving driving safety and reliability.

[Brief description of drawings]

FIG. 1 is an axial fuel and poison distribution map of a fuel assembly according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a fuel assembly according to an embodiment of the present invention.

FIG. 3 is an axial fuel and poison distribution map of the fuel assembly of the second embodiment according to the present invention.

FIG. 4 is an axial fuel and poison distribution map of the fuel assembly of the third embodiment according to the present invention.

FIG. 5 is an axial fuel and poison distribution map of a fuel assembly according to a fourth embodiment of the present invention.

FIG. 6 is a characteristic comparison curve diagram showing a combustion change of an infinite multiplication factor of the fuel assembly of the first embodiment according to the present invention.

FIG. 7 is a comparison curve diagram of the core average axial direction power distribution characteristics during the output operation of the load according to the first embodiment of the present invention.

[Explanation of symbols]

1 ... Fuel assembly, 2 ... Fuel rod, 3 ... Water rod, 4
... channel box, 5 ... upper area, 6 ... central area, 7 ... lower area, 8 ... upper end, 9 ... lower end.

Claims (1)

[Claims] 1. In a fuel assembly loaded in a core of a nuclear reactor composed of a plurality of fuel rods, the average enrichment of the effective fuel portion of the fuel assembly is approximately 1.0 wt% or more and 1.5 wt% or less.
% Or less, and the axial direction is set to three regions of an upper region, a central region and a lower region, and the average enrichment of the upper region is at least the central region and the average enrichment of the central region is at least the lower region. Is a fuel assembly.