JP2000314792A - Initial loading reactor core, and fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an initial loading reactor core capable of flattening an axial-directional output distribution to satisfy a thermal characteristic, by setting an enrichment difference to a proper value relating to a fuel of low enrichment, and provide a fuel assembly loaded onto the initial loading reactor core. SOLUTION: Many kinds of fuel assemblies arranged with plural fuel rods in each inside, and different in enrichment are plurally loaded in an initial loading reactor core. In the initial loading reactor core, a fuel A of low average enrichment out of the fuel assemblies is made to have a difference of 0.5 wt.% or more and 1.0 wt.% or less of between uranium average enrichments in its upper part and its lower part, excepting a neutral uranium portion. For example, the difference in the uranium average enrichment is 0.8 wt.% between an upper fuel region 42 and a lower fuel region 4, to satisfy the range within 0.5 wt.% or more and 1.0 wt.% or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an initially loaded core of a boiling water reactor, and a fuel assembly loaded in the initially loaded core.

[0002]

2. Description of the Related Art Generally, in a light water reactor, water plays an important role not only as a coolant but also as a neutron moderator. In a boiling water reactor, voids due to boiling inside the core are distributed in the axial direction, and the proportion of voids increases toward the upper part of the core. Since a light water reactor is designed to suppress the reaction when the void fraction is high, the axial power distribution of the core average tends to expand in the lower part of the core compared to the upper part. For example, Japanese Patent Application Laid-Open No. 55-47490 proposes a method of flattening the axial power distribution by providing a difference in the uranium average enrichment in a plane perpendicular to the axial direction of the fuel assembly. ing.

[0003] In addition, the above publication discloses that the average enrichment in a plane perpendicular to the axis is almost the same, and the enrichment distribution is changed between the upper and lower parts of the fuel assembly to obtain an axial power distribution. Has been proposed.

[0004] Fig. 8 shows the enrichment distribution along the axial direction of low-enrichment fuel and high-enrichment fuel in an initially loaded core having various enrichments. The upper part of the figure shows the upper part of the core, and the lower part shows the core. At the bottom. The low-enrichment fuel and the high-enrichment fuel are divided into several fuel regions between the upper part and the lower part of the core, and the fuel regions 51, 55, 56, and 60 use natural uranium, and the enrichment is Is 0.71% by weight. In the low-enrichment fuels, the enrichments of the fuel regions 52, 53, and 54 are 2.3 wt%, 2.2 wt%, and 2.0 wt%, respectively.
In the case of the high enrichment fuel, the enrichment of the fuel regions 57, 58, and 59 is 4.2 wt%, 4.2 wt%, and 4.1 wt%, respectively. The enrichment difference (excluding the natural uranium portion) between the upper part and the lower part of the low-enrichment fuel is 0.3 wt% at maximum (the enrichment difference between the region 52 and the region 54). The difference in enrichment (excluding the natural uranium portion) between the upper and lower parts of the high enrichment fuel is 0.
It is 1 wt% (concentration difference between the areas 57 and 58 and the area 59).

[0005]

In order to improve the economy of the initially loaded core, it is necessary to increase the discharge burnup. To increase the discharge burnup, the average enrichment of the core must be increased. It is necessary.

However, as described above, in the conventional initially loaded core, since the enrichment difference between the upper and lower parts of the low enrichment fuel is as small as 0.3 wt% at the maximum, the average enrichment is aimed at high burnup. If the degree is increased, the tendency of the axial power distribution to swell downward in the early stage of combustion is further increased, and there is a problem that thermal characteristics become severe.

An object of the present invention is to set an enrichment difference to an appropriate value for a low enrichment fuel, thereby flattening the axial power distribution and satisfying the thermal characteristics, And a fuel assembly to be loaded into the initially loaded core.

[0008]

In order to achieve the above object, the present invention comprises a plurality of fuel assemblies having a plurality of fuel rods arranged therein and having different enrichments loaded therein. In the initially loaded core, the fuel assembly having the lowest average enrichment among the fuel assemblies has a difference in uranium average enrichment in a plane perpendicular to the axial direction, except for a natural uranium portion,
The upper and lower portions are characterized in that the content is 0.5 wt% or more and 1.0 wt% or less.

Further, the present invention provides an initially loaded core having the same configuration as described above, wherein the fuel assembly having the lowest average enrichment among the fuel assemblies is characterized by an average uranium enrichment in a plane perpendicular to the axial direction. Except for natural uranium, the difference between the highest and lowest points along the axial direction is 0.5 wt% or more and 1.0 wt%.
% Or less.

[0010] Of the fuel assemblies constituting the initially loaded core,
When the difference in the average uranium enrichment of the fuel assembly having the lowest average enrichment is 0.5 wt% or more and 1.0 wt% or less, the axial power distribution can be flattened by the following operation.

FIG. 7 shows the relationship between the difference between the upper and lower enrichment of the fuel assembly and the axial peaking coefficient. The axial peaking coefficient indicates the maximum value (relative value) of the core axial power. The upper limit of the peaking coefficient required to satisfy the thermal limit is 1.40. To satisfy this condition,
It is necessary that the difference between the upper and lower enrichment degrees is 0.5 wt% or more and 1.0 wt% or less.

In order to increase the core average enrichment, the axial enrichment difference of the high enrichment fuel should be as small as possible.
By increasing the enrichment in the upper part of the low enrichment fuel more than before, the power distribution in the axial direction of the core can be flattened. Therefore, the difference between the upper and lower enrichment of the low enrichment fuel is 0.5 wt%.
By setting the content to 1.0 wt% or less, the output in the axial direction can be flattened, and the thermal limit value can be satisfied.

Uranium average enrichment difference is 0.5 wt% or more and 1.0
By loading low-enriched fuel of less than wt% together with high-enriched fuel in the first loaded core, the average enrichment of the entire core is 3.0 wt%.
%. In this case, a fuel assembly having the highest average enrichment among the fuel assemblies is loaded on the outer periphery of the core.

Further, the present invention also relates to a fuel assembly to be loaded in the initially loaded core. That is, in the present invention, in a fuel assembly in which a plurality of fuel rods are arranged inside and initially loaded into a core, the uranium average enrichment in a plane perpendicular to the axial direction, except for the natural uranium portion, 0.5wt at top and bottom
% Or more and 1.0 wt% or less.

Further, according to the present invention, in a fuel assembly having the same structure as described above, the average uranium enrichment in a plane perpendicular to the axial direction is the highest along the axial direction except for natural uranium. It is characterized in that there is a difference between 0.5 wt% and 1.0 wt% at the lowest point and at the lowest point.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an axial enrichment distribution of a fuel assembly according to the present invention. In the initially loaded core, fuel assemblies having different enrichments are loaded. In this figure, the fuel effective portions of the two types of fuel assemblies constituting the core are shown. The low-enrichment fuel A and the high-enrichment fuel B are divided into several fuel regions from above to below the core. In FIG. 1, the upper part shows the upper part of the core, and the lower part shows the lower part of the core.

The low-enrichment fuel A is supplied from the fuel regions 41 to 45
Has been split up. 1st node fuel area 45 and 2
Natural uranium is used in the fuel region 41 of the 3rd and 24th nodes, and the fuel enrichment is low, and the enrichment is 0.71 wt.
%. Fuel region 4 from 16th node to 22nd node
2 has an average enrichment of 2.8 wt%, the average enrichment of the fuel region 43 from the eleventh node to the fifteenth node is 2.6 wt%, and the average enrichment of the fuel region 44 from the second node to the tenth node is 2. 0 wt%. The difference between the upper and lower enrichment is 0.8 at the maximum.
wt% (concentration difference between regions 42 and 44).

In this embodiment, as described above, the difference between the upper and lower enrichment degrees is 0.8 wt% at the maximum, and is 0.5 wt% or more and 1.0 wt%.
Within the following range, the axial power distribution can be flattened, and the thermal limit can be satisfied.

Highly enriched fuel B is supplied from fuel region 46 to 50
Has been split up. First node fuel regions 50 and 2
Natural uranium is used in the fuel region 46 of the 3rd and 24th nodes, and the fuel enrichment is low, and the enrichment is 0.71 wt.
%. Fuel region 4 from 16th node to 22nd node
The average enrichment of the fuel regions 48 at the 7th and 11th nodes to the 15th node is 4.2 wt%, and the average enrichment of the fuel regions 49 at the 2nd to 10th nodes is 4.1 wt%.

FIG. 2 is a longitudinal sectional view of the fuel assembly. As shown in the figure, the fuel assembly includes a fuel rod 61 and an upper tie plate 62 that supports the upper and lower ends of the fuel rod 61, respectively.
And a lower tie plate 63, a plurality of spacers 64 provided in the axial direction to maintain the interval between the fuel rods 61, a channel box 65 surrounding these components, and a channel for fixing the channel box 65 to the upper tie plate 62. It is composed of a fastener 66 and the like.

FIG. 3 shows an example of the arrangement of fuel rods in the low-enrichment fuel according to the present invention. The fuel rods are arranged in a square of 9 rows and 9 columns in the channel box 65 as shown in the figure. Reference numeral 23 denotes a partial length fuel rod, and the effective length of the partial length fuel rod 23 in the axial direction of the fuel rod is shorter than other fuel rods. Numeral 24 is a fuel rod mounting gadolinia. The partial length fuel rod 23 is 1
Fuel rods 2 with gadolinia mounted inside
4 is arranged inside the partial length fuel rod 23. Also,
Reference numeral 25 denotes a water rod, which has a structure through which water flows. In addition, 21 is a control rod.

In FIG. 3, 1, 2, 3, 4, 5 and P, G indicate fuel rod types, and the axial design of each fuel rod is as shown in FIG. Here, the fuel rod types 1, 2, 3, 4, 5 and P, G in FIG. 3 correspond to the fuel rod types 1, 2, 3, 4, 5, and P, G shown in FIG. 4, respectively. I have. Numerical values described in parentheses in FIG. 4 indicate the total number of fuel rods of each fuel rod type. Twelve fuel rod types are arranged inside the fuel assembly, and the upper enrichment is 3.8 wt% and the lower enrichment is 2.2 wt%
It is. The 16 fuel rod types 2 are arranged in the second layer from the outer periphery, and the upper enrichment is 3.0 wt% and the lower enrichment is 1.7 wt%. Two fuel rod types 3 are arranged adjacent to the water rod and 22 at the outermost periphery, and the upper enrichment is 2.6 wt% and the lower enrichment is 2.2 wt%. Eight fuel rod types 4 are arranged adjacent to each corner, and the enrichment is 2.2 wt%. Fuel rod type 5 has four fuel rods, one at each corner, with an enrichment of 1.7 wt.
%.

Eight partial length fuel rods 23 indicated by P are arranged, and the enrichment is 1.7 wt%. Four fuel rods 24 with gadolinia, indicated by G, are arranged, have an enrichment of 1.7 wt%, and a gadolinia concentration of 9 G above.
d, the lower gadolinia density is 10 Gd. When the fuel rods of each type are arranged in this manner, the average enrichment of the fuel assembly of this low enrichment fuel is 2.2 wt%. The difference in enrichment between the upper part and the lower part of the core is 0.8 wt% at the maximum.

FIG. 5 shows an example of the fuel loading pattern of the initially loaded core in which the low-enrichment fuel and the high-enrichment fuel shown in FIG. 1 are loaded in the core. The low-enrichment fuel 8 is supplied to the control cell 3
Control rods are inserted during the operation of the reactor called No. 2 and the reactivity is often adjusted, and the remaining regions other than the core outer peripheral portion 31 are loaded while being mixed with the highly enriched fuel. In FIG. 5, the number of control cells 32 is thirteen, but the number may be further increased. As shown in the figure, by loading the low enrichment fuel and the high enrichment fuel, the core average enrichment can be made 3.0 wt% or more. this is,
In this case, approximately 52% of the area other than the core outer peripheral portion 31 and the control cell 32 is loaded with the highly enriched fuel. Since the enrichment of the highly enriched fuel is 3.7 wt%, the core average enrichment ranges from 3.0 wt% to 3.6 wt%. The lower limit of the core average enrichment of 3.0 wt% was set as the minimum core average enrichment required to improve economic efficiency. Up to this point, if the enrichment of the initially loaded core could be increased, the average unloaded burnup of the initially loaded core fuel assembly would be 35 GW
It can be increased from d / t to the range of 42 GWd / t.

In the example of the fuel loading pattern shown in FIG.
By loading low-enrichment fuel with an upper and lower enrichment of 0.5 wt% or more and 1.0 wt% excluding high-enrichment fuel and natural uranium as shown in the above, the thermal characteristics can be satisfied and the economics of the first loading core Improvement can be achieved.

FIG. 6 shows another example of a fuel loading pattern when the fuel assembly of the present invention is loaded on the initially loaded core. In FIG. 6, a low enrichment fuel 9 is loaded instead of the high enrichment fuel loaded on the core outer peripheral portion 31. By using the low enrichment fuel 9 in the core outer peripheral portion 31, the characteristics and operation method of the core change. Also in such a loading pattern, the control cell 32 is arranged and the control cell 3
Reference numeral 2 denotes a low-enrichment fuel 8. The core average enrichment becomes 3.0 wt% or more when a high enrichment fuel is loaded in about 64% of the area other than the core outer peripheral part 31 and the control cell 32. In FIG. 6, the low enrichment fuels 8 and 9 have the same average enrichment of 2.2% by weight, but the arrangement of the internal fuel rods is different from each other.

In the fuel loading pattern example of FIG. 5, FIG.
By loading low-enrichment fuel with an upper and lower enrichment of 0.5 wt% or more and 1.0 wt% excluding high-enrichment fuel and natural uranium as shown in the above, the thermal characteristics can be satisfied and the economical efficiency of the first loaded core Improvement can be achieved.

[0028]

As described above, according to the present invention,
By making the difference of the average uranium enrichment of the low enrichment fuel 0.5 wt% or more and 1.0 wt% or less, the axial power distribution can be flattened and the thermal characteristics can be satisfied, and the height of the first loaded core can be improved. Burnup can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an axial enrichment distribution of a fuel assembly according to the present invention.

FIG. 2 is a longitudinal sectional view of a fuel assembly.

FIG. 3 is a fuel rod arrangement diagram of the low enrichment fuel according to the present invention.

FIG. 4 is a diagram showing enrichment and gadolinia distribution for each low enrichment fuel rod according to the present invention.

FIG. 5 is a diagram showing an example of a fuel loading pattern of an initially loaded core in which a low enrichment fuel according to the present invention is loaded in the core.

FIG. 6 is a diagram showing another example of a fuel loading pattern of the initially loaded core in which the low enrichment fuel according to the present invention is loaded in the core.

FIG. 7 is a diagram showing a relationship between a difference between the upper and lower enrichment levels of a fuel assembly and axial peaking.

FIG. 8 is a diagram showing an axial enrichment distribution of a low enrichment fuel according to the related art.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 fuel rod type 1 2 fuel rod type 2 3 fuel rod type 3 4 fuel rod type 4 5 fuel rod type 5 7 high enrichment fuel loaded on core outer periphery 8 low enrichment fuel 9 low enrichment fuel loaded on core outer periphery Degree fuel 21 Control rod 23 Partial length fuel rod 24 Gadolinia fuel rod 25 Water rod 31 Outer core 32 Control cell 41-60 Fuel area 61 Fuel rod 62 Upper tie plate 63 Lower tie plate 64 Spacer 65 Channel box 66 Channel fastener 66

Claims (6)

[Claims] In an initial loading core configured by loading a plurality of fuel assemblies having a plurality of fuel rods arranged therein and having different enrichments, an average enrichment among the fuel assemblies is obtained. The lowest fuel assemblies have a difference in average uranium enrichment in a plane perpendicular to the axial direction, except for natural uranium.
An initially loaded core characterized in that the upper and lower portions are not less than 0.5 wt% and not more than 1.0 wt%. 2. An initially loaded core comprising a plurality of fuel assemblies having a plurality of fuel rods arranged therein and having different enrichments loaded therein, wherein the average enrichment of the fuel assemblies is at least one. The lowest fuel assemblies have a difference in average uranium enrichment in a plane perpendicular to the axial direction, except for natural uranium.
An initially loaded core characterized by being at least 0.5 wt% and not more than 1.0 wt% at the highest and lowest points along the axial direction. 3. The initially loaded core according to claim 1, wherein the average enrichment of the entire core is 3.0% by weight or more. 4. The initially loaded core according to claim 3, wherein
An initially loaded core, wherein a fuel assembly having the highest average enrichment among the fuel assemblies is loaded on the outer periphery of the core. 5. An average uranium enrichment in a plane perpendicular to the axial direction of a fuel assembly in which a plurality of fuel rods are arranged and initially loaded in a core, excluding a portion of natural uranium, And a lower portion having a difference of 0.5 wt% or more and 1.0 wt% or less. 6. An average uranium enrichment in a plane perpendicular to the axial direction of a fuel assembly in which a plurality of fuel rods are arranged and loaded in an initially loaded core, except for a natural uranium portion. 0.5wt at the highest and lowest points along the direction
%. A fuel assembly having a difference of not less than 1.0% by weight.