JPH095472A - Reactor core - Google Patents

Abstract

PURPOSE: To improve the core average enrichment without losing the thermal margin by loading a fuel assembly having the highest average enrichment and multiple fuel assemblies having the average enrichment of a half of it or below. CONSTITUTION: Low-enrichment fuel assemblies 2, 3 have the average enrichment of about 1/3 of that of a highest-enrichment fuel assembly 1. A medium- enrichment fuel assembly 4 simulates an assembly existing in an equilibrium core in one cycle, and the assemblies 2, 3 simulate an assembly existing in the equilibrium core in two and three cycles. The assembly 3 contains fuel rods having nearly uniform enrichment distribution on the control rod side and the opposite side, and the assembly 2 arranged on a control cell contains the fuel rods having the lower average enrichment in the region on the control rod side than that on the opposite side. The fuel rod average enrichment of the outermost layer in the control rod side region is lower than that on the opposite side. The output on the control rod side is reduced, and no output peak is attained when a control rod is inserted and the control rod is extracted after burnup proceeds. A margin is generated in the whole thermal characteristic, and the average enrichment can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor core and, more particularly, to a core of a light water cooled nuclear reactor loaded with various kinds of fuel assemblies having different enrichments when initially loaded.

[0002]

2. Description of the Related Art A fuel assembly loaded in a core has 4
About one-third to about one-third is taken out, a new fuel assembly is loaded, and the next operation cycle is started. Therefore, fuel assemblies having different stay periods are mixed in the core. A fuel assembly having a long staying period does not produce much power in the core because combustion advances and the ratio of uranium 235 in the fuel decreases. Normally, the fuel assemblies that have advanced in combustion are sequentially replaced with new fuel.

The initially loaded core is constructed by loading a new fuel assembly which has not progressed in combustion over the entire core. After the completion of one operation cycle, the initially loaded core takes out about 1/4 of the fuel in the core, loads the new fuel, and starts the next operation cycle. Normally, the fuel assembly is taken out after staying for 3 to 4 operation cycles, but in the initially loaded core,
Combustion does not proceed sufficiently and some uranium 235 remains in the core while remaining relatively large, which is economically disadvantageous.

In order to improve such deterioration of fuel economy, an initially loaded core (hereinafter referred to as a multi-enrichment core) composed of a combination of various kinds of fuel assemblies having different average enrichments has been proposed in recent years. ing. In Japanese Unexamined Patent Publication No. 57-197490, when a fuel assembly stays in an equilibrium core for N cycles and is taken out, an N type fuel assembly having a different number of years of stay constitutes an initial loading core, and N type fuel assemblies. Proposes a core configuration with excellent fuel economy by adjusting the enrichment so that each becomes equal to an infinite multiplication factor that varies depending on the length of stay of the equilibrium core.

[0005]

In the multi-enrichment core, the fuel assembly having the highest enrichment is made to have the same enrichment as the fuel assembly loaded in the equilibrium core, and the fuel assembly staying in the one-cycle core is simulated. Therefore, it is composed of a fuel assembly having a lower enrichment and a medium enrichment, and a fuel assembly having a lower enrichment simulating a fuel assembly that has stayed for a few cycles.

[0006] In the equilibrium core, the fuel assembly whose combustion has progressed to a low degree of reactivity is loaded on the outermost layer of the core in the region facing the control rods (hereinafter referred to as control cells) operated during operation. On the other hand, in the initially loaded core, the fuel assembly having the lowest enrichment simulating the fuel assembly that has advanced in combustion is loaded.

A fuel assembly having a high degree of enrichment and a fuel assembly having a low degree of enrichment usually have the same structure equipped with fuel rods, water rods, channel boxes, etc. The cost does not change much. Therefore, in order to improve the fuel economy, it is desirable to increase the take-out burnup of the fuel assembly.

In Japanese Patent Laid-Open No. 60-71987, a fuel assembly having the highest enrichment in the core is loaded in the outermost layer of the core having a low output to reduce the fuel assembly having the lowest enrichment in the core, and Low enrichment fuel assemblies that are taken out early
Proposals have been made to promote combustion sufficiently by loading in the central region of the core with high output, and to improve fuel economy.

However, the fuel assembly loaded in the outermost layer of the core is irradiated with neutrons because two or three inner surfaces of the four side surfaces are adjacent to the fuel assembly but one surface is not adjacent. Faces and non-faces can be created. For this reason, the distortion of the fuel assembly due to neutron irradiation is larger than that of the fuel assembly whose four surfaces are adjacent to the fuel assembly. For this reason, it is difficult to return the fuel assembly loaded once to the outermost core layer to the core inner region, and the output of the outermost core layer is low. Combustion does not progress.

Further, since the control cell is formed in the central portion of the core where the output is high, the arranged fuel assemblies are relatively burned. However, since the control rod is included, when a fuel assembly having a high degree of enrichment is arranged, the axial output distribution becomes large due to the insertion of the control rod, so that the thermal characteristics deteriorate. Also,
Since two of the four side surfaces of the fuel assembly face the control rod, there is a difference in the progress of combustion between the control rod side and the opposite side,
When the control rod is inserted for a long time, a high output peak appears in the fuel rod on the control rod side when the control rod is pulled out.

As described above, in the multi-enrichment core, the position where the fuel assembly with low enrichment is loaded has a special combustion characteristic.

In order to improve the economical efficiency, even in the fuel assembly of the initially loaded core, in order to increase the take-out burnup, the average enrichment of the initially loaded core is increased to increase the number of replacements in the second cycle and the third cycle. It is effective to reduce it. But,
If the uranium enrichment has an upper limit, there is a limit to the further enrichment of the highly enriched fuel assembly. For this reason, it is necessary to increase the average enrichment of the fuel assembly having a low enrichment, but there is a problem that a fuel assembly having a high enrichment cannot be arranged at a position having a special combustion characteristic.

It is an object of the present invention to provide an initially loaded core which can improve the fuel economy by increasing the average enrichment of the initially loaded core without impairing the thermal margin.

[0014]

In order to achieve the above-mentioned object, a fuel assembly suitable for each characteristic is arranged at a position where a low enrichment fuel including a control cell and a core outermost layer is loaded. As shown in FIG. 1, at the position of the control cell in the central region of the core where combustion relatively progresses in the core, a low-concentration fuel assembly having a fuel rod arrangement such that an output peak on the control rod side does not appear is loaded. Thus, the average enrichment of the low enrichment fuel assembly is increased without impairing the thermal margin.

[0015]

The average enrichment of the fuel assembly having the enrichment simulating the fuel assembly staying in the two-cycle or three-cycle core is about half or less of that of the new fuel having no combustion history. In the present invention configured as described above, a plurality of types of fuel assemblies having a low enrichment of which the average enrichment is ½ or less of the maximum enrichment when initially loaded are loaded.

In the fuel assembly arranged in the control cell, the fuel rod enrichment on the side facing the control rod is lowered and the fuel rod enrichment on the side not facing the control rod is raised. This allows
After combustion of the fuel assembly progresses with the control rod inserted,
Even if the control rod is pulled out, a high output peak does not appear in the fuel rod on the control rod side. Further, by arranging this fuel assembly in the outermost layer of the core and arranging the fuel rods with high enrichment on the surface adjacent to the fuel assembly, the output of the outermost layer can be increased and The fuel assembly of the lowermost outer layer can be burned more than the fuel assembly of the conventional configuration.

Since the thermal characteristics of the fuel assembly of the control cell into which the control rod is inserted can be afforded, the average enrichment of the fuel assembly of low enrichment is 0.2% by weight to 0.2% by weight as compared with the conventional one.
It can be increased by about 5% by weight, and the average enrichment of the initially loaded core can be increased. As a result, the number of replacement bodies in the second cycle can be reduced and combustion of the fuel assemblies in the outermost core layer can be promoted, so that fuel economy is improved.

[0018]

Embodiments of the present invention will be described below.

(Embodiment 1) FIG. 1 is a first embodiment of the present invention and shows a loading pattern of an initially loaded core loaded with various kinds of fuel assemblies having different enrichments. In the figure, 1 is a fuel assembly with the highest enrichment in the core, 2 and 3 are fuel assemblies with the lowest enrichment in the fuel assembly, and an average enrichment of about 1/3 of fuel assembly 1 Has become. 4 is an equilibrium core
This is a fuel assembly of medium enrichment simulating a fuel assembly that stayed in a cycle furnace. Both 2 and 3 simulate a fuel assembly staying in a 2 or 3 cycle reactor in an equilibrium core. 2A shows a cross-sectional view of the fuel assembly 3 and FIG. 2B shows a cross-sectional view of the fuel assembly 2. The fuel assemblies of No. 3 and No. 2 have different fuel rod enrichment distributions in the fuel assemblies.
The fuel assembly of No. 3 has almost the same fuel rod enrichment distribution on the control rod side and the other side, whereas the fuel assembly of No. 2 arranged in the control cell is the average of the region a on the control rod side. The concentration is smaller than that on the opposite side b, and the average concentration of fuel rods in the outermost layer in the region a is smaller than the average concentration of fuel rods in the outermost layer in region b. As a result, the output of the region A where combustion is delayed on the control rod side is reduced in advance, so that after combustion progresses with the control rod inserted,
The output peak disappears even when the control rod is pulled out. Since there is a margin in the thermal characteristics of the fuel assembly of the control cell, it is possible to increase the average enrichment of the low enrichment fuel assembly and increase the core average enrichment.

Even when the control rod is not inserted into the control cell, the enrichment in the region facing the highly enriched fuel assembly is high, so that a high-power fuel assembly is produced due to the difference in output between the fuel assemblies. The neutron influx of the is mitigated and the thermal properties are improved.

(Embodiment 2) FIG. 3 shows a second embodiment of the present invention. As shown in FIG. 2, two surfaces adjacent to the control rod side,
Fuel assemblies 2 having different fuel rod enrichment distributions in the outermost layers of two surfaces that are not adjacent to each other are loaded in the control cell and the outermost layer of the core.

The fuel assemblies 2 and 3 are simulated fuel assemblies in which combustion has progressed in the equilibrium core, and both have an average enrichment of 1/2 or less of the fuel assembly having the highest average enrichment of the core. ing. The fuel assembly 3 has a higher average enrichment than the fuel assembly 2, and therefore the core average enrichment is higher than that of the first embodiment. In the core, in the outermost layer of the core, the control rod side (the area a of B fuel assembly in FIG. 2) and the opposite side (the area b of B fuel assembly in FIG. 2) are arranged in the same manner as the control cells. By using fuel assemblies with different enrichment levels in the fuel rods and arranging them so that the highly enriched region b is on the side surface adjacent to the fuel assembly, the output of the outermost core layer is increased and the heat of the core is increased. You can improve your margin. Further, since the output of the outermost layer of the core is increased, the combustion of the fuel assemblies loaded in the outermost layer is promoted and the fuel economy is improved.

(Embodiment 3) FIG. 4 shows a third embodiment of the present invention, in which three types of low enrichment fuel assemblies having an average enrichment of 1/2 of the highest fuel assembly in the core are provided. It is loaded.
In this core, in the core of Example 2, the average enrichment of the fuel assembly in the outermost layer of the core is lower than that of the fuel assembly in the control cell. Since the outermost layer of the core has a large strain of the fuel assembly due to neutron irradiation, the fuel assembly once loaded at this position is taken out without being returned to the core inner region. Further, it is a region where combustion does not proceed so much because the output is low. Therefore, by making the average enrichment of the fuel assembly loaded in the outermost layer region of the core smaller than the others,
Unburned fuel is reduced and fuel economy is improved.

(Embodiment 4) FIG. 5 shows a fourth embodiment of the present invention. In this example, three fuel assemblies with a low enrichment having an average enrichment of ½ of the highest fuel assembly in the core are 3
Type is loaded. The core of this example is composed of a fuel assembly having the highest enrichment and a fuel assembly having an enrichment equal to or less than 1/2 of the highest enrichment, and an intermediate enrichment simulating a fuel assembly that stayed for one cycle in the equilibrium core. The fuel assembly of the degree is not arranged. When the fuel pellet enrichment has an upper limit value and the maximum enrichment of the fuel assembly cannot be raised, it is necessary to increase the fuel enrichment of the maximum enrichment in order to increase the average enrichment of the initially loaded core. However, to ensure thermal safety, the control cell must always be loaded with a low enrichment fuel assembly. Further, in order to configure the control cell in the operation cycle subsequent to the first cycle, which is the initially loaded core, a low enrichment fuel assembly is required, and further, the fuel assembly at the position where the output is low, such as the outermost layer, is required. It is desirable to place a low enrichment fuel assembly in the body. Therefore, a fuel assembly having a low enrichment is always required in the furnace. In order to increase the core average enrichment, it is desirable that all the fuel assemblies have high average enrichment. In this embodiment, even in the case of a fuel assembly having a low enrichment, the fuel rod arrangement and the average enrichment are changed depending on the loading position, so that the core average enrichment can be maximized under the limitation of the fuel pellet enrichment.

(Fifth Embodiment) FIG. 6 shows a fifth embodiment of the present invention. In this example, three fuel assemblies with a low enrichment having an average enrichment of ½ of the highest fuel assembly in the core are 3
Type is loaded. 7A shows a cross-sectional view of the fuel assembly 9 and FIG. 7B shows a cross-sectional view of the fuel assembly 2. In the core of the present embodiment, the fuel assemblies of 9 × 9 lattice arrangement are loaded in the regions other than the outermost layer of the core, and the fuel assemblies of 8 × 8 lattice arrangement are loaded in the outermost layer of the core. . The outermost layer of the core has a large neutron leak, and the output does not increase so much.
It has a large thermal margin compared to other areas. Therefore, 8 × 8
The lattice arrangement has a smaller thermal margin than the 9 × 9 lattice arrangement, but an 8 × 8 lattice arrangement can be arranged in the outermost layer of the core having a large thermal margin. Since the 8 × 8 lattice array has a simpler configuration than the 9 × 9 lattice array, the manufacturing cost is low, so that the economical efficiency can be secured.

[0026]

According to the present invention, since the thermal margin can be secured and the average enrichment can be increased in the initially loaded core, the fuel economy can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a low enrichment fuel assembly according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a second embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a third embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a fourth embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a fifth embodiment of the present invention.

FIG. 7 is a sectional view showing a low enrichment fuel assembly according to a second embodiment of the present invention.

[Explanation of symbols]

1 ... Fuel assembly, 2, 3, 9 ... Low enrichment fuel assembly, 4
... medium enriched fuel assembly, 5 ... control rod, 6 ... channel box, 7 ... water rod, 8 ... fuel rod.

Claims (5)

[Claims] 1. A nuclear reactor core composed of a plurality of fuel assemblies, wherein a plurality of types of fuel assemblies having different average enrichments are loaded, and a first fuel assembly having the highest average enrichment, A reactor core comprising a plurality of types of fuel assemblies having an average enrichment of one half or less of that of the first fuel assemblies. 2. The average enrichment according to claim 1, wherein
A second fuel assembly that is less than or equal to one half of the fuel assembly of the first fuel assembly, and a third fuel assembly that also has an average enrichment of less than or equal to one half of the first fuel assembly. One of them is loaded, and the second fuel assembly and the third fuel assembly are configured by arranging fuel rods of at least one type of enrichment in a grid pattern. The third fuel assembly is a reactor core in which the concentration distribution of fuel rods arranged is different. 3. The average enrichment according to claim 2, wherein
Of the second fuel assembly which is less than or equal to one half of that of the first fuel assembly, and the third fuel assembly of which the average enrichment is less than or equal to one half of that of the first fuel assembly are Reactor cores with different enrichments. 4. The second fuel assembly according to claim 1, 2 or 3, wherein the average enrichment is equal to or less than half of that of the first fuel assembly, and the average enrichment is the same as that of the first fuel assembly. The third fuel assembly, which is less than half of the fuel assembly of
Each of the second fuel assembly and the third fuel assembly is loaded with at least one, and each of the second fuel assembly and the third fuel assembly comprises at least one fuel rod and a water rod. The assembly is a reactor core with different arrangement of fuel rods and water rods. 5. The second fuel assembly according to claim 1, 2, 3 or 4, wherein the average enrichment is equal to or less than half of that of the first fuel assembly, and the average enrichment is the same as that of the second fuel assembly. A nuclear reactor core in which at least one third fuel assembly, which is one half or less of the first fuel assembly, is loaded, and the second fuel assembly is loaded in a control cell.