JPS61128185A - Core for nuclear reactor - 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] [Field of Application of the Invention] The present invention relates to the configuration of the initial loading core of a boiling water reactor installed in a nuclear power plant. The present invention relates to a nuclear reactor suitable for allowing four lines to operate at the same time.

[Background of the invention]

As shown in FIG. 2, the core of an S-water reactor is constructed by arranging a plurality of cells each consisting of one control rod and four fuel assemblies surrounding it.

In general, in a boiling water reactor, the average a11 of the fuel assemblies loaded in the reactor core during initial operation, the so-called initial loading core, is
ii! The i degrees were the same and one type. By the way, in a nuclear reactor, about 1/3 to 1/4 of the total number of fuel assemblies are taken out and replaced with new fuel every cycle, but the average enrichment of the fuel assemblies for the initially loaded core is 2 to 3 cycles. Since the setting is such that combustion is possible within the core, the operation cycle using the initially loaded core fuel assembly (hereinafter referred to as the "first cycle")
The cycles in which the fuel is partially exchanged and continued operation thereafter are referred to as a "second cycle", a "third cycle", and so on. ) During the fuel exchange at the end of 11, combustion had not yet progressed sufficiently and the fuel assembly with a high residual amount of uranium-235 had to be removed from the reactor core, which was uneconomical and uneconomical.

For this reason, in a boiling water reactor, the initial loading core is composed of all combinations of many types of fuel assemblies with different average enrichments, and each cycle is... - Taking out fuel assemblies with a low degree of contraction and replacing them with new fuel assemblies: This increases the average equalized burnup of the initially loaded fuel assemblies and speeds up the transition to the next cycle. Attempts are being made.

The new fuel assembly that is loaded at the beginning of the second cycle is called a replacement fuel assembly, and after the first cycle, the core that is continuously loaded with replacement fuel assemblies for several cycles is This is a cycle in which the fuel components have reached an almost constant state, and the PA characteristics of the previous cycle and the next cycle remain unchanged and are stable. This is called an equilibrium cycle, and the core that has become an equilibrium cycle is called an equilibrium cycle. That's what it means.

In this type of reactor, the thermal characteristics and cycle incremental burnup in the intermediate cycle (hereinafter referred to as the "transition cycle") transitioning from the first cycle to the equilibrium cycle are comparable to those in the equilibrium cycle, or are It is preferable to converge on them. However, when the aggregate average enrichment is one type as in the conventional initial loading core, the transition to the equilibrium cycle takes a long time, and the number of refueling bodies fluctuates in the transition cycle, which is not always satisfactory. There wasn't.

In addition, in the first cycle, there is a start-up test prior to commercial operation of the reactor, so the period until the end of the first cycle is longer, and the burnup in the first cycle is approximately 2000 MW, dZt lower than the burnup in subsequent cycles. Hadocho・gusimusashi 3nai. This was the first loading.

[Purpose of the invention]

An object of the present invention is to provide a cryogenic reactor core having an initial loading core that allows a rapid transition to an equilibrium cycle and is suitable for increasing the extraction burnup of an initial loading fuel assembly. .

[Summary of the invention]

The present inventor constructed an initial loading reactor core with a plurality of fuel assemblies having different average enrichments, and the... 4 degree of contraction,
It has been found that the above object of the invention can be achieved by specifying the number of plants and the expiration period of elm. The outline of the invention can be characterized by the following seven points.

(2) The initial loading core is configured by two or more types (for example, three types) of fuel assemblies with different average enrichments of the assemblies.

■ If the charge exchange ratio after the first cycle is 1/N, then
The type of fuel assembly mentioned above is also approximately N.

(2) A fuel assembly with the lowest z-shrinkage among the fuel assemblies described above (this is referred to as a low enrichment fuel assembly) is arranged at the outermost portion of the core.

■ The low enrichment fuel assemblies include fuel assembly L1 located at the outermost periphery of the core and fuel assembly L located inside the core.
2, and 7 of fuel assembly L2:! 1. is equal to the number of fuel assemblies taken out after the completion of one cycle, fuel assembly L2 is all taken out after the completion of one cycle, fuel assembly L1 stays in the outermost part of the core for two cycles, and fC
After that, that is, after the end of the second cycle.

(4) The four fuel assemblies surrounding the control rods inserted into the reactor core during power operation are composed of a low enrichment fuel assembly L1.

(2) The enrichment of the above-mentioned low enrichment fuel assembly is in the range of about 1.0 weight S to 1.5 weight weight S.

■ Among the fuel assemblies that make up the initial loading core, the enrichment of the fuel assembly with the highest M degree (this is called a high enrichment fuel assembly) is the same as that of the replacement fuel assembly! ! 'Equal to 1 degree.

In addition to increasing the fuel output speed, the limitation on the method of fuel extraction described in (2) contributes to an increase in the extraction burnup.

This technology is known, for example, in Japanese Patent Application Laid-open No. 57-8486 and Japanese Patent Laid-open No. 58
-63887, etc., but this alone cannot sufficiently achieve the object of the present invention, and the following technology is required.

According to ■, the type of fuel initially loaded at the end of the first cycle is the number of batches in the equilibrium core/? E' is equal, so
The transition from the initial loading core to the equilibrium core will be smooth.

Assuming that the total number of fuel loaded bodies is N4 and the number of replacement bodies is N8, the number of batches (this is defined as the reciprocal of the ratio of the number of replacement bodies) is N.

As shown in ■, the concentration of the initially loaded fuel is
In order to make the degree type n't- approximately equal to the number of batches,
nf: It is sufficient to choose an integer that satisfies the following. In other words, the number of replacement wings is N, and the number of replacement parts is N.
The following relationship holds between a-degree class n.

According to (2), the turbidity -rL of neutrons from the core can be reduced, and the burnup can be improved.

This is because most of the neutrons that leak out from the core and are lost are generated from the fuel assemblies at the outermost periphery of the core, while neutrons generated in fuel assemblies located at or above the innermost part of the core can leak out of the core. Therefore, if the outermost part of the core is made up of only low-reactivity fuel assemblies that generate a small number of neutrons, it is possible to reduce the number of neutrons leaking out of the core and improve the burnup. In transition cycle cores and equilibrium cycle cores loaded with replacement fuel, fuel assemblies with advanced fuel are used as low-reactivity fuel assemblies, i.e., fuel assemblies for the first loading are used for the next cycle. This patent describes an example in which the outermost part of the core is composed of high enrichment fuel with various enrichment levels, but this patent describes the arrangement of low enrichment fuel in the outermost part of the core. Since this is a feature, it is a completely different idea. In JP-A-59-15888, highly enriched fuel is placed around the outer periphery of the core in order to flatten the power distribution, but this increases the leakage of neutrons from the core, resulting in loss of reactivity. In order to compensate for this reactivity loss, it is necessary to increase the enrichment level, so the enrichment level required to obtain the same extraction burnup is
The example described in No. 8 is higher than the present invention, and the present invention has better fuel economy.

(2) is particularly important in increasing the burnup of low enrichment fuel. Because the output at the outer periphery of the core is low due to neutron leakage, combustion progresses only about half as much as at the center of the core. Table 1 shows that for fuels with the same enrichment, the average enrichment of the fuel assemblies inside the core is approximately 110 Wd/l, while the average burnup of the fuel assemblies outside the core is approximately 5 QWd/l. K2
The burnup becomes approximately 10() Wd/l at the end of the second cycle, which is equal to the burnup inside the core at the end of the first cycle. For this reason,
If the fuel assembly at the outermost periphery of the core is removed at the end of the first cycle, it will be removed before it is fully depleted, as shown in Table 1, which is uneconomical. For this reason, it is necessary that the fuel placed at the outermost periphery of the core at the time of initial loading stay in the core for at least two cycles. Since the reactivity loss is large when the reactor is allowed to stay in the furnace, two cycles is suitable for the period of stay in the reactor. In addition, it is important that the fuel removed at the end of the first cycle is only low-enrichment fuel in order to increase the removal burnup, so a number of low-enrichment fuels equal to the number of replacement bodies are It is removed from the core and replaced at the end of the first cycle.

According to this, the number of fuel assemblies N 1 b in the 1b group is equal to the number of replacement bodies and Nm, so the relationship according to equation (2) holds true.

(1) As described in Japanese Patent Application Laid-Open No. 56-1386, it is necessary to limit the number of control rods inserted into the reactor core during operation so as to eliminate the need for pattern exchange of control rods. In order to perform such an operation, it is necessary to configure the four fuel assemblies surrounding the control rods inserted into the reactor core during operation with low-reactivity fuel assemblies, and in the present invention, low-reactivity fuel assemblies It is characterized by the adoption of a fuel assembly with a low enrichment degree, and the burnup of the first cycle is 1100 MW, which currently requires only 9 to 12 months of operation period plus start-up time.
It is approximately 100 Wd/l in an e-class reactor.

The replacement fuel assembly has a triple enrichment of F&CH and is approximately 300Wd/l.
10 GWd/ per concentration
It will burn at a rate of l. In contrast, with conventional initially loaded fuel assemblies, the fuel assemblies taken out at the end of the first cycle have an initial enrichment of about 2t and 11-chi, but a burnup of about 100Wd/1. Since the fuel was still burning at a low temperature, it was difficult to tell if it was burning sufficiently, which caused uneconomical results. Therefore, considering that the concentration of the low enrichment fuel assembly taken out at the end of the first cycle is about 100 Wd/l, it is preferable that the enrichment is about 1.0% by weight. Furthermore, due to the expected longer operating period, the burnup of the first cycle is expected to increase to about 15 GWd/l, so the enrichment of low enrichment fuel will be about 1.0 to 1.0 GWd/l.
Five layers! It is best to set it as 壬.

As mentioned in (2), since the enrichment of high-enrichment fuel is removed, the transition to an equilibrium core quickly reaches K.

[Embodiments of the invention]

An embodiment of the present invention will be described below. Figure 1 is 1100
This is a plan view schematically showing 1/4 of the core of an MWe-class boiling water grave reactor. In the figure, 41.42 indicates the control rod,
Four fuel assemblies are loaded around it, one control rod and four fuel assemblies constitute a unit cell, and a reactor core is constructed by arranging a plurality of these unit cells. Control rods are classified into control rods 41, which are inserted into the reactor core during normal operation and for the purpose of adjusting the reactivity of the reactor core, and control rods 42, which are normally withdrawn from the reactor core and inserted into the reactor core only when the reactor is stopped. .

Fuel assemblies are classified into two or more types with respect to their average enrichment, and in the example shown in FIG. 1, are classified into three types. In this example, the total number of fuel loaded bodies is 764 bodies, the target average number of replacement bodies in the equilibrium core is 212 bodies, and according to the above formula (1), the type of enrichment is 764/212 = 16t. - The largest integer that does not exceed the same as the replacement fuel assembly, approximately 3.0% by weight, and the number of fuel assemblies is 248. The fuel assembly 2 is a medium enrichment fuel assembly with an enrichment level of approximately 12% by weight and a total of 212 fuel assemblies.

The remaining fuel assemblies are low-enrichment fuel assemblies with an enrichment of about 1.4 times I-1, with 31 being placed at the outermost periphery of the core and 32 being low-enrichment fuel assemblies.
is the other low enrichment fuel assembly arranged inside the reactor core. Total number F of low enrichment fuel assemblies 3.32 arranged inside the core
1212 bodies, which are removed at the end of the first cycle.

The number of 310 low enrichment fuel assemblies is 92.

A reactor core composed of such fuel assemblies operates as follows. First, when the first cycle of operation is completed, 3
This is carried out for the purpose of removing the 212 low enrichment fuel assemblies placed inside the reactor core as shown in 2.3 and quickly transferring them to the balanced core instead. The reason why the low enrichment fuel assemblies placed at the outermost periphery of the core are not removed at this time is partly due to the above-mentioned reason.This method can increase the burnup of the fuel assemblies placed at the outermost periphery of the core. .

Therefore, the fuel assembly taken out at the end of the first cycle always has a small amount of residual uranium-235.

At the end of the second cycle operation, 31.92 low-enrichment fuel assemblies and some of the medium-enrichment fuel assemblies, 120 of which were placed at the outermost part of the core, were removed, for a total of 212 fuel assemblies. of fuel assemblies are replaced with replacement fuel assemblies having an enrichment of approximately 3. Similarly, at the end of the third cycle operation,
The remaining 92 medium enrichment fuel assemblies and 120 high enrichment fuel assemblies, 212 in total, will be replaced with replacement fuel assemblies with a concentration of about 1a and 3ch. The third cycle core configured in this manner is an equilibrium core because the core purchase is the same as in the fourth cycle and subsequent cycles, in which fuel was continued to be replaced by 212 cores every year.

The number of replacement bodies at the end of the series remains constant at 212. When replacing fuel assemblies in this way, the fuel assemblies with the least amount of uranium-235 remaining are always taken out.
Unlike the conventional initial loading core, fuel with a high residual amount of uranium-235 and not sufficiently burned is no longer removed, so uranium can be used effectively and fuel economy is improved.

Figure 3 of Table 2 compares the change in burnup of surplus reactivity in each cycle.Only in the first cycle, the cycle incremental burnup is large due to the startup test period, but the surplus between each cycle The change in reactivity is small, and the change in burnup of excess reactivity after the 3rd cycle is the same, ? It can be said that the mind is in equilibrium.

As shown in Table 2, the reason why the core quickly converges to the equilibrium core is that the number of refueling bodies after the first cycle is the same.

Figure 4 compares the change in burnup of excess reactivity in the transition cycle when conventional fuel assembly enrichment is set to one type. Equilibrium is becoming difficult to achieve. In addition, in the conventional initially loaded core, which showed excess reactivity in Figure 4, the average extracted burnup was about 17 GWd/l, whereas
In this example, with the same initial core average enrichment, the average extracted burnup is about 210 Wd/l, which is a strange increase of about 23 inches.

Furthermore, in this embodiment, only the control rod 41 surrounded by the four Lee-enrichment fuels is used because the combustion change in excess reactivity is small and flat in each cycle as shown in FIG. Therefore, single pattern operation without the need for control rod pattern exchange is possible from the first cycle.

In particular, in this reactor core, the surplus J reactivity is as high as 1.3 Δ, and the control capacity of 1 ton of town 81 is approximately 0.1 Δ. 41 is 13, and the number of cells constituted by the four low enrichment fuel assemblies 32 is also 13.

Furthermore, in JP-A-58-223092, the initially loaded fuel assembly is divided into N groups with respect to enrichment, and the first group (1≦i<N-
An invention is described in which fuel assemblies belonging to 1) are taken out in the first cycle, but in this case, the relationship between the number of fuel assemblies in each group and the number of taken out at the end of each cycle is as described above. According to the example in the publication, as shown in Table 3, the number of fuel assemblies in group i and the number of removed assemblies at the end of the first cycle are the same, so this invention is completely different from the present invention shown in Table 2. It is clear that it belongs to In addition, in the example described in the above publication,
Since the number of bodies to be replaced in the first cycle and the second cycle is different, it is difficult to quickly shift to an equilibrium core. However, in the present invention, by making the number of bodies to be replaced in each cycle the same, This allows for a prompt transition to an equilibrium core.

Table 3 [Effects of the Invention] According to the present invention, the average discharge burnup of the initially loaded fuel assembly can be increased by about 20%, increasing fuel economy. Further, the transition from the initially loaded reactor to the equilibrium core becomes rapid, and from the third cycle onward, the same operation as in the equilibrium cycle becomes possible.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a nuclear reactor core according to an embodiment of the present invention, Figure 2 is a diagram showing unit lattice cells constituting the reactor core, and Figure 3 is a diagram showing the first cycle and transition cycle based on the present invention. FIG. 4 is a diagram showing combustion changes in surplus reactivity in the first cycle and transition cycle in a conventional initial loading core. 1...High enrichment fuel assembly, 2...Medium enrichment fuel assembly, 3...Low enrichment fuel assembly (for P central circle arrangement)
, 31...Low enrichment fuel assembly (for placement at the outermost periphery of the core: 32...Low enrichment fuel assembly (for control cell), 41
...Control rods inserted into the reactor core during operation, 42...p! Control rods that are not inserted into the reactor core during conversion.

Claims (1)

[Claims] 1. In a nuclear reactor core consisting of a fuel assembly and a control rod,
The fuel assemblies to be loaded are classified into a plurality of groups based on the average enrichment of the fuel assemblies, and the average enrichment of the fuel assemblies of each group is increased in the order of the first group, the second group, ... the nth group, and the highest The fuel assemblies belonging to the first group with low enrichment are further classified into group 1a, which is arranged at the outermost periphery of the core, and group 1b, which is arranged inside the core except for the outermost periphery of the core. Part 1
Only fuel assemblies belonging to group a are arranged, and the number N_i_b of fuel assemblies belonging to group 1b is the total number of fuel loaded bodies N
When _t, N_t/(n+1)<N_i_b≦N_
A nuclear reactor core characterized by being t/n. 2. The nuclear reactor core according to claim 1, wherein only the fuel assemblies belonging to group 1b are taken out at the end of the first cycle. 3. A patent claim characterized in that all four fuel assemblies adjacent to the control rods inserted into the reactor core during power operation and used for power adjustment are fuel assemblies belonging to the group 1b. A nuclear reactor core according to item 1 or 2. 4. Claims 1 and 2, characterized in that the average enrichment of the fuel assemblies belonging to the first group is in the range of 1.0% to 1.5% by weight, or The nuclear reactor core described in paragraph 3. 5. The atoms according to claims 1 to 4, wherein the average enrichment of the fuel assembly belonging to the n-th group is equal to the average enrichment of the replacement fuel assembly. Furnace core. 6. The number of fuel assemblies to be replaced at the end of each cycle from the first cycle to the equilibrium cycle is constant and approximately equal to the number of fuel assemblies N_i_b of the 1b group. Nuclear reactor core of Items 1 to 5.