JPS62179690A - Nuclear reactor core - 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 a nuclear reactor core, and particularly to an initial loading core of a boiling water reactor installed in a nuclear power plant.

[Background of the invention]

The core of a boiling water reactor is shown in Figure 8.

It is constructed by arranging a plurality of cells each consisting of one control rod 3 and four fuel assemblies 1 surrounding it. 2
1 shows fuel rods constituting the fuel assembly 1.

Generally, in a boiling water reactor, the average enrichment of the fuel assemblies loaded into the reactor core during initial operation, the so-called initial loading core, is the same and of one type. By the way, in a nuclear reactor, approximately 173 to 1/4 of the total number of fuel assemblies are removed and replaced with new fuel every cycle, but the average enrichment of the initially loaded core fuel assemblies is within the core for 2 to 3 cycles. Since the setting is such that combustion is possible, the operation cycle using the initially loaded core fuel assembly (hereinafter referred to as the "first cycle"), and the subsequent cycle in which fuel is partially replaced and continued operation, is called the "first cycle". 2
cycle", "third cycle", etc. ) Refueling at the end of the reactor is uneconomical because the fuel assembly, in which combustion has not yet progressed sufficiently and has a high residual amount of uranium-235, must be removed from the reactor core.

Load urine at the beginning of the operating cycle after the second cycle The new fuel suspension assembly that is installed is called a replacement fuel assembly.

After the first cycle, the thermal characteristics of the previous cycle and the next cycle continue to be stable for several cycles without changing, and this is called an equilibrium cycle. It's called a cycle.

In such a reactor, the thermal characteristics and cycle incremental burnup in the intermediate cycle transitioning from the first cycle to the equilibrium cycle (hereinafter referred to as the "transition cycle") are comparable to those of 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 also took a long time, and the number of refueling bodies varied greatly during the transition cycle, which was not always satisfactory.

For this reason, in boiling water reactors, the initial core is constructed by combining many types of fuel assemblies with different average enrichments, and in each cycle, the fuel assemblies with lower average enrichments are taken out and used as new fuel. Attempts have been made to increase the average discharged burnup of the initially loaded fuel assembly and to speed up the transition to the primary cycle by replacing it with the fuel assembly.

This technique is disclosed in, for example, Japanese Patent Laid-Open No. 57-8486.

On the other hand, in order to lengthen the operating cycle and increase burnup,
It is necessary to increase the average enrichment even in the initially loaded core.
As mentioned above, when many types of fuel assemblies with different average enrichments are combined to form the initial loading core, the difference in enrichment between the fuel assemblies becomes large, and the difference between high-condensity fuel assemblies and low-condensity fuel assemblies increases. As a result of the increased difference in nuclear properties from the enrichment fuel assembly, it is no longer necessarily desirable from the standpoint of burning fuel efficiently.

[Purpose of the invention]

An object of the present invention is to provide an initial loading core that is optimal for prolonging the M cycle and increasing burnup.

[Summary of the invention]

The present invention provides a nuclear reactor core in which an initial loading core is constructed by combining many types of fuel assemblies with different average enrichments.
The present invention is characterized in that at least some of the types of fuel assemblies among the various types of fuel assemblies have different moderator-to-fuel ratios in combination with average enrichment.

The present invention was made based on the results of the inventor's detailed study of the relationship between the moderator-to-fuel ratio and the neutron multiplication factor, and by quantifying changes in the moderator-to-fuel ratio with respect to changes in enrichment. , the results of the study will be explained below.

In general, the relationship between the moderator-to-fuel ratio and the neutron infinite multiplication factor is:
It is characterized as follows.

In addition, in light water reactors such as boiling water reactors.

Water not only acts as a coolant, but also has the function of moderating neutrons and acts as a moderator.

In nuclear reactors using slightly enriched uranium, determining the moderator-to-fuel ratio is particularly important in determining the nuclear properties of the reactor core. First, the moderator-to-fuel ratio affects the neutron absorption rate in the fast neutron energy region and the resonant neutron energy region. As the moderator-to-fuel ratio increases, the proportion of epithermal neutrons that are absorbed into the fuel material decreases and the proportion of neutrons that reach thermal energy increases. (This is called a softening of the energy spectrum.) In a mature neutron reactor that uses slightly enriched uranium, most of the absorption reactions of epithermal neutrons are capture reactions into parent materials such as 286U, and the energy generated by fission is does not contribute to the generation and multiplication of neutrons; on the other hand, as thermalization increases, the fissile material in the fuel, which is the largest thermal neutron absorber, e.g.
Because absorption into 11U increases, the proportion of neutrons that cause thermal neutron fission among all neutrons increases. As a result, assuming that all thermal neutrons are absorbed by the fuel material.

As the moderator-to-fuel ratio increases, the neutron infinite multiplication factor increases monotonically. However, in reality, increasing the moderator-to-fuel ratio will also increase the absorption of thermal neutrons into the moderator, reducing the fraction of thermal neutrons that undergo fission and decreasing the infinite neutron multiplication factor. It has an effect.

When the moderator-to-fuel ratio is small, the former effect is significant, and increasing the moderator-to-fuel ratio results in an increase in the neutron infinite multiplication factor. However, as this ratio increases, the thermalization of neutrons becomes saturated, and the increase in thermal neutron absorption into the moderator offsets the decrease in the absorption of epithermal neutrons into 28δU. As a result, the relationship between the moderator-to-fuel ratio and the infinite neutron multiplication factor is generally characterized as shown in FIG.

In other words, the infinite neutron multiplication factor is, as shown in Figure 7,
There will be a maximum point for changes in the moderator-to-fuel ratio, but if the moderator-to-fuel ratio is smaller than the maximum point of the infinite neutron multiplication factor, it is usually said that there is insufficient moderation, and if it is larger, the , is said to be an excessive deceleration.

From the point of view of fuel economy, it is desirable to set the moderator-to-fuel ratio at the point where the infinite neutron multiplication factor is maximum, but in reality, for safety reasons, the moderator-to-fuel ratio should be set slightly below the maximum point. Optimized in the state of

By the way, if we change the average enrichment of the fuel.

Since the content of fissile material in the fuel changes and the moderator-to-fuel ratio changes, if the fuel lattice shape is not changed, the infinite neutron multiplication factor can be changed from the optimal point by changing the average enrichment. 1 Normally, the average enrichment of the initially loaded core is approximately 2% by weight, and the core lattice shape is selected to optimize the moderator-to-fuel ratio for this average enrichment. However, if the initial loading core is composed of many types of fuel assemblies with different average enrichments, the high-enrichment fuel assemblies will be in a state of insufficient moderation, and the low-enrichment fuel assemblies will be in a state of excessive moderation. Therefore, the deceleration state is not necessarily optimal.

Based on these facts, in the present invention, in an initial loading core composed of many types of fuel assemblies with different average enrichments, for high enrichment fuel assemblies, the fuel assemblies are arranged so that the amount of moderator increases. For low enrichment fuel assemblies, the shape of the fuel assemblies was chosen to reduce the amount of moderator. In this explanation, we have focused on the amount of moderator, but if we focus on the amount of uranium (including not only 28δU but also other isotopes such as 28δU), the amount of uranium in a low enrichment fuel assembly can be expressed as follows: This means that the amount of uranium was greater than the amount of uranium in the high-enrichment fuel assembly.

[Embodiments of the invention]

Examples will be described below.

Figures 1 to 4 are explanatory diagrams of one embodiment, and Figure 1 is 1100.
This is an explanatory diagram of a MWa-class boiling water reactor (BWR).The reactor core is composed of 764 fuel assemblies and 193 control rods. /4 cross section is shown. Figures 2 to 4 are plan cross-sectional views of fuel assemblies used in the core of Figure 1. In these figures, 11 is a high enrichment fuel assembly, 12 is an intermediate high enrichment fuel assembly, and 13 is a high enrichment fuel assembly. intermediate low enrichment fuel assembly; 14 is a low enrichment fuel assembly; 31 is an operating core;
.

The control rods inserted therein, ie, the control rods inside the control cell, 32 indicate the control rods outside the control cell.

In this embodiment, the core includes a high enrichment fuel assembly 11, an intermediate high enrichment fuel assembly 12, and an intermediate low enrichment fuel assembly 13.
．． The low enrichment fuel assembly 14 is made up of four types of fuel assemblies each having a different average degree of contraction. Assuming that the operating period after the start-up test is 15 months, the enrichment of the high enrichment fuel assemblies 11 is approximately 3.4% by weight, 208, and the enrichment of the intermediate high enrichment fuel assemblies 12 is 1! , the degree of contraction is approximately 2.6% by weight and 208 bodies, the degree of contraction of intermediate low enrichment fuel assembly 13 is approximately 2.0% by weight and 204 bodies, and the low enrichment fuel assembly 14 is approximately 1.2% by weight] -44 bodies, and the average enrichment of the initially loaded core is about 2.4% by weight.

In this core, 92 low enrichment fuel assemblies 14 are arranged at the outermost part of the core, and the remaining 52 are arranged as control cells so as to surround control rods, 4 of each. The remaining high:1 compression degree fuel assemblies 11, intermediate high:ja compression degree fuel assemblies 12, and intermediate low enrichment fuel assemblies 13 are almost uniformly arranged in the reactor core. Fuel assemblies with low average enrichment are removed from the core and replaced with new fuel. In particular, since the high enrichment fuel assembly 11 is taken out last, the average burnup of the taken out fuel is approximately 38 GWd/l.
bawl.

The average enrichment of the fuel assemblies used in this core is approximately 1
．． The range varies from 2% by weight to approximately 3.4% by weight, and the average burnup for extraction ranges from approximately 6 GVd/l to approximately 38G.
It varies up to Wd/l.

For this reason, if fuel assemblies with the same shape as in the past are used, the high enrichment fuel assemblies 11 will have insufficient deceleration.
On the other hand, the low enrichment fuel assembly 14 is not preferable in terms of reactivity because the moderation is excessive.6 Therefore, in this reactor, three types with different water to uranium volume ratios are prepared by changing the fuel rod configuration. The fuel assembly is used.

FIG. 2 shows an intermediate low enrichment fuel assembly 12 and an intermediate low enrichment fuel assembly 12.

FIG. 3 is a diagram showing the shape of an intermediate low enrichment fuel assembly 13.

Hereinafter, it will be referred to as a type 2 fuel assembly. The Type 2 fuel assembly consists of 62 fuel rods 2 containing fissile material and two water rods 4 arranged in an 8x8 grid, covered with a channel box 5.
This is a fuel assembly used quasi-iIA in WR.

FIG. 3 is a diagram showing the shape of the high enrichment fuel assembly 11-, which is hereinafter referred to as a type 1 fuel assembly. The type 1 fuel assembly has 60 fuel rods 5 containing fissile material and one large-diameter water rod 6 arranged in the center with four fuel rods removed in a lattice pattern. is covered with a channel box 5. Type 1 fuel assemblies have an increased water to uranium volume ratio due to the increased cross-sectional area of the water rods compared to Type 2 fuel assemblies. In addition, FIG. 6 is a schematic explanatory diagram of the high enrichment fuel assembly 11, in which 2 is a fuel rod, 6 is a large diameter water rod, 1
7.18 indicates the upper and lower tie plates, and 19 indicates the fuel spacer.

FIG. 4 is a diagram showing the shape of the low enrichment fuel assembly 14,
Hereinafter, it will be referred to as a type 3 fuel assembly. A type 3 fuel assembly is composed of 64 fuel rods 2 and a channel box 5, and does not include water rods.
Since the water to uranium volume ratio of the fuel assembly is smaller than that of the type 2 fuel assembly, excessive moderation of neutrons can be alleviated.

Incidentally, the cross-sectional area of the water rods in this example is approximately 8.0 cd for the type 1 fuel assembly, and approximately 3.0 cd for the type 2 fuel assembly with two water rods each having a cross-sectional area of approximately 1.6 d.
2 ci, Type 3 fuel assembly is O. Therefore, when comparing the water to run volume ratio, type 1 fuel assemblies are approximately 2
．． 86. Type 2 fuel assemblies are approximately 2.80, and Type 3 fuel assemblies are approximately 2.70.

In addition, in the embodiment shown in Fig. 1, type 3
Fuel assemblies are placed. This is because the fuel assembly at the outermost periphery of the core has water on one or two of its four sides, so it has a relatively large amount of moderator, and especially the moderator-to-fuel ratio. This is because there is no need to increase the cross-sectional area of the water rod in order to increase the water rod. Furthermore, the fuel assemblies at the outermost periphery of the core are also low-smoke fuel assemblies and have a relatively low output, which has the effect of reducing leakage of neutrons from the core. On the other hand, low-enrichment fuel assemblies are also used within control cells, in order to lower the output within the control cell and alleviate sudden changes in fuel output due to control rod operations. .

In addition, as a type 1 fuel assembly, a high enrichment fuel assembly 151 in which the number of fifth water rods is increased to 4 or more instead of a large diameter water rod, for example, JP-A-50-28
601 may be used; in this case too, the cross-sectional area of the water rods is increased compared to the Type 2 fuel assembly, so the water to uranium volume ratio is increased and the The insufficient deceleration of the enriched fuel assembly can be alleviated, and the reactivity characteristics are improved.

In addition, in the above-mentioned example, an example was shown in which the shapes of both the high-enrichment fuel assembly and the low-enrichment fuel assembly were different from the shape of the commonly used Type 2 fuel assembly. However, for example, only high enrichment fuel assemblies are type 2.
Even when this invention is partially applied, such as using a type 1 fuel assembly different from the fuel assembly and using a type 2 fuel assembly as the low enrichment fuel assembly, the effect is the same as in the case of the above-mentioned embodiment. Although it is not as good as that, it works in the same way, and you can get a certain effect.

According to these examples, it is possible to optimize the moderator-to-fuel ratio of fuel assemblies with different average enrichments used in the initial loading core, making it possible to maintain optimal reactivity characteristics and This has the effect of increasing the extraction burnup of the loaded core.

〔Effect of the invention〕

The nuclear reactor core of the present invention can provide an initial loading core that is optimal for long-term operation cycle ratios and high burnup, and has great industrial effects.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing the configuration of an embodiment of the nuclear reactor core of the present invention, FIGS. 2 to 4 are plan sectional views of a fuel assembly used in the reactor core of FIG. 1, and FIG. is a plan cross-sectional view of a fuel assembly used in another embodiment of the nuclear reactor core of the present invention, FIG. 6 is an explanatory diagram of the configuration of the fuel assembly of FIG. 3, and FIG. 7 is a moderator in the reactor core. FIG. 8, which is a diagram showing the relationship between fuel ratio and infinite neutron multiplication factor, is a plan cross-sectional view of a unit lattice cell constituting the boiling water reactor core. DESCRIPTION OF SYMBOLS 1... Fuel assembly, 2... Fuel rod, 4... Water rod, 5... Channel box, 6... Large diameter water rod, 11... High enrichment fuel assembly, 12
... Intermediate high enrichment fuel assembly, 13... Intermediate low enrichment fuel assembly, 14... Low enrichment fuel assembly.

Claims (1)

[Scope of Claims] 1. In a nuclear reactor core in which an initial loading core is constructed by combining many types of fuel assemblies with different average enrichments, at least some types of fuel among the many types of fuel assemblies are provided. In the aggregate, a nuclear reactor core characterized by different moderator-to-fuel ratios in combination with average enrichment. 2. The nuclear reactor core according to claim 1, wherein among the multiple types of fuel assemblies, fuel assemblies with higher enrichment levels have a larger moderator-to-fuel ratio. 3. The nuclear reactor core according to claim 1 or 2, wherein the moderator to fuel ratio is determined by the cross-sectional area of the water rod. 4. The nuclear reactor core according to claim 3, wherein the cross-sectional area of the water rod is determined by a large diameter water rod inserted after removing four fuel rods. 5. The nuclear reactor core according to claim 3, wherein the cross-sectional area of the water rods is determined by increasing or decreasing the number of water rods inserted after removing fuel rods. 6. Any one of claims 1 to 5, wherein the nuclear reactor core is a nuclear reactor core in which a fuel assembly with a low moderator-to-fuel ratio is arranged at its outermost periphery. The nuclear reactor core described.