JPS60238784A - Fuel aggregate - 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 [Technical Field of the Invention] The present invention relates to a fuel assembly used in a boiling water nuclear reactor or the like.

[Technical Background of the Invention] Generally, a fuel assembly for a boiling water nuclear reactor is composed of a plurality of fuel rods.

As shown in FIG. 2, the fuel rod 1 is covered on the outside with a fuel cladding tube 2 made of a zirconium alloy.

The upper and lower ends of the fuel cladding tube 2 are sealed by an upper end plug 3 or a lower end plug 4.

Inside the fuel cladding tube 2, a large number of short cylindrical fuel pellets 5 made of sintered powder of uranium oxide or the like are loaded.

FIG. 3 shows the cross section of the fuel assembly.

Inside the channel box 6, 62 fuel rods 1 and two water rods 7 are arranged in a grid of 8 rows and 8 columns. Light water at a relatively low temperature is flowing in the water lot 7, and even when the light water 8 in the channel box 6 is boiling, a moderating effect on neutrons is ensured. Note that the number of fuel rods 1 and water rods 7 in the channel box 6 varies depending on the reactor.
It is not limited to these values.

Now, the fuel material forming each fuel pellet 5 of the fuel rod 1, that is, uranium, is a fissile material, uranium-23.
A concentrated version of 5 is used. The enrichment of uranium-235 is uniformly distributed in the height direction of the fuel assembly, for example, about 3.0.
Weight %.

The reason for increasing the enrichment of the fuel material in this way is to ensure a high degree of reactivity so that the fuel assembly can burn for a long time in the reactor core.

By the way, the line segment A in FIG. 4 represents the relationship between the burnup of this fuel material and the infinite multiplication factor. As understood from this figure, the infinite multiplication factor decreases linearly as combustion progresses.

However, with such an infinite multiplication factor characteristic, the infinite multiplication factor is excessive from the early stage to the middle stage of combustion, and the shutdown margin necessary for shutting down the reactor cannot be obtained.

In order to resolve this contradiction, in each of the fuel assemblies shown in FIG. Contains toxic substances.

Gadolinia has the ability to absorb neutrons, but as it is irradiated with neutrons, it absorbs neutrons.

It has the property of decreasing yield capacity. 'For this reason, as shown by line B in FIG. 4, when fuel pellets containing gadolinia are used, the infinite multiplication factor at the early stage of combustion is reduced, and the necessary reactor shutdown margin is obtained. Furthermore, in the final stage of combustion when the infinite multiplication factor decreases, the neutron absorption capacity of gadolinia is sufficiently reduced, so that the infinite multiplication factor is not adversely affected.

The fuel rods loaded with Gadolinian fuel pellets (hereinafter referred to as Gadolinian fuel rods) have the role of averaging the infinite multiplication factor during the combustion period.

An example of gadolinia design in a conventional fuel assembly is shown below.

FIG. 5 shows the distribution of the number of Gadolinian fuel rods in a conventionally used fuel assembly. Line segment C1 in the figure shows the case of the most basic fuel assembly, and the number of Gadolinian fuel rods is a uniform number (7) regardless of the height of the fuel assembly.

In addition, for line segment C2, in order to flatten the power distribution in the height direction within the core, the number of Gadolinian fuel rods was set to 6 in the upper half of the fuel assembly, one less than in the lower half. An example of a fuel assembly design is shown.

Figure 6 shows the content of gadolinia contained in them; 3.5% in the upper half of the fuel assembly;
In the bottom half, it is 4.0%.

Thus, the gadolinia content is relatively smaller in the upper half of the fuel assembly than in the lower half. This is to reduce the amount of unburned gadolinia in the upper part of the core, where reactivity is low and combustion is relatively delayed during power operation, and to flatten the power distribution in the height direction within the core. be.

[Problems with the Background Art] Incidentally, in order to improve the operating rate and fuel economy of nuclear power plants, it is effective to increase the enrichment of the fuel assembly and lengthen the operating cycle length. be.

However, as already explained, if an attempt is made to enrich the uranium-235 higher than the conventional one, the infinite multiplication factor will again become excessive, causing the problem that the reactor shutdown margin will become smaller, especially at the beginning and end of the operating cycle. . In order to ensure margin for reactor shutdown, it becomes necessary to include a larger amount of burnable poison in the fuel assembly. However, when the content of burnable poison is increased, in the conventional fuel assembly, a situation arises in which the burnable poison remains again at the end of the cycle.

As a result, if the reactivity loss at the end of the cycle increases, the cycle length becomes shorter, causing problems such as lowering the reactor operating rate and fuel economy.

[Objective of the Invention 1] In view of the above circumstances, the present invention provides a method that can secure sufficient reactor shutdown margin necessary for shutting down a nuclear reactor even if the enrichment level of nuclear fuel material is increased, and also improves reactor operation rate and fuel economy. The purpose of the present invention is to provide a fuel assembly that does not significantly reduce the fuel efficiency.

[Summary of the Invention] That is, in the present invention, a burnable poison high concentration region is formed in a part of the upper part of a fuel assembly in which the burnable poison content of gadolinia is higher than in other regions.

This burnable poison high concentration area is preferably located in an area that includes a position where the neutron flux distribution in the core height direction when the core is cold when loaded with fuel assemblies that do not form this area is the highest. However, it may be a region in this vicinity. By setting the burnable poison high concentration region in this way, it is possible to secure a margin for reactor shutdown due to this high concentration region.

In addition, since the concentration of the burnable poison is low in the upper part other than this area, the residual pot of the burnable poison can be made smaller at the end of the cycle, and no reactivity loss occurs. Therefore, the reactor's operating efficiency and fuel economy are not impaired.

[Embodiments of the invention] The present invention will be described in detail below with reference to Examples.

i The fuel assembly of this example, as shown in FIG.
In the channel box 6, 62 fuel rods and 2 water rods are arranged in a grid of 8 rows and 8 columns. The average enrichment of uranium-235 in the fuel assembly is approximately 3.3% by weight, which is higher than conventional fuel assemblies, and the axial distribution of the enrichment is uniform C in the height direction.

Figure 7 shows the distribution of the number of gadolinia fuel rods, and Figure 1 shows the content of gadolinia contained therein.

In this example, from 19/24 to 21/2 of its effective length.
The range up to 24 is the high concentration area of burnable poisons. In this region, the number of Gadolinian fuel rods is 7, and the concentration of Gadolinia contained therein is 5.5% by weight.

Also, the upper region of the fuel assembly adjacent to this region, that is, the effective length of the fuel assembly from 11/24 to 19/
In the region between 24 and 22/24 tops, the number of gadolinian fuel rods is 7, and the gadolinia concentration is 4
．． It is 0% by weight. Therefore, in the burnable poison high concentration area, the gadolinia concentration is 1.5 type area % higher than in the adjacent area. Also, 11 from the bottom end of the fuel assembly.
/24, the number of gadolinia-filled fuel rods is 8, and the gadolinia concentration is 4.5% by weight.

In the fuel assembly of this example configured as described above, a burnable poison high 81 degree region is formed in the upper part excluding the upper end and the vicinity thereof, where the gadolinia content in the gadolinian fuel pellet is higher than in other parts. are doing. Therefore, it is possible to efficiently suppress the excessive reactivity of the reactor core in a cold state, and the necessary reactor shutdown margin can be secured.

The reason for this will be explained below.

That is, in general, in a boiling water nuclear reactor, light water, which is a coolant, generates voids in the reactor core, forming a void distribution. Since the void fraction is large in the upper part of the core, neutrons are poorly slowed down in the upper part of the core, and the reaction rate with neutrons of nuclear fuel material is low.

For this reason, there is a large amount of unburned fuel material in the upper part of the core. In addition, regions with high void ratios have a large accumulation of plutonium-239, which is a fissile material. As a result,
During a cold shutdown of a nuclear reactor where void generation is eliminated, the reactivity in the upper part of the reactor core becomes excessively high, making the margin for reactor shutdown difficult.

Therefore, in order to suppress the reactivity in the upper part of the reactor core from becoming excessively high, the fuel assembly of the example forms a region with high concentration of burnable poisons in the upper part of the reactor core, and efficiently secures reactor shutdown margin. ing.

Next, in order to explain the present invention in detail, the core characteristics during cold shutdown of the nuclear reactor will be compared with a fuel assembly based on a conventional design concept.

The average enrichment of uranium-235 in the conventional fuel assembly to be compared is 3.3% ω%, which is the same as in the embodiment of the present invention, and the number of gadolinia-containing fuel rods is also the same as in the embodiment of the present invention, that is, The distribution is shown in Figure 7. The gadolinia content is distributed as shown in Figure 8, with 4.0% by weight in the upper part of the fuel assembly and 5% by weight in the lower part.

As described above, the conventional fuel assembly does not have a burnable poison high concentration area in the upper part of the fuel assembly as in the present invention.

A comparison will be made of the neutron flux distribution in the height direction of the core during cold shutdown in a core configured as described above and made up of fuel assemblies according to the embodiment of the present invention and a core made up of conventional fuel assemblies. Figure 9 shows the results.

Line segment C5 is the case of the embodiment of the present invention, and line segment C6 is the case of the conventional fuel assembly. As you can see from these,
When the reactor is cold, the reactivity in the upper part of the core becomes excessively high, resulting in a significant upward shift in the neutron flux distribution.

On the other hand, in the embodiment of the present invention, a region with a high concentration of burnable poison is formed in the upper part of the fuel assembly, so the peak of the neutron flux density in the upper part of the core is smaller than in the case of a conventional fuel assembly. ing.

In other words, this is the result of suppressing the reactivity in the upper part of the core when it is cold, and this effect improves the reactor shutdown margin.

Moreover, a high concentration region of burnable poisons is formed in the upper part of the core where the C1 neutron flux density is highest in the cold state, that is, in the range of 19/24 or 621/24 of the effective length of the core, which improves efficiency. The degree of reactivity can be well suppressed, and the reactor shutdown margin can be improved.

In addition, in the present invention, since the burnable poison 8 concentration region is limited to only a part of the upper part of the fuel assembly, the amount of unburned gadolinia is minimized and the reactivity due to unburned remains is reduced. Reduces losses.

Therefore, problems such as a decrease in the reactor operating rate and fuel economy due to a shortened cycle length do not occur.

Incidentally, the burnable poison height (fi1 degree line area size) is not limited to the area of the embodiment of the present invention, and may be expanded or reduced depending on the required degree of reactor shutdown margin.

Similarly, the gadolinia content in the region may also be increased or decreased. Furthermore, it goes without saying that the distribution of Gadolinian fuel rods and the enrichment distribution of uranium-235 in the height direction of the fuel assembly are not limited to those of the example.

Further, in the present invention, enriched uranium is used as the nuclear fuel material, but the present invention is not limited to this.

For example, a fuel material that is a mixture of plutonium-239, which is a nuclear fission material, and uranium has been developed, and the present invention can also be applied to such a material.

[Effects of the Invention] As explained above, in the present invention, the concentration of burnable poison is made higher in a partial region of the upper part of the fuel assembly than in the lower part of the fuel assembly. Therefore, even if the fuel material is highly enriched, the reactivity in the upper part of the core does not become excessively high in the cold state, and the reactor shutdown margin can be improved.

Furthermore, since excessive burnable poisons are not placed in the entire upper part of the core, the presence of burnable poisons at the end of the cycle can be minimized and reactivity loss can be reduced. Therefore, it is possible to eliminate problems such as a decrease in the operating rate and fuel economy of the nuclear reactor due to a shortened cycle length.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the distribution of gadolinia concentration in the fuel assembly of the present invention, Fig. 2 is a partial sectional view of a fuel rod, Fig. 3 is a cross-sectional view of the fuel assembly, and Fig. 4 is a combustion Figures 5 and 6 are diagrams showing the relationship between degree and infinite multiplication factor, respectively. 7 is a diagram showing the distribution of the number of gadolinian fuel rods in a fuel assembly according to an embodiment of the present invention, and FIG. FIG. 9 is a diagram showing the distribution of gadolinia content in the fuel rod, and is a diagram comparing the neutron flux distributions of the embodiment of the present invention and the conventional fuel assembly in the cold state. 1・・・・・・・・・・・・Fuel rods 5・・・・・・・・・・・・ Fuel pellets Patent attorney Sa Suyama - Figure 1. Force triniy 1 degree (tt7.) Fig. 2 Fig. 5 Gase Sonia Shido (lord)

Claims (3)

[Claims] (1) A fuel assembly characterized in that a burnable poison high concentration region is formed in a part of the upper region in which the content of burnable poison contained in the fuel pellets is higher than in other parts. (2) The fuel assembly according to claim 1, wherein the burnable poison high concentration region is formed in the upper part excluding the upper end. (3) A burnable poison high concentration region is formed in a core loaded with a fuel assembly that does not form this region, and includes the position where the neutron flux distribution in the height direction of the core in a cold state is highest. The fuel assembly according to claim 1, characterized in that: