JPS61272686A - Faster breeder - 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 fast breeder reactors, and in particular to fast breeder reactors that perform so-called breeding in which new fissile material is produced by absorbing neutrons generated by plant cos fission into a fuel parent material. This feature allows for effective use of fuel. The core of such a fast breeder reactor is generally formed in a cylindrical shape, and the core is surrounded by axial and radial blankets containing a fuel parent material as a main component. The reactor core is loaded with enriched uranium or plutonium-enriched uranium as fuel, and the blanket is loaded with, for example, natural uranium or depleted uranium as a fuel parent material. Useful fissile material is then produced by this fuel parent material capturing neutrons leaking from the core.

However, fast breeder reactors with such characteristics.

Plant costs are higher than light water reactors that generate the same output. The most effective way to reduce plant costs is to design a small breeder reactor for the entire plant, especially the core system.

By the way, if the core is downsized while preserving the core power, the average power density (output per unit volume) of the core increases. On the other hand, the upper limit of the power density is determined from the fuel usage limit. Therefore, in order to downsize the core, it is necessary to flatten the power distribution of the core as much as possible and increase the average power density while suppressing the maximum value. For this reason, various efforts have been made to flatten the output distribution.

For example, there is a core concept called a homogeneous core.

(Refer to "Fast Breeder Reactor" by Hiroshi Yasunari, published by the same publication, p. 44) As shown in Figure 7, this means that the enrichment of fissile material in the outer core region IB, where neutron leakage is large, is higher than that in the inner core region IA. By increasing the height, the power distribution of the core is flattened.

In general, a nuclear reactor loses its reactivity due to an increase in output at reactor startup and combustion after achieving rated output. Therefore, in order to compensate for these reactivity losses, the reactivity is usually increased by loading extra fuel into the reactor at the initial stage of combustion, and new fuel is added to the reactor by fuel exchange after a predetermined firing period has elapsed. j・1 This is to prevent the reactor from shutting down due to a drop in reactivity until it is loaded into the core.

On the other hand, nuclear reactors are equipped with control rods containing neutron-absorbing substances, which are inserted into the reactor core during operation to reduce excess reactivity and keep the reactor at just the right level of criticality.

In the case of fast breeder reactors, control rods are often inserted from the top of the core, and their insertion depth is greatest at the beginning of combustion when reactivity is high, decreases as combustion progresses, and is minimal at the end of combustion. .

In the above-mentioned homogeneous core, the role of the control rods 4A is not only to reduce excess reactivity but also to smooth out the power distribution. That is, assuming the power distribution when the control rod 4A is fully withdrawn, the control rod 4A is inserted where the power density is maximum.

However, with this method, if the reactivity value per control rod 4A is increased by increasing the insertion depth of the control rod 4A or increasing the concentration of the neutron absorbing substance, the control rod 4A
Therefore, it is necessary to reduce the reactivity value per control rod and operate with a large number of control rods.

[Purpose of the invention]

The present invention has been made in view of the above, and its purpose is to provide a fast breeder reactor that can flatten the core power distribution throughout the combustion cycle and that can be operated with a small number of control rods. It is in.

[Summary of the invention]

A feature of the present invention is that in a fast breeder reactor in which a cylindrical core is surrounded by an external bracket mainly composed of a fuel parent material,
The core is divided into three regions: inner region A, outer region B, and control rod adjacent region C, and the fissile material enrichment e, l e, I QC for each region is expressed as eA<ec≦e. The point is that we made it so.

[Embodiments of the invention]

This example is for a case in which a core fuel containing a mixed oxide, mainly a blanket fuel containing depleted uranium, is loaded and liquid sodium is used as a coolant, but the present invention can also be applied to cases where fuels and coolants other than those mentioned above are used. It is possible to apply.

FIG. 1 is a configuration explanatory diagram showing one embodiment of a core of a fast breeder reactor of the present invention, in which (a) is a horizontal sectional view of the core, and (b) is a vertical sectional view of the core. In Figure 1, IA, IB, IC
The core regions indicated by are the inner region, outer region, and control rod adjacent region, respectively, and the plutonium enrichment of the fuel is IB and IC.

In order of increasing IA. These core regions are surrounded by a radial blanket 2 and an axial blanket 3, each of which has a fuel parent substance as its main component. Note that the design parameters and operating conditions of this example are as shown in Table 1.

That is, the reactor power is approximately 2500 MW, the equivalent core diameter and the core height are 327 G, 1. OOam,
The axial blanket thickness and radial blanket thickness are 35G and 31an, respectively, and the fuel smear density is 87% theoretical density in the core and 91% theoretical density in the blanket. The plu-1-nium enrichment of the core fuel is determined by the inner region,
14% and 20% in the outer area and control rod adjacent area, respectively.
, 18%.

The number of fuel assemblies is 132°12 in each region above.
6.78, a total of 336 control rods, and the number of control rods is 1 adjustment rod.
3. There are 6 starting rods and 6 backup system slots, totaling 25 pieces. The refueling interval is one year, the capacity factor is 80%, and the number of refueling batches is 3 for the core, 4 for the inner radial blanket, and 5 for the outer radial blanket.

Figure 2 is a diagram showing the power distribution in the core radial direction when control rods are inserted in the early stage of the combustion cycle in comparison with a conventional homogeneous core. is the seventh
In the case of the conventional homogeneous core shown in the figure, curve C shows the case of the conventional homogeneous core of FIG.

The numbers of adjustment bars used here are 13, 1.8, and 13 for a, b, and c, respectively. Assuming that the number of regulators is constant, in the embodiment shown in FIG. 1, when the conventional homogeneous reactor 7' is fully drawn out, the maximum linear output increases by about 3% compared to the final stage of the conventional homogeneous reactor. However, at the end of the combustion cycle, the fissile material plutonium-239 accumulates in the outer blanket, so its output increases, and the output in the core region decreases relatively, resulting in a maximum linear output of about 6% less than at the beginning of combustion. . From the above, the maximum linear output throughout the combustion cycle is reduced by about 3% in the embodiment of FIG. 1 compared to the conventional homogeneous core. This increases the thermal margin of the fuel and improves operational performance, or it can be designed under conditions where the maximum linear output remains constant, reducing the core volume by approximately 3% compared to conventional methods and reducing plant costs. . Additionally, 18 adjustment rods were required in conventional cores to flatten the power distribution.
The number can be reduced to 13. As a result, the reactivity of the control rods inserted into the reactor core during operation can be reduced by about 30%, and the permissible fluctuation range of the control rods during earthquakes can be increased by about 30%. Therefore, if the rigidity of the structural material such as a reactor vessel is small,
By surrounding the adjustment rod with relatively highly enriched fuel,
The reactivity value of the conditioning rod increased by approximately 10%. Therefore, the concentration of neutron absorbing material to obtain the same reactivity value as the conventional one can be reduced by about 10% compared to the conventional homogeneous core.

The above effects are achieved by increasing the neutron flux around the control rods, which was too small in a conventional homogeneous core, by placing highly enriched fuel there, and flattening the overall neutron flux distribution. This is due to a number of reasons.

Next, other embodiments of the present invention will be described.

FIGS. 3 to 6 are explanatory diagrams of the configuration of a core showing other embodiments of the present invention, and each is a vertical sectional view of the core.

In the embodiment shown in FIG. 3, the outer region IB in FIG.
The fuel enrichment in the control rod adjacent region IC is made equal to that of the control rod adjacent region IC, and almost the same effect as that shown in FIG. 1 can be obtained.

1B. This flattens the core axial power distribution, reduces the maximum linear power, and increases the thermal margin of the fuel. Also, if the maximum line output is constant,
This makes it possible to downsize the reactor core, which has a significant effect on reducing plant costs. Furthermore, it has an effect similar to that of FIG.

In the embodiment shown in FIG. 5, in FIG.
In D, the core height of the area adjacent to the control rods is reduced to 0.3 to 0.7 of the total core height from the side where the control rods are inserted, and the reduced part is replaced in the inner area. It is. This flattens the axial power distribution at the time of control rod mid-insertion in the early and middle stages of the combustion cycle, increasing the thermal margin of the fuel or making it possible to downsize the reactor core. Together, the same effects as in the case of FIG. 1 can be obtained.

In the embodiment shown in FIG. 6, in FIG.
-In E, the core height of the area adjacent to the control rods is reduced to 0.3 to 0.7 of the total core height from the side where the control rods are inserted, and the reduced part is replaced with the inner area IA. This is what I did. As a result, FIG.

An effect similar to that of FIG. 5 can be obtained.

〔Effect of the invention〕

According to the present invention, the neutron flux in the area around the control rod increases and the neutron flux in the area away from the control rod decreases, thereby flattening the neutron flux distribution in the core. Therefore, the power distribution is flattened and the maximum linear power is reduced by about 3% compared to a conventional homogeneous core. Furthermore, the above effect eliminates the need for adjustment rods for flattening the power distribution, which were required in conventional homogeneous cores, and the total number of adjustment rods can be reduced by about 30%. This reduces the reactivity of the control rods inserted into the core during operation, and increases the permissible fluctuation range of the control rods during earthquakes by about 30%. Therefore, the rigidity of structural materials such as reactor vessels can be reduced, which has a great effect on reducing the size and weight of the plant, that is, reducing the plant cost by 1. Furthermore, when using control rods with the same neutron-absorbing material, the reactivity value of the control rods increases by about 10% compared to a conventional homogeneous core. That is, the concentration of the neutron absorbing material can be reduced by about 10% to obtain the same reactivity value.

Currently, mixed oxides of plutonium and uranium are used as fuel for fast breeder reactors, but there is a possibility that they will shift to carbide or nitride fuels in the future. In this case, compared to oxide fuel, the fuel substance concentration in the fuel is higher, and the heat transfer properties are better, so the fuel volume ratio is also increased, and the internal conversion ratio is increased.

Therefore, depending on the design, the reactivity of the reactor core increases with combustion. However, the absolute value of the fluctuation range of the reactivity is smaller than that when using oxide fuel. Therefore, reactivity control is possible with a small number of control rods made of a substance that has positive reactivity when inserted into the reactor core. Such control rods include void tubes and fuel assemblies. In this case, the power distribution of the reactor core will be flatter if the enrichment of fissile material in the fuel located around the control rods is not very high. Therefore, the enrichment can be the same as or smaller than that of the inner core region.

[Brief explanation of the drawing]

FIG. 1 is a configuration explanatory diagram showing one embodiment of the core of a fast breeder reactor of the present invention, FIG. 2 is a relationship diagram between core radial distance and aggregate output for explaining the effect of FIG. 3. Figs. 3 to 6 are diagrams illustrating the configuration of one reactor core showing other embodiments of the present invention, respectively, and Figs. 7 and 8 are configuration diagrams showing conventional homogeneous cores. (a) is a horizontal sectional view of the core, and FIGS. 7(b) and 8 are vertical sectional views. IA...Inner core area, IB...Outer core area, I
C... Control rod adjacent core region, 2... Radial blanket, 3... Axial blanket, 4... Control rod,
4A...Adjustment offering, 4B...Backup system rod, 4C...
Starting stick, 5...storium follower.

Claims (1)

[Claims] 1. A reactor core region having fuel enriched with fissile material;
In the reactor core region, the reactor core region is defined as a first core region in which the outer blanket region surrounds the outside of the core region and includes fuel mainly made of a fuel parent material, and a plurality of control rods are inserted into the core region. and a second core region surrounding the outside of the first core region, and a third core region surrounding a plurality of control rods inserted into the core region during operation,
Enrichment of fissile material in each core region e_1, e_2, e
A fast breeder reactor characterized in that _3 satisfies e_1<e_3<e_2. 2. The height of the third core region is 0.3 to 0.7 of the total core height in the axial direction from the side where the control rods are inserted, and the core region is adjacent to the third core region in the axial direction. The fast breeder reactor according to claim 1, wherein: is the first core region. 3. The fast breeder reactor according to claim 1 or 2, wherein the second and third core regions have the same enrichment of fissile material.