JPH04335189A - Core structure of fast breeder reactor - Google Patents

Abstract

PURPOSE:To obtain core structure for a fast breeder reactor which suppresses the lowering of economy by keeping coolant void reactivity to be zero or negative even in a high power core, or improves economy by utilizing effectively and positively an unused region in the center of an annular type core. CONSTITUTION:Core region 3 is formed by arranging a multitude of fuel assemblies 2 loaded with fissile material in multi-annular shape. In the inner side of the fuel assemblies 2 in the core region 3, a multitude of absorber assemblies 10 loaded with neutron absorber material are arranged to provide a neutron absorber region 11 in an annular type core.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core structure of a fast breeder reactor using liquid metal as a coolant, and more particularly to a core structure of a liquid metal cooled fast breeder reactor which is an improved core structure of a fast breeder reactor.

0003

[Prior art] A fast breeder reactor that uses liquid metal as a coolant is
The core consists of a large number of fuel assemblies filled with fissile material, and sodium is primarily used as a coolant to remove heat from the fuel.

In this liquid metal cooled fast breeder reactor, if the flow rate of the coolant decreases for some reason, the temperature of the coolant increases, the density of the coolant decreases, and reactivity is introduced. The reactivity of fast breeder reactors depends on the output scale of the reactor, and is negative in small reactors, but positive in medium and large reactors, which may increase reactor output due to a decrease in coolant density.

[0005] In a fast breeder reactor, sodium, which is a coolant, does not normally evaporate and form voids, but in the unlikely event that sodium becomes voids, the reactor can be safely shut down and The design maintains the safety of the reactor core.

[0006] The response of a nuclear reactor when sodium, which is a coolant, becomes void is that if the reactor core is small, the leakage of neutrons from the core is large, resulting in negative reactivity as described above. The reactor core can be safely shut down.

On the other hand, when the core of a fast breeder reactor becomes larger, neutron leakage from the core decreases, and the reactivity when the coolant sodium becomes void becomes positive. In order to determine whether the core can be safely shut down in this positive reactivity state, it is necessary to conduct a detailed analysis including other reactivity factors to confirm the safety of the core.

The positive reactivity caused by sodium, which is a coolant, can be reduced by increasing neutron leakage from the core and softening the neutron spectrum. Measures have been taken to soften the neutron spectrum by loading absorbing materials, lowering and flattening the core height to increase neutron leakage, or loading neutron moderators such as beryllium or zirconium hydride. It's here. In any case, if the void reactivity of sodium, which is a coolant, can be reduced, the safety and reliability of the reactor core will be further improved, and this will be of great value in the safety design of the reactor.

[0009]

[Problem to be Solved by the Invention] However, in a nuclear power plant such as a fast breeder reactor, it is preferable in terms of power generation cost to design the core output to be extremely small from the standpoint of core safety. do not have.

[0010] Furthermore, in order to make the reactor core reactivity negative even if the coolant sodium becomes void, it is necessary to use a small reactor with a core output of, for example, about 100 MWe; If it exceeds the limit, a problem arises in that the voiding reactivity of sodium becomes positive.

On the other hand, in the core of a fast breeder reactor with a large output, as a method to make the void reactivity of sodium, which is a coolant, zero or negative, a large number of fuel assemblies containing a large amount of fissile material are arranged in a circumferential direction. An annular core arranged in a ring is considered, but in order to increase the leakage of neutrons from the core, it is necessary to increase the core surface area, which increases the core size and reduces economic efficiency. There's a problem.

The present invention has been made in consideration of the above-mentioned circumstances, and provides a core structure of a fast breeder reactor that maintains the void reactivity of the coolant to zero or negative even in a high-power reactor core, and suppresses the decline in economic efficiency. The purpose is to provide

Another object of the present invention is to provide a core structure for a fast breeder reactor that improves economic efficiency by effectively and proactively utilizing the unused area in the center of the annular reactor core. [Structure of the invention]

[0014]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the core structure of the fast breeder reactor according to the present invention has a core structure in which a large number of fuel assemblies loaded with fissile material are arranged in a polycyclic manner. A neutron absorbing region is provided in which a large number of absorbing assemblies loaded with a neutron absorbing material are arranged on the inner peripheral side of the fuel assemblies in the core region.

Further, in order to solve the above-mentioned problems, the core structure of the fast breeder reactor according to the present invention has a neutron absorption region configured by arranging a large number of absorption aggregates in a ring, and As described in claim 2, a plurality of inner fuel assemblies are arranged on the circumferential side, and the effective axial loading length of the fissile material of the inner fuel assembly is equal to the effective axial loading of the fuel assemblies in the core region. In addition, as described in claim 3, a TRU fuel assembly having a minor actinide nuclide is arranged, and as described in claim 4, a spent fuel assembly is arranged. You may.

[0016]

[Operation] In the core structure of the fast breeder reactor according to the present invention, absorption assemblies containing neutron absorbing substances are arranged on the inner peripheral side of the core region in which a large number of fuel assemblies are arranged in a polycyclic manner. Even in a core with a relatively small surface area, the leakage of neutrons from the core can be increased, and even in a high-power core, an increase in core size can be suppressed.

[0017] Furthermore, when the radial thickness of the neutron absorption region of the absorption aggregate exceeds a certain level, the effect of arranging the absorption aggregate tends to be saturated. Therefore, the number of the absorption aggregates loaded is determined from the viewpoint of reducing the void reactivity of sodium and suppressing the increase in core size.In this case, there is an unused area inside the neutron absorption area where the absorption aggregates are arranged. occurs.

In this unused area, (1) a plurality of inner fuel assemblies having an effective axial loading length shorter than the effective axial loading length of the fissile material in the fuel assembly;
2) By placing TRU fuel assemblies containing minor actinide nuclides such as neptinium, americium, and curium, or (3) by placing used fuel assemblies, it is possible to effectively utilize unused areas, resulting in improves economic efficiency.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the core structure of a fast breeder reactor according to the present invention will be described below with reference to the accompanying drawings.

FIGS. 1A and 1B are a sectional view and a longitudinal sectional view showing an example of the core structure of a fast breeder reactor according to the present invention, and this fast breeder reactor is equipped with an annular core 1. be.

The annular core 1 has a large number of fuel assemblies (outer fuel assemblies) 2 loaded with fissile material arranged in a polycyclic ring having concentric circles to form an annular core region 3. The upper part of the fuel assembly 2 is provided with a gas plenum region 4, while the lower part thereof is formed with an axial blanket region 5 of parent material. In this core region 3, control rods 6 that reduce the void reactivity of sodium, which is a coolant, and control the reactor output, and cavity channels 7 that do not allow sodium to flow are arranged. The cavity channels 7 are arranged in rows (annularly) along the circumferential direction, and are intended to increase the surface area of the core in order to increase the leakage of neutrons from the core. A radial blanket region 8 is formed on the outer peripheral side of the annular core region 3, and radial blanket fuel assemblies 9 are arranged in rows in the circumferential direction in this radial blanket region 8.

Furthermore, a large number of absorption assemblies 10 are arranged in an annular manner on the inner peripheral side of the fuel assembly 3 loaded in the annular core region 2, thereby forming an annular neutron absorption region 11. The width of the neutron absorption region 11 is appropriately determined from the viewpoint of reducing the void reactivity of sodium, which is a coolant, and suppressing the increase in core size.
An unused area is formed inside the .

A large number of inner fuel assemblies 13 are arranged in this unused inner core region 12. The inner fuel assembly 13 has a flat structure in which the effective loading length of fissile material in the core axial direction is shorter than the effective loading length of the outer fuel assembly 2 in the core axial direction. By flattening the inner core region 12, the core surface area effective for neutron leakage can be increased. Further, a large number of control rods 6 are distributed in the inner core region 12.

The upper part of the inner core region 12 is formed as a sodium plenum region 4, and the lower part thereof is formed as a lower shaft blanket region 5.

In the core structure of a fast breeder reactor, in order to reduce the void reactivity of sodium, which is a coolant,
It is necessary to increase neutron leakage from the reactor core, and it is required to increase the surface area of the core that is effective for neutron leakage.

In this embodiment, by forming an annular-shaped neutron absorption region 11 on the inner peripheral side of each outer fuel assembly 2 in the core region 3, this neutron absorption region 11
By absorbing neutron leakage from the core region 3, it is possible to increase the surface area of the core that is effective against neutron leakage.

In this embodiment, an inner core region 12 is formed inside the annular-shaped neutron absorption region 11, and this inner core region 12 is used as an inner fuel assembly 13 having a short effective loading length of fissile material in the core axial direction. , it is possible to further increase the surface area of the core that is effective for neutron leakage.

Furthermore, by arranging a large number of cavity channels 7 in the annular core region 3, for example in a single row, the surface area of the core can be further increased.

In this annular core 1, the void reactivity of sodium in the annular core region 3 is significantly reduced, and the void reactivity of sodium in the inner core region 11 is reduced by the fissile material in the inner fuel assembly 12. Since the axial loading length of the annular core region 3 is significantly reduced due to the short flattening, and heat is also generated in the inner core region 11, the amount of heat generated in the annular core region 3 is small, and as a result, The size of the annular core region 3 can be reduced.

FIG. 2 shows a second embodiment of the core structure of a fast breeder reactor according to the present invention.

The core structure of this fast breeder reactor is an annular core 1A, and its basic configuration is the same as the annular core 1 of the first embodiment. The same configurations are given the same reference numerals and their explanations will be omitted.

A fundamentally different configuration from the annular core 1 shown in FIG. 1 is that a transuranium element fuel assembly (hereinafter referred to as a TRU fuel assembly) 20 having a minor actinide nuclide is provided in the inner core region 12A. The point is to arrange a large number of them. The minor actinide nuclide is neptium-23
7 (237Np) and americium-241 (241
Am), Americium-243 (243Am), Curium-242 (242Cm), Curium-2
It is obtained by removing plutonium (Pu) from each nuclide of transuranium elements (TRU) such as 44 (244Cm).

In this annular core 1A, TRU fuel assemblies 20 are arranged in the inner core region 12A, while diluted fuel assemblies mainly made of stainless steel, for example, are arranged in order to control the reactivity of the inner core region 12A. 21 are distributed in the inner core region 12A. Furthermore, in order to improve the breeding characteristics of the reactor core, a large number of blanket aggregates 22, 22 filled with parent substances are added to the annular neutron absorption region 1.
1A of absorbent aggregates 10 are arranged in rows in the circumferential direction so as to be sandwiched.

[0034] In this annular core 1A, the void reactivity of sodium, which is the coolant in the annular core region 3, can be significantly reduced, as well as in the one shown in the embodiment. In the case of the reactor core, neutrons generated in the annular core region 3 reach the unused inner core region 12A, and minor actinides, which are radioactive waste with a long half-life in the TRU fuel assembly 20, react with the neutrons. , it is possible to dissipate it as a radioactive substance with a short half-life and generate electricity.

FIG. 3 shows a third embodiment of the core structure of a fast breeder reactor according to the present invention.

Annular core 1 shown in this example
The basic core structure of B is the same as that shown in the first embodiment, and the substantially different configuration is that spent fuel assemblies 25 are placed in the inner core region 12B to wait for decay heat to decrease. Absorption assemblies 26 are installed in the inner core region 12B loaded with spent fuel assemblies 25 to prevent nuclear reactions.
The point is that they are distributed in a distributed manner.

In this annular core 1B as well, the void reactivity of sodium in the annular core region is significantly reduced, and in the case of the conventional core structure, the spent fuel assemblies 25 are placed outside the core. In contrast, in the annular core of this example, the diameter of the reactor vessel does not increase and the diameter of the reactor vessel increases accordingly. 25 can be stored.

The core structure of the fast breeder reactor according to the present invention is not limited to the above embodiments, but is arranged so that the void reactivity of sodium becomes approximately zero or negative when the coolant voids. Any core configuration is sufficient.

[0039]

Effects of the Invention In the core structure of the fast breeder reactor according to the present invention, absorption assemblies containing neutron absorbing substances are arranged on the inner peripheral side of the core region in which a large number of fuel assemblies are arranged in a polycyclic manner to absorb neutrons. Since the area is provided, even in a core with a relatively small surface area, neutrons from the core can be absorbed and neutron leakage can be increased, and even in a high-power core, an increase in core size can be suppressed.

[0040]Also, an unused state occurs inside the neutron absorption region loaded with a large number of absorption aggregates, but this unused state is actively used as an inner core region, and (1) (2) TRU fuel assemblies having minor actinide nuclides such as neptinium, americium, and curium; (3) By arranging used fuel assemblies, it is possible to make effective use of unused areas, and as a result, economic efficiency is improved.

[Brief explanation of drawings]

FIG. 1 (A) and (B) are a cross-sectional view and a longitudinal cross-sectional view showing a first embodiment of a core structure of a fast breeder reactor according to the present invention.

FIG. 2 is a cross-sectional view showing a second embodiment of the core structure of a fast breeder reactor according to the present invention.

FIG. 3 is a cross-sectional view showing a third embodiment of the core structure of a fast breeder reactor according to the present invention.

[Explanation of symbols]

1, 1A, 1B Annular core 2 Fuel assembly (outer fuel assembly) 3 Core region 4 Sodium plenum region 5 Axial blanket region 6 Control rod 7 Cavity channel 8 Radial blanket region 9 Radial blanket assembly 10 Absorption assembly 11 Neutron absorption regions 12, 12A, 12B Inner core region 13 Inner fuel assembly 20 TRU fuel assembly 21 Dilution assembly 22 Blanket assembly 25 Spent fuel assembly 26 Absorption assembly

Claims (4)

[Claims] Claim 1: A core region is formed by arranging a large number of fuel assemblies loaded with fissile material in a polycyclic manner, and a large number of fuel assemblies loaded with neutron absorbing material are arranged on the inner peripheral side of the fuel assemblies in this core region. A core structure of a fast breeder reactor characterized by having a neutron absorption region in which aggregates are arranged. 2. The neutron absorption region is configured by arranging a large number of absorption assemblies in a ring shape, and arranging a plurality of inner fuel assemblies loaded with fissile material on the inner peripheral side of each absorption assembly,
2. The core structure of a fast breeder reactor according to claim 1, wherein the effective axial loading length of the fissile material loaded into the inner fuel assembly is shorter than the effective axial loading length of the fuel assembly in the core region. 3. The neutron absorption region according to claim 1, wherein the neutron absorption region is constructed by arranging a large number of absorption assemblies in a ring shape, and a TRU fuel assembly having a minor actinide nuclide is arranged on the inner peripheral side of each absorption assembly. Core structure of fast breeder reactor. 4. The fast breeder reactor according to claim 1, wherein the neutron absorption region is constructed by arranging a large number of absorption assemblies in a ring shape, and a spent fuel assembly is arranged on the inner peripheral side of each absorption assembly. core structure.