JPH04335191A - Core structure for fast breeder reactor - Google Patents

Abstract

PURPOSE:To obtain a core structure for a fast breeder reactor capable of keeping the core reactivity of void to be zero or negative and obtaining a bore with larger power. CONSTITUTION:A core region 22 is formed by loading a multitude of fuel assembly charged with fissile material. Between these fuel assemblies in the core region 22, cavity assemblies 33 having a cylindrical coolant flow path 40 inside and charged with such gas as inert gas are loaded. In these cavity assemblies 33, coolant connection holes are provided for enabling the coolant to flow in and out the coolant flow path 40. The coolant surface in the coolant flow path is so set as to vary in the core axial direction due to the variation of the coolant pressure.

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 has an improved core structure.

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.

The present invention has been made in consideration of the above-mentioned circumstances, and provides a core for a fast breeder reactor that can obtain a core with a larger output while maintaining the void reactivity of the core due to the coolant to zero or negative. The purpose is to provide structure. [Structure of the invention]

0012

[Means for Solving the Problems] In order to solve the above-mentioned problems, the core structure of a fast breeder reactor according to the present invention includes a core region formed by loading a large number of fuel assemblies filled with fissile material. A cylindrical coolant flow path is formed between the fuel assemblies in the core region, and a cavity assembly filled with gas such as an inert gas is loaded, and this cavity assembly is used to carry the coolant. A coolant communication hole through which coolant can be taken in and taken out is provided in the flow path, and the coolant liquid level in the coolant flow path is set to fluctuate in the core axial direction due to changes in the pressure of the coolant.

Furthermore, in order to solve the above-mentioned problems,
As described in claim 2, the coolant communication hole to the coolant flow path of the cavity assembly is provided at a position above the coolant inflow hole to the fuel assembly.

[0014]

[Operation] In the present invention, a cylindrical coolant flow path is formed inside between each fuel assembly in a core region loaded with a large number of fuel assemblies, and an enclosed gas such as an inert gas is filled. The cavity assembly was loaded, and the coolant flow path inside the cavity assembly was designed so that the liquid level of the coolant fluctuated due to changes in the pressure of the coolant. Compared to a core that is not loaded, this can further improve safety and reliability.

[0015] Possible causes of voiding of the sodium coolant are: when the coolant flow rate decreases and the sodium temperature rises and boils, and when a large amount of gas mixes with the coolant outside the core and flows into the core. Conceivable.

In the case of a decrease in coolant flow rate, the decrease in coolant pressure within the cavity assembly causes the enclosed gas to expand, causing the liquid level in the coolant channels to drop below the core top level, forming a cavity. Simultaneously or subsequently, the coolant in the fuel assembly also tries to void due to the rise in coolant temperature, but since the inside of the cavity assembly is voided, neutrons generated in the fuel assembly are mainly transferred from the cavity assembly. It leaks upwards into the core, resulting in negative reactivity.

[0017] When gas flows in from outside the core, the coolant communication holes in the cavity assembly are formed above the coolant inlet holes in the fuel assembly. By flowing in from the fuel assembly, the same effect as in the case of reducing the coolant flow rate occurs.

[0018]

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.

FIG. 1 is a sectional view showing an example of a liquid metal cooled fast breeder reactor equipped with a core structure according to the present invention. The fast breeder reactor has a reactor safety vessel 2 provided in a cavity wall 1a of a reactor building 1, and a reactor vessel 3 is housed within this safety vessel 2, so that it has a double vessel structure.

[0020] Inside the reactor vessel 3, a reactor core 4 loaded with nuclear fuel is housed. The reactor core 4 is installed immersed in sodium 5, which is a liquid metal coolant, filled in the reactor vessel 3, and is supported by a core support structure 6. Inside the reactor vessel 3, there is an upper plenum 7 with this support structure 6 as a boundary.
and a lower plenum 8. Above the reactor core 4, an upper core mechanism 10 containing a control rod drive mechanism, etc. is installed, and around the reactor core 4 there is an intermediate heat exchanger that exchanges heat between the primary coolant and the secondary coolant. A circulation pump 12 that forcibly circulates the reactor vessel 3 and the primary coolant inside the reactor vessel 3 is installed.

The intermediate heat exchanger 11 and the circulation pump 12 are supported in a suspended state by an upper lid 13 and a roof slab 14 that cover the upper part of the reactor vessel. A rotary plaque 15 is rotatably provided in the center of the roof slab 14, and a small rotary plug 16 is rotatably supported at an eccentric position of the rotary plug 15. Reference numeral 17 is a cover gas space, and this space 17 is filled with cover gas, which is an inert gas.

On the other hand, the core portion 4 of the fast breeder reactor includes a core 20 having a core configuration shown in FIG. This core 20 has a core region 22 in the center loaded with a large number of fuel assemblies 21 (see FIGS. 3 and 4) filled with fissile material, and a diameter A radial blanket region 24 is formed in which a directional blanket assembly 23 is annularly loaded, and a neutron shielding region 26 is formed outside this radial blanket region 24 in which a neutron shield 25 is loaded.

Fuel assembly 21 loaded in core region 22
is the upper axis blanket region 2 loaded with the parent substance on the top.
8, a lower shaft blanket region 29 is formed at the bottom thereof, and a gas plenum region 30 is provided above the upper shaft blanket region 28. Below the lower shaft blanket region 29 is configured as a gas plenum region or neutron shielding region 31.

Furthermore, between each fuel assembly 21 in the core region 22, a cavity assembly 3 is provided between each fuel assembly 21 as shown in FIGS. 1 and 2.
Multiple units of 3 are loaded. The fuel assembly 21 and the cavity assembly 33 are supported on an upper support plate 35 of a core support plate 34, and a high pressure plenum 37 is formed between the upper support plate 35 and the lower support plate 36. The high-pressure plenum 37 is supplied with sodium 5, which is a primary coolant discharged from a circulation pump.

A cylindrical coolant flow path 40 is formed inside the cavity assembly 33, and this flow path 4
0 is formed as a cavity channel 41 filled with sealed gas. The gas to be filled is suitably an inert gas such as argon, but is not particularly limited.

The coolant passage 40 of the cavity assembly 33 communicates with the high pressure plenum 37 via a coolant communication hole 44 formed in the leg portion (entrance nozzle portion) 43. is formed above the coolant inflow hole 44 formed in the entrance nozzle portion 43 of the fuel assembly 21 .

The liquid level (free liquid level) of the coolant flowing into the coolant flow path 40 of the cavity assembly 33 is caused to fluctuate in the core axial direction by the pressure of the coolant in the high pressure plenum 37. During normal operation of the reactor, the coolant liquid level is higher than the core top level (the upper end of the core region 22);
In the event of a coolant flow rate reduction accident, the internal shape of the coolant channel 40 and the amount of sealed gas are set so that the coolant liquid level becomes low. Specifically, the coolant liquid level in the coolant flow path 40 is set to be above the upper shaft blanket region 28 during steady operation, and to be located near the axial center of the reactor core 20 when the coolant flow rate decreases. , the leakage of neutrons from the fuel assembly 21 to the cavity assembly 33 can be increased, and the negative reactivity effect can be increased, but there is no particular restriction.

In FIG. 3, UP indicates the core upper end position (upper surface position of the core region), CP the core center position, and LP the core lower end position.

Next, the operation of this embodiment will be explained.

In fast breeder reactors, the causes of the voiding of the sodium coolant are the case where the sodium temperature rises due to a decrease in the coolant flow rate and boils, and the case where a large amount of gas such as cover gas is generated outside the reactor core. A case can be considered in which the coolant flows into the reactor core 20.

When the coolant flow rate decreases, the coolant pressure within the high pressure plenum 37 decreases, and with this pressure decrease, the coolant pressure within the cavity assembly 33 also decreases, causing the enclosed gas to expand. Due to the expansion of this sealed gas, the coolant flow path 40
The liquid level moves below the core top level and the cavity channel 41 increases.

In this case, the inside of the cavity assembly 33 first becomes void as the pressure of the coolant inside the cavity assembly 33 decreases. Simultaneously or subsequently with this void formation, the inside of the fuel assembly 21 also becomes void due to the rise in coolant temperature.

However, since the coolant flow path 40 in the cavity assembly 33 is voided, the neutrons generated in the fuel assembly 21 leak from the cavity assembly 33 to the upper part of the core through the cavity channel 41. , a negative reactivity effect is obtained.

Next, consider the case where gas flowing in from outside the core mixes with the coolant and flows into the core.

In this case, as shown in FIG. 3, the coolant communication holes 44 of the cavity assembly 33 are provided vertically above the coolant inlet holes 46 of the fuel assembly 21. On the other hand, the gas mixed in the coolant discharged from the circulation pump 12 to the high-pressure plenum 37 is lighter than the coolant, so it accumulates in the lower part near the upper support plate 35 .

Therefore, the gas accumulated in the lower part of the upper support plate 35 flows into the cavity assembly 33 before flowing into the fuel assembly 21, thereby voiding the coolant flow path 40 therein. Subsequently, the gas mixes with the coolant and flows into the fuel assembly 21, but the same effect as in the case where the coolant flow rate is reduced is produced.

[0037] If the mixing of gas into the coolant outside the core is not considered, the positional relationship between the coolant communication holes 44 of the cavity assembly 33 and the coolant inlet holes 46 of the fuel assembly 21 does not matter.

Furthermore, by loading the cavity assemblies 33 between the fuel assemblies 21 in the core region 22, neutrons generated in the fuel assemblies 21 are prevented from leaking into the cavity channels 41 in the axially upper part of the cavity assemblies 33. In order to facilitate this, a neutron absorber 48 may be placed above the cavity channel 41 as shown in FIG.

As a result, the sodium void reactivity of the core of this embodiment can be reduced to zero or negative compared to a core without a cavity assembly.

For example, in the case of a flat core with a core height of 60 cm and a core diameter of approximately 300 cm, the sodium void reactivity at the height of the core after 365 days of combustion without a cavity assembly is approximately 3 $. , cavity assembly 3
3 is arranged as shown in Figure 4, the sodium void reactivity in the same size core is such that the cavity assembly becomes void from the height of the upper shaft blanket region (35 cm thick) 30 to the core center height, and the fuel assembly If the fuel assembly 21 was voided up to the core height, the cost would be approximately -0.5$, the cavity assembly 33 was voided to a further 30 cm above the height of the upper shaft blanket region 30, and the fuel assembly 21 was voided up to the core height. In this case, it will be about -2$. code 4
Control rods 9 are distributed in the core region 22.

Next, another embodiment of the present invention will be described with reference to FIGS. 5 and 6.

In addition to the core structure shown in FIG. 4, the core structure of the liquid metal cooled fast breeder reactor shown in this embodiment includes a core region 2 which is configured by loading a large number of fuel assemblies 21.
A plurality of cavity aggregates 33 are centrally arranged in the central part of 2. The other core configurations are basically the same as those shown in FIGS. 2 and 4, so the same reference numerals are given and the description thereof will be omitted.

In the core structure of the fast breeder reactor shown in FIG. 5, the cavity assembly 33 arranged in the center of the core in the axial direction of the reactor prevents neutrons toward the center of the reactor when sodium, which is a coolant, voids. The structure is such that the leakage effect is promoted.

Furthermore, by loading the cavity assembly 33 adjacent to the control rods 9 disposed in the core region 22 of the flat core or in an annular manner along the circumferential direction, neutron leakage in the direction of the control rods can be prevented. It is also possible to actively use At that time, as shown in FIG. 6, if a gas plenum region 51 is arranged between the upper axial blanket region 50 and the core region 22 of the fuel assembly 21, or if the axial blanket region 50 is deleted, leakage in the upper direction can be prevented. Neutrons are more likely to leak through the plenum region.

[0045] In the embodiment of the present invention, an example was shown in which a cavity assembly was arranged between each fuel assembly in the core region, but the arrangement of the cavity assembly and the structure inside the cavity assembly were similar to the above example. It is not limited. The cavity assembly may be arranged so that the sodium void reactivity becomes almost zero or negative when sodium, which is a coolant, voids, and its shape and configuration are set so as to satisfy the above effects. good.

[0046]

As described above, in the core structure of the fast breeder reactor according to the present invention, a large number of fuel assemblies are loaded to form a core region, and between the fuel assemblies in this core region, A cylindrical coolant flow path is formed inside, and a cavity assembly filled with a sealed gas such as an inert gas is loaded, and this cavity assembly is a coolant that can be inserted into and taken out of the coolant flow path. A communication hole is provided so that the coolant liquid level in the coolant flow path changes in the core axial direction due to changes in the pressure of the coolant, so when the flow rate of coolant circulating in the reactor vessel decreases, the coolant level changes in the reactor core axis direction. As the pressure decreases, the coolant pressure within the cavity assembly decreases and the enclosed gas expands, initially voiding the cavity assembly and lowering the liquid level in the coolant flow path. Due to this lowering of the liquid level, neutrons generated in the fuel assembly mainly leak upward from the core from the cavity assembly, making it possible to maintain the sodium hoide reactivity due to the coolant to zero or negative, allowing the reactor to generate a larger power core. In addition, the safety and soundness of the reactor can be sufficiently maintained even in a large reactor core.

Furthermore, if the coolant communication hole to the coolant flow path of the cavity assembly is provided above the coolant inflow hole to the fuel assembly, gases such as cover gas outside the core will mix into the coolant. Even in this case, the mixed gas is stored upwards due to the density difference with the coolant, and flows in through the coolant communication hole of the cavity assembly before the fuel assembly, so the same effect as in the case of reducing the coolant flow rate occurs. be effective.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing a liquid metal cooled fast breeder reactor equipped with a core structure according to the present invention.

FIG. 2 is a core configuration diagram showing an embodiment of the core structure of the fast breeder reactor according to the present invention.

FIG. 3 is a diagram showing the relationship between a fuel assembly and a cavity assembly in the core structure shown in FIG. 2;

FIG. 4 is a cross-sectional view showing the core structure of the fast breeder reactor according to the present invention.

FIG. 5 is a cross-sectional view showing another embodiment of the core structure of the fast breeder reactor according to the present invention.

FIG. 6 is a core layout configuration diagram showing the core structure of the fast breeder reactor shown in FIG. 5;

[Explanation of symbols]

1 Reactor building 3 Reactor vessel 4 Reactor core 5 Sodium (coolant) 7 Upper plenum 8 Lower plenum 10 Upper core mechanism 11 Intermediate heat exchanger 12 Circulation pump 13 Upper cover 14 Roof slab 15 Rotating plug 20 Core 21 Fuel assembly 22 Core region 24 Diameter blanket region 26 Neutron shielding region 28 Upper shaft blanket region 29 Lower shaft blanket region 30 Gas plenum region 31 Gas plenum region or neutron shielding region 33
Cavity assembly 34 Core support plate 35 Upper support plate 36 Lower support plate 37 High pressure plenum 40 Coolant channel 41 Gas cavity 44 Coolant communication hole 46 Coolant inflow hole 48 Neutron absorber 49 Control rod 50 Axial blanket 51 Gas plenum

Claims (2)

[Claims] Claim 1: A reactor core region is formed by loading a large number of fuel assemblies filled with fissile material, and an internal cylindrical coolant flow path is formed between the fuel assemblies in this core region. A cavity assembly filled with a sealed gas such as an active gas is loaded, and this cavity assembly is provided with a coolant communication hole through which the coolant can be taken in and taken out in the coolant flow path. A core structure of a fast breeder reactor characterized in that the coolant liquid level in the flow path is set to fluctuate in the axial direction of the core. 2. The core structure of a fast breeder reactor according to claim 1, wherein the coolant communication hole to the coolant flow path of the cavity assembly is provided at a position above the coolant inflow hole to the fuel assembly.