JPH05164873A - Reactor core of fast reactor - Google Patents

Abstract

PURPOSE:To improve the safety and to obtain a larger output by forming the lid opening mechanism of a gas cavity container at the upper side of a void aggregate with a shape memory alloy. CONSTITUTION:When the flow of the cooling material of an atomic reactor is reduced abnormally, the flow of the cooling material 5 flowing in to the inside of a void aggregate 33 is also reduced. As a result, the temperature of the cooling material 5 heated by a heat generating substance 47 rises abnormally. The cooling material 5 heated abnormally heats an expansion metal fitting 38 which consists of a shape memory alloy abnormaly. The metal fitting 38 restores its memorized shape and is contracted. As a result, a lid 45 is pulled to the outer side of a gas cavity container 42 and opened. As a result, a sealed gas in the container 42 is released to the inside of a lower opening container 46 to depress the liquid level of the cooling material, and a void space is formed. Consequently, neutrons generated in a fuel aggregate 21 are leaked to the upper side of the reactor core through the void space formed in the cooling material passage 40 of the aggregate 33, and a negative reactivity is applied to the reactor core.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fast breeder reactor core for cooling liquid metal, and more particularly to a fast breeder reactor core having an improved core structure.

[0002]

2. Description of the Related Art A fast breeder reactor using liquid metal as a coolant,
The core is constructed by loading a large number of fuel assemblies filled with nuclear fuel material, and sodium is mainly used as a coolant for removing heat from the fuel.

In this fast breeder reactor, if the coolant flow rate decreases for some reason, the coolant temperature rises, the coolant density decreases, and reactivity is injected. The reactivity of the fast breeder reactor depends on the power output of the reactor, and is negative for small reactors, but positive for medium and large reactors.
Reducing the coolant density may increase the reactor power.

In a fast breeder reactor, sodium, which is a coolant, usually does not evaporate to form voids. However, in the unlikely event of an accident, even if sodium forms voids, the reactor can be safely shut down, It is designed to maintain core safety.

As a response of the reactor when sodium, which is a coolant, is voided, when the core is small, a large amount of neutron leaks from the core, resulting in a negative reactivity as described above. The core can be safely stopped.

On the other hand, when the core of a fast reactor becomes large, neutron leakage from the core decreases and the reactivity when sodium as a coolant is voided becomes positive. In this positive reactivity state, it is necessary to confirm the safety of the core by performing a detailed analysis including other reactivity factors to determine whether or not the core can be safely stopped.

The positive reactivity of sodium as a coolant can be reduced by increasing neutron leakage from the core and softening the neutron spectrum. Therefore, conventionally, in order to suppress and reduce the reactivity, neutron absorbing material is loaded in the core, or the height of the core is reduced to achieve flattening to increase neutron leakage, or beryllium or hydrogen. Means have been taken to soften the neutron spectrum by loading neutron moderators such as zirconium oxide.

[0008]

If the void reactivity of sodium, which is a coolant, can be reduced, the safety and reliability of the core will be further improved, which is very effective in the safety design of the nuclear reactor.

However, in a nuclear power plant such as a fast reactor, it is not preferable from the viewpoint of core safety in designing the plant that the core output is designed to be extremely small in terms of power generation cost.

Further, even if sodium, which is a coolant, should be voided, in order to make the reactivity of the core negative, it is necessary to make the core output a small reactor of, for example, about 100 MWe. If it exceeds, the problem that the voiding reactivity of sodium becomes positive arises.

The present invention has been made in consideration of the above points, and by suppressing the void reactivity of the core due to the coolant, when the flow rate of the coolant is reduced, when the core is overpowered, and bubbles are introduced into the core. In order to provide a fast reactor core that can improve the safety and reduce the positive reactivity input factor in the case of an abnormality such as time, while improving the safety. There is.

[0012]

In order to achieve the above object, in the present invention, a core region formed by loading a plurality of fuel assemblies filled with fissile material, and a fuel constituting the core region It consists of a voided region formed by a voided aggregate inserted between the aggregates, and the voided aggregate that constitutes this voided region contains a lower open container inside the main body and the inside of this lower open container. A gas cavity container in which a gas is pressurized and sealed is disposed on the upper part of the container, and an opening mechanism made of a shape memory alloy for opening and closing the opening / closing lid of the gas cavity container at a predetermined temperature is provided. To provide a core of a fast reactor.

[0013]

In the core of the fast reactor thus constructed, the void assembly is inserted between the fuel assemblies filled with the fissile material. In this voided assembly, a lower open container is housed inside a main body, and a gas cavity container in which a gas is pressurized and sealed is arranged in an upper part inside the lower open container. An opening / closing lid is installed in this gas cavity container. The opening / closing lid is opened by an opening mechanism formed of a shape memory alloy that opens at a predetermined temperature. Simultaneously with this opening, the pressurized enclosed gas is opened. The liquid level of the coolant in the voided assembly is pushed down by the opened gas. The liquid surface of the pressed coolant is adjusted to be at least below the upper surface of the core region.

When the coolant of the nuclear reactor decreases abnormally, the flow rate of the coolant flowing into the inside of the voided assembly also decreases,
The temperature of the coolant rises abnormally compared to the temperature during normal operation. Due to this temperature rise, the opening mechanism formed of the shape memory alloy is abnormally heated. Due to such abnormal heating, the opening mechanism formed of the shape memory alloy restores the original memory shape, and the opening mechanism performs the opening operation. In this case, a gas cavity formed by the enclosed gas is formed above the liquid surface of the coolant in the voided assembly. Neutrons generated in the fuel assembly leak to the upper part of the core through this gas cavity. Due to this neutron leakage, a negative reactivity is injected into the core. As described above, by reducing the factor for injecting the positive reactivity into the core, it is possible to provide the core of the fast reactor in which safety can be improved and a core having a larger output can be obtained.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the core of a fast 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 fast breeder reactor having a core structure according to the present invention. The fast breeder reactor has a safety container 2 provided inside a cavity wall 1 of a reactor building 1, and a reactor container 3 is housed in this safety container 2 to have a double container structure.

A nuclear reactor vessel 3 accommodates a core portion 4 loaded with nuclear fuel. The core part 4 is installed in a state of being 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. The reactor core 3 is divided into an upper plenum 7 and a lower plenum 8 by the core support structure 6. Above the core portion 4, a core upper part mechanism 10 containing a control rod drive mechanism and the like (not shown) is installed. On the other hand, around the core 4,
Intermediate heat exchanger for exchanging heat between the primary coolant and the secondary coolant
A circulation pump 12 for forcibly circulating 11 and the primary coolant in 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 portion of the reactor vessel 3. A rotary plug 14 is rotatably provided at 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. A cover gas space 17 is formed above the upper plenum 7. The space 17 is filled with a cover gas which is an inert gas.

FIG. 2 shows the first of the core of the fast reactor according to the present invention.
FIG. 3 is a core configuration diagram showing the example of FIG. FIG. 3 is a cross-sectional view showing the core of the fast reactor shown in FIG. Fast reactor core 4
Includes a core 20 having a core configuration shown in FIG. The core 20 has a core region 22 in the center thereof, in which a large number of fuel assemblies 21 filled with fissile material are loaded, and outside the core region 22, a radial blanket in which a radial direction 23 is annularly loaded is formed. A region 24 is formed, and a neutron shielding region 26 loaded with a neutron shield 25 is formed outside the diameter blanket region 24.

An upper shaft blanket region 28 having a parent substance loaded thereon is provided above the fuel assembly 21 loaded in the core region 22.
Lower shaft blanket regions 29 are formed in the lower portions thereof, and a gas plenum region 30 is provided above the upper shaft blanket regions 28. Below the lower shaft blanket region 29 is a gas plenum region or a neutron shielding region 31. Further, a plurality of voided assemblies 33 are loaded between the fuel assemblies 21 in the core region 22. Further, control rods 52 are arranged in the core region 22 in a horizontally dispersed manner.

FIG. 4 is a structural diagram showing the relationship between the fuel assembly 21 and the voided assembly 33 in the core of the fast reactor shown in FIG.
In the figure, UP represents the core upper end position (upper surface position of the core region), and LP represents the core lower end position. Fuel assembly
21 and the voided assembly 33 are upper support plates of the core support plate 34.
A high pressure plenum 37 is formed on the upper support plate 35 and the lower support plate 36. The high pressure plenum 37 is supplied with sodium 5 which is the primary coolant discharged from the circulation pump.

Inside the container body 56 of the voided assembly 33, a cylindrical lower open container 46 having an opening at the lower end is incorporated. A coolant passage 40 is formed inside the container body 56 and the lower open container 46. This lower open container
A gas cavity 41 is formed above the 46. The gas cavity 41 is formed by being housed in the gas cavity container 42. A pressurized enclosed gas is enclosed in the gas cavity container 42. An inert gas such as argon is suitable as the enclosed gas, but is not particularly limited. In addition, the gas cavity container 42 has a hook 3
9 and 39 are installed. One of the hooks 39 is installed in the gas cavity container 42 and the other is installed in the opening / closing lid 45. Around the opening / closing lid 45, there is a V-shaped groove 57 so that the thickness of the opening / closing lid 45 is thinner than that of the gas cavity container 42. An expandable metal fitting 38 made of a shape memory alloy is installed and connected between the hooks 39, 39. The expandable metal fitting 38 made of this shape memory alloy is formed of an alloy that does not recover to the memory shape at the temperature during normal operation. Because of this
Is a hook 39 in an extended state compared to the original memory shape,
It is formed so as to connect between 39.

At the bottom of the lower open container 46, there is a heat generating material.
47 are installed. The exothermic substance 47 is formed of a substance that generates heat by neutron absorption or gamma ray absorption, for example, a substance such as boron carbide, tungsten, or tantalum. Pyrogen 47
Is coated with a substance having a small chemical reaction with the coolant 5 such as stainless steel, if necessary, so as not to cause a chemical reaction with the coolant.

The coolant passage 40 of the voided assembly 33 communicates with the high pressure plenum 37 through the coolant communication holes 44 formed in the entrance nozzle portion 43 which is the leg portion of the container body 56. Further, the coolant passage 40 communicates with the upper plenum 7 through the coolant outlet holes 48 formed in the voided assembly 33.

As shown in FIG. 5, the gas cavity container 42
When the enclosed gas in the gas cavity container 42 is released to the inside of the lower open container 46, the coolant liquid level in the lower open container 46 is the upper surface (UP) of the core region 22.
Is set to come at least at the bottom. Further, the coolant communication hole 44 of the voided assembly 33 is formed vertically above the coolant communication hole 53 of the fuel assembly 21. Next, the operation of this embodiment having such a configuration will be described.

In the fast breeder reactor, when sodium as a coolant is voided as a factor causing the sodium temperature to rise due to a decrease in the coolant flow rate and boils, an abnormal increase in core power, that is, overpower, occurs and the sodium temperature increases. May rise and boil, or a large amount of gas such as cover gas may flow into the coolant outside the core and flow into the core 20.

During normal operation, the coolant 5 flows from the high pressure plenum 37 into the voided aggregate 33 through the coolant communication holes 44 formed in the entrance nozzle portion 43. The coolant 5 that has flowed into the voided aggregate 33 is heated by the exothermic substance 47, and then rises inside the coolant passage 40 to heat the expandable metal fitting 38, and the coolant outflow port 48 is placed above. Outflow to plenum 7.

On the other hand, when the coolant flow rate of the nuclear reactor is abnormally reduced, the flow rate of the coolant 5 flowing into the voided aggregate 33 is also reduced. As a result, pyrogens
The temperature of the coolant 5 heated at 47 rises abnormally compared to the normal operating temperature. The abnormally heated coolant 5 rises inside the coolant passage 40 and abnormally heats the expandable metal fitting 38. The elastic metal fitting 38 restores its original memory shape and is shortened in comparison with the length during normal operation. When the stretchable metal fitting 38 is shortened, the hooks 39, 39 connected to both ends thereof are attracted to each other. Since one of the hooks 39 is installed on the opening / closing lid 45, this opening / closing lid 45 is a gas cavity container.
Pulled to the outside of 42. The periphery of the opening / closing lid 45 is formed by a V-shaped groove 57, and the structural strength is lower than that of the other parts of the gas cavity container 42. Therefore, the V-shaped groove 57 around the opening / closing lid 45 is cut, and the opening / closing lid 45 is cut. 45 is open. Due to this opening, the enclosed gas in the gas cavity container 42 is released into the lower open container 46. This gas cavity container
As shown in FIG. 5, when the enclosed gas in the gas cavity container 42 is opened to the inside of the lower open container 46, the liquid level of the coolant in the lower open container 46 is the core region 22 as shown in FIG.
Is set to be at least lower than the upper surface UP. Therefore, in this case, the coolant liquid level in the lower open container 46 inside the voided assembly 33 is at least lower than the upper surface UP of the core region 22. In this way
A void space 54 is formed by the enclosed gas above the liquid surface of the coolant in the coolant channel 40. As a result, the fuel assembly
The neutrons generated at 21 are the coolant channels of the voided aggregate 33.
Leakage to the upper part of the core through the void space formed in 40, and negative reactivity is injected into the core of the fast reactor.

Further, when the output of the nuclear reactor rises abnormally, the calorific value of the exothermic substance 47 also rises. As a result, the temperature of the coolant 5 heated by the exothermic substance 47 rises abnormally as compared with the temperature during normal operation. Therefore, also in this case, the same effect as that in the case where the coolant flow rate of the reactor is abnormally reduced can be obtained.

Further, the gas flowing from outside the core is cooled by the coolant 5.
In the case where the gas is mixed and flows into the core, the gas mixed into the coolant 5 and flowing into the core has a lower specific gravity than the coolant, and therefore is accumulated in the lower portion of the upper support plate 35. On the other hand, the coolant communicating hole 44 of the voided assembly 33 is formed vertically above the coolant communicating hole 53 of the fuel assembly 21. As a result,
The gas accumulated in the lower part of the upper support plate 35 is
Before flowing into the voided aggregate 33. The gas flowing into the voided aggregate 33 has a lower specific gravity than that of the coolant, and therefore is accumulated in the upper portion of the coolant passage 40 in the lower open container 46. That is, also in this case, the void space 54 as shown in FIG. 5 is formed in the upper part of the coolant channel 40. Therefore,
The same effect as when the coolant flow rate of the reactor is abnormally reduced can be obtained.

By loading the voided assemblies 33 between the fuel assemblies 21 in the core region 22, the neutrons generated in the fuel assemblies 21 enter the gas cavities 41 in the axially upper part of the voided assemblies 33. A neutron absorber 49 may be arranged above the gas cavity 41 as shown in FIG. Further, instead of being placed in the lower open container 46 as indicated by the reference numeral 47, it is also possible to place the heat generating substance in the lower portion of the container main body 56 as indicated by the reference numeral 55. As described above, according to this embodiment, the sodium void reactivity of the core of the fast reactor can be reduced to zero or negative as compared with the core having no voided aggregate.

For example, the core height is 60 cm and the core diameter is about 30.
In the case of 0 cm flat core, when there is no cavity assembly
The sodium void reactivity at the core height after 365 days of combustion is about $ 3. The sodium void reactivity in the core of the same size when the voided aggregates 33 are arranged as shown in FIG. 3 shows that the voided aggregates are voided from the height of the upper axial blanket region (thickness 35 cm) 30 to the core center height. Turned into
When the fuel assembly 21 is voided to the core height,
That's $ 0.5. When the voided assembly 33 is voided to a height of 30 cm above the height of the upper shaft blanket region 28 and the fuel assembly 21 is voided to the core height, −2
It becomes a dollar. Therefore, when the flow rate of the coolant drops, when the output is high,
In the event of an abnormality such as when air bubbles flow into the core, a negative reactivity of the core is obtained, and safety is improved.

Even in the unlikely event that fuel melts and the fuel in the core falls downward, the exothermic substance arranged in the lower part of the voided assembly absorbs many neutrons. , The system after fuel melting can be kept subcritical, which helps prevent the spread of accidents.

Next, another embodiment of the present invention will be described with reference to FIGS. 6 and 7. FIG. 6 is a core configuration diagram showing a second embodiment of the core of the fast reactor according to the present invention. Figure 7
FIG. 7 is a cross-sectional view showing a core of the fast reactor shown in FIG. 6. In the core of the fast reactor shown in this embodiment, in addition to the core structure shown in FIGS. 2 and 3, a voided assembly 33 is provided at the center of a core region 22 configured by loading a large number of fuel assemblies 21. It is a collection of multiple pieces that are collectively arranged. Another core configuration is shown in FIG.
Also, since it is basically the same as that shown in FIG. 3, the same reference numerals are given and description thereof is omitted.

In the fast reactor core shown in FIG. 7, neutron leakage toward the center of the core is caused by voiding aggregates 33 collectively arranged in the center of the core in the axial direction of the core when sodium as a coolant is voided. It is structured to promote the effect.

Further, the voided aggregate 33 is loaded adjacent to the control rod 52 arranged in the core region 22 of the flat core,
It is also possible to positively utilize the neutron leakage in the direction of the control rod 52 by loading annularly along the circumferential direction. At this time, as shown in FIG. 6, if a plenum region 51 is arranged between the upper blanket region 50 of the fuel assembly 21 and the core region 22, or if the axial blanket region 50 is deleted, leaked neutrons in the upper direction Are more likely to leak through the plenum area.

In the embodiment of the present invention, an example in which the voided assemblies are arranged between the fuel assemblies in the core region is shown. However, the arrangement of the voided assemblies and the structure of the voided assemblies are as described above. It is not limited to the example of. The voided aggregate may be arranged so that the sodium void reactivity becomes small when the sodium as the coolant is voided, and the shape and configuration thereof may be set so as to satisfy the above-mentioned action. Further, the present invention can obtain the safety improving effect in the same manner as described above not only in a large-sized furnace but also in a small-sized furnace which originally has a small void reactivity.

[0038]

As described above, according to the present invention,
By reducing the reactivity of the core by the coolant,
In the event of abnormalities such as when the flow rate of the coolant drops, when the core is overpowering, or when bubbles are flowing into the core, reducing the factor that causes the positive reactivity of the core to improve safety while increasing power output It is possible to provide a core of a fast reactor capable of obtaining the core.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of a fast reactor equipped with a core according to the present invention.

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

FIG. 3 is a cross-sectional view showing a core of the fast reactor shown in FIG.

FIG. 4 is a configuration diagram showing a relationship between a fuel assembly and a voided assembly of the core shown in FIG.

5 is a configuration diagram showing an open state of a gas cavity container of the voided assembly shown in FIG. 4. FIG.

FIG. 6 is a core configuration diagram showing a second embodiment of the core of a fast reactor according to the present invention.

7 is a cross-sectional view showing the core of the fast reactor shown in FIG.

[Explanation of symbols]

 5 ... Sodium 20 ... Core 21 ... Fuel assembly 22 ... Core region 24 ... Diameter blanket region 26 ... Neutron shielding region 28, 50 ... Upper shaft blanket region 29 ... Lower shaft blanket region 30, 51 ... Gas plenum region 31 ... Gas plenum region or Neutron shielding area 33 ... Voided assembly 38 ... Expandable metal fitting 40 ... Coolant flow path 41 ... Gas cavity 42 ... Gas cavity container 45 ... Open / close lid 46 ... Lower open container 56 ... Vessel body

Claims (1)

[Claims] 1. A void region formed by a core region formed by loading a plurality of fuel assemblies filled with fissile material, and a voided assembly inserted between fuel assemblies forming the core region. The voided assembly, which is composed of a region and constitutes the voided region, accommodates a lower open container inside the main body, and arranges a gas cavity container in which a gas is pressurized and sealed in an upper part inside the lower open container, A core of a fast reactor comprising an opening mechanism made of a shape memory alloy for opening and closing the opening / closing lid of the gas cavity container at a predetermined temperature.