JPH04320992A - Neutron absorber - Google Patents

Abstract

PURPOSE:To extend life and shorten the length of neutron absorber element used in a liquid metal cooled fast breeder reactor by charging liquid metal precisely in a space formed with a porous plug as a pressure control mechanism without requiring the processing to raise the temperature and pressure of the primary coolant and the like at the time of loading in reactor. CONSTITUTION:In a chamber 6 below a middle plug 5 in a clad 3 sealed at both ends with end plugs, neutron absorber material 14 is contained. In a cylindrical support 12 as a pressure control mechanism provided in an upper chamber 8 and spaces 12a, 12b which are provided in series in the inside and formed with a plurality of permeable partition walls, metal pieces the same as the primary coolant of the reactor or having the equivalent characteristics which is solid at the ambient temperature and is liquefied at the temperature used in a reactor and the like, is charged. Furthermore, the surface of the metal pieces is coated with a low melting metal and the like.

Description

[Detailed description of the invention]

[Object of the invention]

0002

[Industrial Application Field] The present invention relates to a neutron absorbing element for a neutron shield used in a fast breeder reactor using liquid metal as a coolant, and in particular, the present invention relates to a neutron absorbing element for a neutron shield used in a fast breeder reactor using liquid metal as a coolant. Concerning absorption elements.

0003

[Prior Art] Fig. 4 is a vertical cross-sectional view showing a conventional neutron absorption element designed to have a long life and short length and is used in a liquid metal cooled fast breeder reactor. Boron (B4C) pellets 2 are stored in a lower chamber 6 surrounded by a lower end plug 4 and an intermediate end plug 5 at the lower part of a cylindrical cladding tube 3. The upper part of the intermediate end plug 5 has an upper chamber 8 surrounded by the cladding tube 3 and the upper end plug 7, and the upper end plug 7 is provided with a gas discharge hole 9 and communicates with the outside. Further, a communication pipe 10 is installed in an upper chamber 8 of the intermediate end plug 5 and communicates with a lower chamber 6 containing boron carbide pellets 2.

Further, a cylindrical support 12 is fixed to the lower surface of the gas discharge hole 9 provided in the upper end stopper 7 as a pressure regulating mechanism, in which three porous plugs 11, which are permeable partition walls, are arranged in series. The ends of the gas discharge holes 9 are sealed with high-temperature solder 13, and helium (He) gas is sealed inside the neutron absorption element 1.

[0005] This neutron absorption element 1 for fast breeder reactor is:
When loaded into a nuclear reactor and heated by the surrounding primary coolant and raised to a predetermined temperature, the high temperature solder 13 melts.
A liquid metal (e.g., sodium,
melting point approximately 98° C.). As a result, the porous plugs 11 are wetted with the primary coolant, and the spaces between the porous plugs 11 and the upper chamber 8 are filled with the primary coolant.

[0006] When a nuclear reactor is started, the core temperature rises due to a nuclear reaction, the temperature of the primary coolant also rises, and neutrons are emitted. Since helium gas is generated from the boron carbide pellet 2 by the (n, α) reaction due to these neutrons, the helium gas pressure inside the neutron absorption element 1 increases to try to release this helium gas. This process is
First, the differential pressure ΔP maintained at the lowest porous plug 11 in the pressure regulating mechanism (this is the surface of the primary coolant (not shown) filled on the porous plug 11 when helium gas passes through the internal gap of the porous plug 11. This corresponds to the force that overcomes the tension and is determined by the characteristics of the porous plug 11.)
When the pressure reaches the above level, the helium gas passes through the porous plug 11. Therefore, when N porous plugs 11 are arranged in series, by setting the holding differential pressure NΔP higher than the primary coolant pressure P0 in any transient state of the reactor, it is possible to Accordingly, the helium gas inside the neutron absorption element 1 can be released and the primary coolant can be prevented from entering the lower chamber 6, so that the boron carbide pellets 2 in the lower chamber 6 and the liquid metal of the primary coolant can be prevented. There is no direct contact between the two and a long lifespan can be maintained.

[0007]

[Problems to be Solved by the Invention] In the conventional neutron absorbing element 1, when the neutron absorbing element 1 is loaded into a nuclear reactor (not shown) and the neutron absorbing element 1 is raised to a predetermined temperature by the surrounding primary coolant, the high temperature solder 13 melts. When the primary coolant enters the interior, the porous plug 11 of the pressure regulating mechanism becomes wetted by the primary coolant, and eventually the pressure P1 in the lower chamber 6 and the pressure P0 of the external primary coolant become equal, resulting in a balance. do. (This condition can be easily set by design.) However, in order to maintain the pressure here, it is necessary to fill the space formed by the porous plugs 11 arranged in series with the primary coolant. In order to allow the primary coolant to penetrate through the air gap, it is necessary to wet the porous plug 11 with the primary coolant.

Stainless steel is generally used as the material for the porous plug 11, but when the primary coolant is sodium, it usually takes a long time at a high temperature of about 300° C. or higher to wet the stainless steel with sodium. It is necessary to keep it in contact with sodium for some time. However, since the porous plug 11 is formed of a large number of fine holes, it is necessary to bring the plug into a higher temperature state in order to wet the voids with sodium. However, when the neutron absorbing element 1 is loaded into a nuclear reactor, the neutron absorbing element 1 itself generates almost no heat inside the reactor, and sodium is usually kept at a temperature of about 200°C to maintain its liquid state before reactor startup. For,
It has been difficult to obtain the temperature and pressure conditions necessary for the internal voids of the porous plug 11 to be wetted with sodium.

Furthermore, when the space between the porous plugs 11 is filled with primary coolant at the stage when the neutron absorption element 1 is loaded into the reactor, it is necessary to ensure that the space is filled with the primary coolant. There was no way to check.

When the reactor is put into operation without the primary coolant being filled in the space between the porous plugs 11, the pressure P1 in the lower chamber 6 of the neutron absorbing element 1 temporarily decreases to the outside due to the operation of the reactor. The pressure becomes equal to the pressure P0 of the primary coolant, and the internal and external pressures are balanced, but by operating in this state for a long time, the wettability of the porous plug 11 by the primary coolant changes due to pressure changes inside and outside the neutron absorption element 1. However, when the neutron absorbing element 1 is stopped, the internal pressure decreases as the temperature of the neutron absorbing element 1 decreases during cooling, resulting in a pressure difference between the inside and outside, and as a result, the primary coolant flows into the boron carbide pellets 2 in the lower chamber 6. There is a possibility that the life of the neutron absorbing element 1 may be shortened by penetrating the neutron absorbing element 1. As a countermeasure to this problem, there has been a proposal to lengthen the upper chamber 8 and the communication pipe 10, but this has the drawback of increasing the overall length of the neutron absorbing element 1, and solving these problems has been a problem.

The object of the present invention is to solve the above-mentioned problems, and to shorten the lifespan of the neutron absorbing element so that the space between the porous plugs, which is a pressure regulating mechanism, can be reliably filled with liquid metal at the time of loading the neutron absorbing element into the nuclear reactor. The object of the present invention is to provide a neutron absorption element with good pressure regulation function and good manufacturability. [Structure of the invention]

001
2]

[Means for Solving the Problems] A cladding tube whose both ends are sealed with end plugs, and an intermediate end plug having a communicating tube that communicates with each other through the cladding tube divides the chamber into an upper chamber and a lower chamber, and either one of the chambers is divided into an upper chamber and a lower chamber. A cylindrical support body in which a neutron absorbing material is housed in one chamber and a plurality of permeable partition walls of a pressure regulating mechanism are arranged in series in the other chamber with a space between them, one of which is provided at the end plug. A neutron absorption element fixed in communication with the discharge hole, which is the same as or equivalent to the primary coolant of a nuclear reactor, which is solid at room temperature and liquid at the operating temperature inside the reactor, etc. It is characterized by being loaded with metal pieces having the following properties. Further, the periphery of this metal piece is coated with a low melting point metal or the like.

[0013]

[Operation] When this neutron absorption element is loaded into a nuclear reactor, heat from the primary coolant surrounding the neutron absorption element causes the permeation partition wall provided in the cylindrical support of the pressure regulating mechanism and the metal pieces loaded in the upper chamber to Upon melting, the space between the permeable partitions is filled with liquid metal. Therefore, it is not necessary to bring the primary coolant to a high temperature and high pressure state in order to fill the permeation partition wall or the upper chamber with the primary coolant.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. Note that the same components as those in the conventional example described above are given the same reference numerals and detailed explanations are omitted. Figure 1 is a longitudinal cross-sectional view of a neutron absorption element for a fast breeder reactor, and this neutron absorption element 14 is made of boron carbide (B4C), which is a neutron absorption material.
Pellets 2 are stored in a lower chamber 6 surrounded by a lower end plug 4 and an intermediate end plug 5 at the lower part of a cylindrical cladding tube 3. The upper part of the intermediate end plug 5 has an upper chamber 8 surrounded by the cladding tube 3 and the upper end plug 7, and the upper end plug 7 is provided with a gas discharge hole 9 and communicates with the outside. Note that a communication pipe 10 passes through the intermediate end plug 5 and opens at the upper part of the upper chamber 8, and communicates with the lower chamber 6 which is planted and contains the boron carbide pellets 2.

Further, on the lower surface of the gas discharge hole 9 provided in the upper end plug 7, there are three porous plugs 11 which are permeable partition walls.
A cylindrical support 12 in which a, 11b, and 11c are arranged in series is fixed to constitute a pressure regulating mechanism, and the gas discharge hole 9
The end portion of the neutron absorbing element 14 is sealed with high-temperature solder 13 to isolate it from the outside air, and the inside of the neutron absorbing element 14 is filled with helium gas. Note that the porous plugs 11a and 11b
, 11c is a permeable slit network made of stainless steel or sintered metal, and the passage of helium gas and the primary coolant is appropriately suppressed by penetrating the liquid metal or interposing it on the surface.

Furthermore, a space 12a is formed between the porous plug 11a and the porous plug 11b arranged in the cylindrical support 12, and a space 12b is formed between the porous plug 11b and the porous plug 11c. and,
Inside the upper chamber 8 is loaded a sodium piece 15a, which is a metal piece made of the same material or substantially the same properties as the primary coolant of a nuclear reactor, which is solid at room temperature and liquid at high temperatures when used inside a nuclear reactor. It is configured as follows.

Next, the operation of the above configuration will be explained. Note that FIG. 1 shows the state of the neutron absorption element 14 before it is loaded into the reactor, FIG. 2 is a vertical cross-sectional view showing the state when it is initially loaded into the reactor, and FIG. 3 shows the final state inside the reactor. FIG. 3 is a vertical cross-sectional view showing a balanced state.

The manufactured neutron absorbing element 14 in the state shown in FIG. 1 has helium gas sealed inside and is isolated from the outside air by high-temperature solder 13 that seals the end of the gas discharge hole 9. 8 and three porous plugs 11a, 11b
, 11c of the cylindrical support 12,
Each of the solid sodium pieces 15a is loaded into each of the solid sodium pieces 12b.

When this neutron absorption element 14 is loaded into a nuclear reactor (not shown), the surrounding area is immersed in liquid sodium 15 which is a primary coolant of about 200°C, so the neutron absorption element 14 is heated by the heat from this sodium 15. The solid sodium pieces 15a (melting point: about 98° C.) in 14 easily melt and become fluid between the surfaces of the porous plugs 11b and 11c, the bottom of the upper chamber 8, and the lower end of the cylindrical support 12. Intervenes in liquid form. As time passes further, the high temperature solder 13 at the end of the gas discharge hole 9 melts, and as a result, the primary coolant sodium 15 is released from the outside via the gas discharge hole 9 due to the pressure difference between the inside and outside of the neutron absorbing element 14. They invade the upper chamber 8. This sodium 15 is already in a liquid state in the upper chamber 8 between the porous plugs 11b, 11c and the lower end of the cylindrical support 12.
5a, and as a unit, once invades the cylindrical support 12 and the upper chamber 8 as shown in FIG.

After that, when the reactor is started, the temperature of the surrounding sodium 15 increases due to the nuclear reaction, and in the neutron absorption element 14, the boron carbide pellets 2 in the lower chamber 6 absorb neutrons. Since helium gas is generated by the (n, α) reaction, the pressure inside the lower chamber 6 is P
1 rises and tries to release this helium gas. The pressure of this helium gas is transferred to the upper chamber 8 through the communication pipe 10.
In addition, each space 12 of the upper chamber 8 and the cylindrical support 12
Push out the sodium 15 in a, 12b, and 12c to the outside. As a result, the pressure between the inside of the neutron absorption element 14 and the pressure of the sodium 15 which is the primary coolant outside is balanced, and as shown in FIG.
Porous plugs 11a, 11b, 11 with 12b and 12c
Liquid sodium 15 exists above c, and a helium gas space is formed.

In the state shown in FIG. 3, when the boron carbide pellet 2 absorbs neutrons and further generates helium gas, this gas becomes bubbles at the lower end of the cylindrical support 12 due to the pressure, and the cylindrical support 12 The sodium rises inside the neutron absorption element 14 and reaches the porous plug 11c at the lower end, passes through the porous plugs 11b and 11a in response to the external pressure, and is released from the gas release hole 9 to the outside sodium 15. The external sodium 15 pressure is again balanced.

[0022] Note that the metal pieces, which are initially solid and have substantially the same properties as the primary coolant that is preloaded into the neutron absorbing element 14 and become liquid during reactor operation, are melted and eventually become primary coolant after reactor operation. Considering that metallic sodium is normally used as the primary coolant in nuclear reactors, it is desirable to use metallic sodium of the same material. However, since this metal sodium is active, it usually needs to be handled in an inert atmosphere, which has the drawback of requiring care and making the handling process complicated. However, by covering the area around this metal sodium piece with a metal that has a lower melting point than sodium, such as solder, and a material that does not adversely affect sodium, which is the primary coolant, when melted, it is stored so that it does not come into direct contact with the outside air. And handling during loading work can be facilitated.

Further, in the above embodiment, the neutron absorbing material was explained as boron carbide pellets, but the neutron absorbing effect and the effects of the present invention will not change even if boron carbide powder is used.

[0024]

As described above, according to the present invention, in a neutron absorption element used in a liquid metal cooled fast breeder reactor, by loading fusible metal pieces into the pressure regulating mechanism in advance during the manufacturing process,
The pressure regulation function is quickly ready when the reactor is loaded, and the reliability of the pressure regulation function of the neutron absorption element during start-up operation is improved, resulting in a long service life, shortened lifespan, and improved pressure regulation function and productivity. This has the effect of

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing one embodiment of a neutron absorption element according to the present invention.

FIG. 2 is a longitudinal cross-sectional view showing the intrusion state of primary coolant when the reactor is loaded in FIG. 1;

FIG. 3 is a longitudinal cross-sectional view showing a balanced state of internal and external pressures in FIG. 1;

FIG. 4 is a vertical cross-sectional view of a conventional neutron absorption element.

[Explanation of symbols]

2... Boron carbide pellets, 3... Cladding tube, 4... Lower end plug,
5... Middle end plug, 6... Lower chamber, 7... Upper end plug, 8... Upper chamber, 9... Gas release hole, 10... Communication pipe, 11a, 11b, 1
1c...porous plug, 12...cylindrical support, 12a, 1
2b, 12c...Space, 13...High temperature solder, 14...Neutron absorption element, 15...Sodium, 15a...Sodium piece.

Claims (1)

[Claims] 1. A cladding tube whose both ends are sealed by end plugs is divided into an upper chamber and a lower chamber by an intermediate end plug, and the two chambers are communicated with each other by a communicating tube attached to the intermediate end plug, so that either of the above-mentioned chambers is connected. One chamber accommodates a neutron absorbing material, and the other chamber has a pressure regulating mechanism.One of the cylindrical supports has a plurality of permeable partition walls arranged in series to form a space between them, and one end plug is connected to the end plug. In the neutron absorption element fixed in communication with the gas release hole provided in the
In the space formed by the permeable partition walls arranged in series within the cylindrical support, a material similar to the primary reactor coolant, which is solid at room temperature and liquid under the temperature conditions of use in a nuclear reactor, etc.
or a neutron absorbing element loaded with metal pieces having equivalent properties.