JP2003014883A - Double pipe structure in steam generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double pipe structure of a steam generator of sodium heating reducing the frequency of sodium-water reaction events. SOLUTION: The steam generator vertically arranges double pipes 8 filled with heat medium of liquid metal which is heavier than sodium, in the medium layer 14 between a pipe wall 13 contacting sodium and a pipe wall 12 contacting water/steam by penetrating an upper plenum 9 and a lower plenum 10. Protection pipes 15 are provided to open the upper end of the medium layer 14 of the double pipes to the upper plenum 9 and close the connection to the lower plenum 10 by surrounding the lower end of the medium layer 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double-tube structure of a steam generator which exchanges heat between sodium and water to generate high-pressure steam, and more particularly to a double-tube steam generation used in a liquid metal cooling fast breeder reactor. Regarding the structure of the vessel.

[0002]

2. Description of the Related Art In a liquid metal cooling fast breeder reactor, in order to prevent contact between sodium and water, when a system / equipment containing sodium and a system / equipment containing water are placed in close proximity, As a general rule, a triple physical barrier is designed between them, or a double physical barrier is installed between the two, which is a leakage medium treatment measure. However, a single wall heat transfer tube is often used in a steam generator of a liquid metal cooled fast breeder reactor, which was an exception to the above design principle. Of course, also in the steam generator, in order to secure the reliability against the sodium-water reaction, a triple-walled heat transfer tube or a double-walled heat transfer tube provided with measures for treating the leakage medium may be used.

The double-walled heat transfer tube is used by filling a space between the walls with a heat transfer medium having a good heat transfer property. Helium gas is often used as the heating medium, but liquid such as molten metal may be used. Japanese Unexamined Patent Publication No. 54-16762 discloses a steam generator using a double tube in which a liquid metal such as helium gas or mercury is used as a heat medium and enclosed in an intermediate layer. In the double-tube steam generator in which the heat medium is enclosed between the tube walls, the structure of the double heat transfer tube is easy, the heat medium is surely enclosed without any restriction on the posture, and the amount of the enclosed medium is small. However, even if the pipe wall is damaged and high-pressure steam enters, it is difficult to detect the damage.

In a steam generator in which a double-walled heat transfer tube is installed vertically, water / steam side tube plate (steam outlet tube plate and feed water inlet tube plate) and sodium side tube plate (upper part) are provided at the upper and lower ends of the steam generator. There is a plenum formed between the sodium tube plate and the lower sodium tube plate. An inner-outer tube gap or an intermediate layer formed between the outer tube and the inner tube of the double tube is opened to the plenum. FIG. 6 is a cross-sectional view showing a lower end portion of a double pipe structure in a conventional steam generator. The inner tube is welded to the feedwater inlet tubesheet, the outer tube is welded to the lower sodium tubesheet, and the middle layer of the inner and outer tubes is open to the lower plenum.

However, in a steam generator having a structure in which a double tube intermediate layer is connected to a plenum, when water / steam side tube wall is damaged, high-pressure steam of 100 atm or more is injected into the intermediate layer. Therefore, water / steam penetrates the upper plenum and lower plenum through the middle layer, and the water / steam that enters the plenum
The steam enters the middle layer of another double tube connected to the plenum. Therefore, if the sodium-side tube wall is broken in any heat transfer tube, the water / steam that has entered the plenum will enter the sodium side from the damaged part and cause a sodium-water reaction.

On the other hand, if the space between the inner and outer tubes is filled with a liquid metal such as a lead-bismuth melt, which is heavier than water / steam, the water / steam that has escaped to the upper plenum will not penetrate into other heat transfer tubes. In addition, by connecting the plenum for liquid metal and the intermediate layer, when the pipe wall on the steam side is damaged and water / steam invades, water etc. can be collected and detected in the upper plenum, and the sodium side When the pipe wall is broken and the liquid metal heat transfer medium leaks out, it can be detected by lowering the liquid level of the plenum.

When the heat medium metal such as lead bismuth is difficult to mix with sodium, the leak accident can be quickly detected by collecting the liquid metal leaking to the sodium side in the recess and detecting it. In particular, lead bismuth has a low melting point, a high heat transfer efficiency, is difficult to react with sodium and water, and has a large specific gravity, so that the head pressure is large, and sodium does not penetrate into the intermediate layer without applying pressure.

However, even in the steam generator having a structure in which the intermediate layer of the double pipe is connected to the plenum for liquid metal,
Water and steam that have entered the lower plenum pass through the middle layer of other double pipes and escape to the upper plenum. Therefore, if the sodium side tube wall is damaged at some heat transfer tube at the same time, or if the sodium side tube sheet is damaged before the water / steam side tube wall is damaged, the water / steam that has entered the lower plenum Will enter the sodium side from the damaged part and cause a sodium-water reaction.

Since the frequency of tube wall breakage is 10 ⁻⁶ to 10 ⁻² times per furnace per year, a sodium-water reaction event occurs in a steam generator in which double-walled heat transfer tubes are connected by upper and lower plenums. The frequency of occurrence of is estimated to be about 10 ⁻⁶ times even if it is small. According to the design criteria of a nuclear reactor, it is necessary to design on the assumption that an event will occur as an event within the design criteria at an event occurrence frequency of 10 ⁻⁷ times / reactor / year or more. It is preferable to reduce the frequency.

[0010]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is to provide a double tube structure for reducing the frequency of sodium-water reaction events in a sodium-heated steam generator.

[0011]

In order to solve the above problems, the double pipe structure of the present invention is a liquid metal heavier than sodium in the intermediate layer sandwiched between the pipe wall in contact with sodium and the pipe wall in contact with water / steam. In a steam generator in which a double tube filled with the heat medium of (1) is vertically arranged through the upper plenum and the lower plenum, the upper end of the intermediate layer of the double tube is opened to the upper plenum, and the lower end of the intermediate layer is It is characterized in that a protective tube is provided around the intermediate layer to block the flow with the lower plenum. The flow-blocking protection tube may be formed by welding a vertically divided cylindrical member, or may be a bellows tube.

When the double pipe structure of the steam generator according to the present invention is used, the gap between the inner and outer pipes of the double pipe is filled with the liquid metal heavier than sodium, so that water and steam do not circulate through the upper plenum. Moreover, since a protection tube is provided at the lower end of the heat transfer tube to prevent water and steam from entering the lower plenum, there is no wraparound via the lower plenum. Sodium in a steam generator to which the present invention is applied
The frequency of water reaction events is almost 1 in a steam generator equipped with several thousand double-walled heat transfer tubes, because it is limited to the case where damage occurs on both the water / steam side and sodium side of the same heat transfer tube.
0 can be estimated to be ^-9 times / furnace-years or so. This value can be treated as a nonstandard event in the design standard, and it becomes possible to reduce the equipment cost and maintenance cost.

If a vertically divided cylindrical member is welded as the protective tube, it is possible to relatively easily attach the heat transfer tube to the tube sheet by butt welding even in a steam generator in which many heat transfer tubes are densely packed. You can Also, when a bellows tube is used as the protective tube, the protective tube can be fixed to the tube sheet relatively easily even in a narrow plenum space.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to examples. FIG. 1 is a sectional view schematically showing an embodiment of a double pipe structure of a steam generator of the present invention, FIG. 2 is an enlarged sectional view showing a lower end portion of the double pipe of the present embodiment, and FIG. Is an exploded view of the protective tube used in the present embodiment, FIG. 4 is an enlarged sectional view showing the upper end portion of the double tube of the present embodiment, and FIG. 5 is a sectional view showing another example of the protective tube.

The steam generator shown schematically in FIG.
000 double wall heat transfer tubes arranged vertically with a diameter of 3 m and a length of 25
It is a heat exchanger of about m. Sodium at about 550 ° C returned from the nuclear reactor is supplied to the shell side, and the water on the tube side is heated to form steam at about 500 ° C (176 atm) and sent to a steam turbine for power generation. The steam generator is equipped with a shell 1 so that liquid sodium from the reactor is supplied from the upper nozzle and returned from the lower nozzle. An upper water chamber 2 for accumulating steam at the upper end of the body 1 and a lower water chamber 3 for supplying water to the lower end are provided. A steam outlet tube sheet 4 is formed on the bottom of the upper water chamber 2, and a water supply inlet tube sheet 6 is formed on the ceiling of the lower water chamber 3. Further, an upper sodium tube sheet 5 is provided on the ceiling of the body 1 and a lower sodium tube sheet 7 is provided on the bottom thereof. An upper plenum 9 is formed between the steam outlet tube sheet 4 and the upper sodium tube sheet 5, and a lower plenum 10 is formed between the feed water inlet tube sheet 6 and the lower sodium tube sheet 7.
Are formed.

A heat transfer tube 8 is provided between the upper water chamber 2 and the lower water chamber 3.
Has been passed. The heat transfer tube 8 is a double-walled heat transfer tube including an inner tube 12 and an outer tube 13, and the inner tube 12 is connected to the steam outlet tube plate 4 and the feed water inlet tube plate 6. The upper end of the outer tube 13 is connected to the upper sodium tube plate 5, and the lower end is connected to the water supply inlet tube plate 6. Water and steam circulate in the inner pipe 12, and sodium flows outside the outer pipe 13. Further, an inner-outer tube gap 14 having a width of, for example, about 0.4 mm is formed between the inner tube 12 and the outer tube 13, and is filled with a liquid metal such as lead bismuth which is heavier than sodium. The steam generator is installed vertically on the frame by means of a support skirt 11 provided on the barrel 1.

FIG. 2 is a cross-sectional view showing a mounted state at the lower end of the double-walled heat transfer tube 8. The inner pipe 12 is butt-welded to the stub 16 provided on the feed water inlet pipe plate 6 from the inside of the inner pipe, and the outer pipe 13 is butt-welded to the stub 17 provided on the lower sodium pipe plate 7 from the outside. Further, at the position of the lower plenum 10, cylindrical members divided into two as shown in FIG. 3 are joined together by sandwiching the inner pipe 12 and connected by welding in the axial direction to form a cylindrical shape, and further, the end face is lower. The stub 19 formed on the lower edge of the heat transfer tube hole of the sodium tube sheet 7 and the stub 18 provided on the upper side at the corresponding position of the feed water inlet tube sheet 6 are circumferentially welded to each other to form the inner tube 12 And the protective tube 15 surrounding the. The protection tube 15 forms a shielding wall between the inner and outer tube gaps 14 and the lower plenum 10 so that the heat medium filling each of the inner and outer tube gaps 14 is filled with the lower plenum 10.
It is intended to prevent communication.

FIG. 4 is a cross-sectional view showing a mounting state at the upper end portion of the double wall heat transfer tube 8. The inner tube 12 is welded to the steam outlet tube plate 4 by fillet welding, and the outer tube 13 is welded to the upper sodium tube plate 5.
Weld the square meat. The inner-outer tube gap 14 communicates with the upper plenum 9, and the liquid metal filling the inner-outer tube gap 14 overflows into the upper plenum 9 to form a liquid surface on the upper part. The heat transfer tubes 8 are attached, for example, sequentially from the center position of the steam generator toward the outside.

The steam generator having the heat transfer tube structure of this embodiment is used by filling the inner and outer tube gaps 14 with a liquid metal heavier than sodium and water / steam, such as lead bismuth. The inner and outer tube gaps 14 are connected to each other by the upper plenum 9, but the lower ends thereof are closed by protective tubes 15, respectively, so that they cannot flow out to the lower plenum 10. Therefore, when the inner tube 12 is damaged, high-pressure steam enters the inner-outer tube gap 14, rises to the upper plenum 9, and once floats on the surface of liquid metal such as lead bismuth, it may return to the heat transfer tube 8 again. Absent. Even if water / steam moves downward in the inner / outer tube gap 14, it does not flow through the lower plenum 10 to other heat transfer tubes because of the protection tube 15. Therefore, even if the outer tube 13 of the other heat transfer tube is damaged, water / steam does not come into contact with sodium, and a sodium-water reaction event does not occur.

That is, in the steam generator of this embodiment, the sodium-water reaction occurs only when both the inner tube 12 and the outer tube 13 of the same double-walled heat transfer tube 8 are damaged. Therefore, in a steam generator having several thousand heat transfer tubes, the sodium-water reaction event occurrence frequency will be reduced by about 3 orders of magnitude, and it can be estimated that it is about ^10-9 times / reactor / year under normal conditions. it can. For this reason, the sodium-water reaction event in the steam generator of the present embodiment is an event outside the design criteria, so the device cost and maintenance cost are reduced.

Since lead bismuth has a specific gravity of about 10 and a large water head pressure in the inner / outer tube gap 14, it flows out to the sodium side when the outer tube 13 is damaged. The plenum may be pressurized if there is a risk of insufficient head pressure depending on the location of damage. It should be noted that such a damage accident can be detected by the liquid level drop of lead-bismuth. Further, it is also possible to detect breakage of the outer tube by detecting lead bismuth leaked into sodium.

FIG. 5 is a cross-sectional view showing a mounting state at the lower end of the double-walled heat transfer tube when a bellows type protection tube is used in this embodiment, and FIG. The process is shown, and (b) shows the completed state. Before welding the heat transfer tube, it is set so that the bellows-type tubular protective tube 22 that can be expanded and contracted when the tip of the inner tube 12 is projected to the lower plenum. The inner pipe 12 is butt-welded to the stub 16 provided on the feed water inlet pipe plate 6 from the inside of the inner pipe, and the outer pipe 13 is butt-welded to the stub 17 provided on the lower sodium pipe plate 7 from the outside.

As shown in FIG. 5 (a), the bellows type protection tube 22 is welded at its upper end edge to a stub 19 formed on the lower edge of the lower sodium tube sheet 7, and then in FIG. 5 (b). )
The bellows are extended and the lower end edge is welded to the stub 18 of the water supply inlet tube sheet 6 as shown in FIG. The bellows type protection tube 22 is the inner tube 12
Inner / outer tube gap 14 and lower plenum 1 sandwiched between outer tube 13 and outer tube 13
A shield wall is formed between the inner and outer tube gaps 0 so that the heat medium filling each of the inner and outer tube gaps 14 does not communicate with the lower plenum 10. Since the bellows-type protection tube 22 has fewer welded portions than the protection tube 15 shown in FIG. 2, the reliability of on-site construction is improved.

In the above embodiment, the case of aiming to reduce the frequency of sodium-water reaction events has been described for the steam generator of the liquid metal cooled fast breeder reactor.
It is apparent that the present invention can be applied to a device in which a double-walled heat transfer tube is used in a device that causes an unfavorable reaction due to contact between fluids for heat exchange, and exhibits a great effect.

[0025]

EFFECTS OF THE INVENTION By using the double tube structure of the steam generator of the present invention, in a heat exchanger handling two kinds of fluids which are difficult to mix, the frequency of contact reaction of the two kinds of fluids is remarkably increased. It is possible to reduce the facility cost of the plant and secure high safety.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing an example of a double pipe structure of a steam generator of the present invention.

FIG. 2 is an enlarged cross-sectional view showing a lower end portion of the double pipe of this embodiment.

FIG. 3 is an exploded view of a protection tube used in this example.

FIG. 4 is an enlarged sectional view showing an upper end portion of the double pipe of the present embodiment.

FIG. 5 is a cross-sectional view showing another example of the protection tube used in this embodiment.

FIG. 6 is a cross-sectional view showing a lower end portion of a conventional double pipe.

[Explanation of symbols]

1 torso 2 Upper water chamber 3 Lower water chamber 4 Steam outlet tube sheet 5 Upper sodium tube sheet 6 Water supply inlet tube plate 7 Lower sodium tube sheet 8 heat transfer tubes 9 Upper plenum 10 Lower Plenum 11 Support skirt 12 inner tube 13 outer tube 14 Inner and outer tube gap (middle layer) 15,22 Protection tube 16, 17, 18, 19 stub

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F28F 9/18 G21C 1/02 GDFE G21C 1/02 GDF G21D 1/00 S

Claims (3)

[Claims] 1. A double tube in which an intermediate layer sandwiched between a tube wall in contact with sodium and a tube wall in contact with water / steam is filled with a heating medium made of a liquid metal heavier than sodium, and vertically penetrates through an upper plenum and a lower plenum. In the steam generator arranged in, the upper end of the intermediate layer of the double tube is opened to the upper plenum,
A double pipe structure, wherein a protective tube is provided at a lower end portion of the intermediate layer so as to surround the intermediate layer and block a flow with the lower plenum. 2. The double pipe structure according to claim 1, wherein the protective pipe is formed by welding a vertically divided cylindrical member. 3. The double pipe structure according to claim 1, wherein the protection pipe is a bellows pipe.