JPH04268495A - Removing equipment for core decay heat - Google Patents

Abstract

PURPOSE:To lessen a temperature difference between the inside and outside of each part and to reduce a thermal stress to the minimum by make each part have a double tube structure. CONSTITUTION:A heat exchanger tube 14b, a downcomer part 14a, a lower communication tube 14c and a upper communication tube 14d of a PRACS heat exchanger element are provided, and an inner tube 18 is provided in the downcomer part 14a which has inlet and outlet tubes of PRACS made to communicate with the upper communication tube 14d and the downcomer part 14a through a ring header and making secondary sodium flow through, and in the lower communication tube 14c, so as to make these tubes have a double tube structure.

Description

[Detailed description of the invention]

[Purpose of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a core decay heat removal device for a fast breeder reactor.

0002

2. Description of the Related Art FIG. 4 is a longitudinal sectional view of an intermediate heat exchanger and a decay heat removal device incorporated therein in a conventional tank-type fast breeder reactor. The intermediate heat exchanger is suspended and supported within the reactor vessel from a roof slab 1 (not shown) that closes the upper opening of the reactor vessel. The intermediate heat exchanger has a body shell 2 which engages the roof slab 1 by means of an upper end flange 2a. The inside of the reactor vessel is filled with liquid metal sodium 3, and a primary inlet window 2b is provided in a portion of the main body shell 2 below the liquid level 3a of the liquid metal sodium 3. Furthermore, an upper tube plate 5a having an opening 5c in the center is provided approximately at the center in the axial direction within the main body shell 2. Furthermore, a lower tube plate 5b is provided at the lower part of the main body trunk 2. An electric heating tube bundle 4 is attached to the upper tube sheet 5a and the lower tube sheet 5b. A hanging portion 5d having a shape corresponding to the shape of the lower end of the main body shell 2 is provided below the lower tube plate 5b, and this hanging portion 5d and the lower end of the main body shell 2 are connected by a bellows 6.

The opening 5c of the upper tube plate 5a is connected to the lower end of a riser pipe 7 which is concentric with the main body shell 2 and passes through a biological shield 9 that closes the upper end of the main body shell 2. A downcomer pipe 8 concentric with the main body shell 2 is arranged. In addition,
The lower end of the downcomer pipe 8 is open to the space above the lower tube plate 5b. Further, the downcomer pipe 8 fluid-tightly penetrates the upper end of the riser pipe 7 that protrudes above the roof slab 1 and is connected to a main pipe (not shown) through which liquid metal sodium flows.
Similarly, it is connected to the main pipe above the roof slab 1. Furthermore, the biological shield 9 includes the ascending pipe 7.
An inlet pipe 10 and an outlet pipe 11 pass through both sides of the pipe, and a toroidal inlet ring header 12 and an outlet ring header 13 are respectively connected to the inner ends of these pipes. Between the inlet ring header 12 and the outlet ring header 13 is a core decay heat removal device (Pr
imary Reactor Auxil-iary
Cooling System (hereinafter referred to as PRACS) is connected by a large number of heat transfer tubes 14b of the heat transfer section 14. The PRACS heat transfer section 14 includes a downcomer 14a that hangs down from the inlet ring header 12 to a position above the upper tube sheet 5a.
and an upper communication pipe 14 communicating with the outlet ring header 13.
d, a heat exchanger tube 14b wound in a helical coil shape,
It has a lower communication pipe 14c that communicates the downcomer 14a and the heat transfer tube 14b. In addition, in the figure, 15 is secondary sodium, 16 is PRACS secondary sodium, and 17 is
indicate the respective drain pipes.

[0004] The operation of the conventional intermediate heat exchanger and PRACS having the above structure will be explained below. During normal operation of the nuclear reactor, primary sodium 3 heated to a high temperature in a reactor core (not shown) flows into the main body shell 2 through the primary inlet window 2b. This primary sodium 3 flows along the outside of the PRACS heat exchanger tubes 14b, and flows into the heat exchanger tube bundle 4 below from the upper tube sheet 5a. While flowing down inside the heat transfer tube bundle 4, it exchanges heat with the outer secondary sodium 15 of the heat transfer tube bundle 4, becomes low temperature, passes through the lower tube plate 5b, and flows out of the main body shell 2 from the lower end of the main body shell 2. On the other hand, the secondary sodium 15 flows down the downcomer pipe 8 from the secondary main pipe (not shown) and flows into the lower end of the heat transfer tube bundle 4. Then, it exchanges heat with the primary sodium 3 flowing within the heat transfer tube bundle 4, becomes high temperature, rises through the annular cross-sectional space between the riser pipe 7 and the downcomer pipe 8, and flows into the secondary main pipe. 2
The next sodium is then sent to a steam generator (not shown).
Here, it participates in steam generation, becomes low temperature, is pressurized by a secondary main circulation pump (not shown), and is refluxed to the main body shell 2. In other words, a closed loop is formed.

[0005] The core decay heat removal device is activated when the above-mentioned heat transport to the steam generator becomes impossible for some reason. That is, when heat transport to the steam generator becomes impossible, PRACS secondary sodium 16 flows in from the PRACS inlet pipe 10 and is distributed to a large number of PRACS heat transfer pipes 14b by the inlet ring header 12,
Flowing down the downcomer portion 14a of the heat transfer section 14, the heat transfer tube 1
4b, it exchanges heat with the primary sodium 3 flowing around it and becomes high temperature, and the outlet ring header 13 and the outlet pipe 11
It flows into the PRACS piping (not shown) via the. Next, the PRACS secondary sodium 16 is guided to an air cooler (not shown), where it exchanges heat with air to become low temperature, is pressurized by an electromagnetic pump provided in the PRACS pipe, and returns to the inlet pipe 10.

[0006]

Problems to be Solved by the Invention The conventional decay heat removal apparatus having the above structure had the following problems when operating PRACS. That is, since the PRACS heat transfer section 14 is located in the main flow path of the primary sodium 3 during normal operation, water from the intermediate heat exchanger, which is the main cooling equipment, via the steam generator,
When it becomes impossible to remove heat using steam, it is possible to start up the PRACS electromagnetic pump and start the air cooler without special flow path switching, so it is evaluated from a reliability standpoint. However, the disadvantage is that the amount of primary sodium flowing during the decay heat removal operation is too large. The details of the above problem will be specifically explained below using a 10 million Kwe class fast breeder reactor as an example.

If a 10 million Kwe class fast breeder reactor is configured with four loops, the amount of heat exchanged per loop during rated operation will be 600 to 650 MWe. The amount of heat exchanged during decay heat removal operation is 1 compared to the amount of heat exchanged during rated operation.
It is about 2 to 20 MWe, which is about 3% of that during rated operation. On the other hand, the circulation flow rate of primary sodium during decay heat removal operation is 8 to 15% of the rated circulation flow rate because the motor of the primary circulation pump is operated as a pony motor, which is too large compared to the required exchange heat amount. Become. As a result, there is not much difference in temperature between the primary sodium flowing into the PRACS heat exchanger tube bundle and the primary sodium flowing out from it. On the other hand, PRACS's 2
The temperature of the secondary sodium depends on the relative relationship between the heat transfer area of the air cooler and the PRACS heat transfer section, but in order to avoid increasing the diameter of the PRACS secondary piping, the temperature difference between the inlet and outlet should be set to 200
It is common to set it to °C. An example is as follows. exchange heat amount
18MWc primary inlet temperature/outlet temperature
520/495°C Secondary inlet temperature/outlet temperature
495/275°C

That is, the temperature at the low temperature side of the PRACS heat exchanger tube bundle is 220°C.
This results in a large temperature difference. In addition, in the above example, an electromagnetic pump is used to circulate secondary sodium to perform forced circulation, but if the electromagnetic pump fails, natural circulation of secondary sodium occurs,
The circulating flow rate decreases significantly and the temperature difference occurring on the low temperature side of the PRACS heat exchanger tube becomes larger. When a large temperature difference occurs on the low temperature side as described above, various problems arise as described below. That is, 1. Thermal stress occurs because there is a large temperature difference between the inner and outer surfaces of the tube. This thermal stress is greatly influenced by the high heat transfer coefficient of the liquid metal sodium used as the coolant. In the case of the temperature difference of 220C in the above specific example, the thermal stress on the heat exchanger tube surface is 19K.
It is about g/mm 2 . This value becomes even larger when secondary sodium circulates naturally, which may impair the integrity of the heat exchanger tube. 2. The PRACS heat transfer tube is provided with a support for the purpose of preventing vibrations due to fluid force, but since this support is provided outside the heat transfer tube, its temperature follows that of primary sodium. On the other hand, the temperature of the heat exchanger tube is primary sodium,
Since the temperature is approximately intermediate to that of secondary sodium, there is a considerable difference in thermal expansion between the support and the heat exchanger tube, and there is a possibility that a large forced displacement will act on the heat exchanger tube. 3. Since the PRACS lower connecting pipe 14c is located directly above the intermediate heat exchanger upper tube sheet 5a, the temperature of the primary sodium flowing into the intermediate heat exchanger varies depending on the area where the PRACS lower connecting pipe is directly above the upper tube sheet 5a and the area where it is not. There is a risk that there will be a large difference. This temperature difference may damage the integrity of the intermediate heat exchanger.

The present invention was made based on the above-mentioned circumstances, and an object of the present invention is to provide a core decay heat removal device capable of alleviating the influence of the temperature difference between primary sodium and secondary sodium. [Structure of the invention]

0010

[Means for Solving the Problems] The core decay heat removal device of the present invention includes a cylindrical main body shell suspended and supported within the reactor vessel from a roof slab that closes the upper opening of the reactor vessel, and this main body. an upper tube sheet that partitions a heat transfer section provided in the shell and a heat transfer tube bundle section that exchanges heat between primary sodium and secondary sodium; a lower tube sheet provided below the heat transfer tube bundle; and the main body. A downcomer pipe and a riser pipe through which secondary sodium passes through a biological shield attached to the upper end of the body and exchanges heat with the heat exchanger tube bundle, and a heat exchanger tube and a downcomer part of the heat transfer part provided outside the riser pipe. , which has a lower communication pipe, an upper communication pipe, and 6 inlet pipes and outlet pipes that communicate with the upper communication pipe and the downcomer portion via a ring header and allow secondary sodium to flow therethrough,
It is characterized in that inner tubes are provided in the downcomer portion and the lower communication tube and have a double tube structure.

[0011]

[Operation] In the core decay heat removal device of the present invention having the above configuration, the downcomer section and the lower connecting pipe have a double pipe structure, and since they are kept at a temperature close to that of primary sodium, the temperature difference between their inner and outer surfaces is small. thermal stress can be alleviated to a large extent. .

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of essential parts of an embodiment of the present invention, and FIGS. 2 and 3 are sectional views of constituent members of the embodiment. Figure 1 is PRACS in Figure 4
The area around the heat transfer section 14 is taken out and shown, and in this figure, above the upper tube plate 5a, there is a PR around the riser pipe 7.
An ACS heat transfer section 14 is configured. An inlet ring header 12 and an outlet ring header 13 are provided above the PRACS heat transfer section 14. A downcomer section 14a is provided around the riser pipe 7 connected to the inlet ring header 12, and a lower communication pipe 14c is provided connected to the downcomer section 14a and located above the upper tube plate 5a. A heat transfer tube 14b whose center line is shown by a dashed line in the figure is connected to the lower communication tube 14c, and the heat transfer tube 14b and the outlet ring header 13 are connected by an upper communication tube 14d. Further, an inner pipe 18 is provided in contact with the inner peripheral surfaces of the downcomer portion 14a and the lower connecting pipe 14c.

FIG. 2 shows how the inner pipe 18 is installed on the inner peripheral surfaces of the downcomer portion 14a and the lower connecting pipe 14c. In this figure, the inner tube 18 is made of metal and is in close contact with the downcomer portion 14a and the lower communication tube 14c, which have a double tube configuration.

The operation of the above embodiment will be explained below. During PRACS operation, secondary sodium is
Via the S inlet pipe 10, the inlet ring header 12, the downcomer part 14a, the lower connecting pipe 14c, and the heat transfer tube 14b.
The upper connecting pipe 14d, the outlet ring header 13, and the plax outlet pipe 11
It flows into an air cooling device (not shown) via the. on the other hand,
The primary sodium 3 undergoes heat exchange in the PRACS heat transfer section 14, passes through the upper tube plate 5a, and flows into the heat transfer tube bundle section. The movements of the primary sodium and secondary sodium described above are similar to those in conventional devices of this type. In the above embodiment, the downcomer portion 14a and the lower communication pipe 14c
are provided with an inner tube 18, so their temperature is 1
The temperature is said to be close to that of sodium chloride.

As described above, the downcomer part 14a and the lower connecting pipe 14c have a double pipe structure, and because they are set at a temperature close to the primary sodium 3, the temperature difference between the inside and outside surfaces is small, and thermal stress is greatly reduced. can be relaxed. In addition, the downcomer portion 14a and the downcomer portion 5 have the lowest temperature.
Since the metal temperature of b is close to the primary sodium temperature, the temperature difference with the heat exchanger tube support is reduced. Therefore, thermal stress is relaxed and the integrity of the heat exchanger tube is improved. In addition, the lower connecting pipe 1 located directly above the intermediate heat exchanger upper tube plate 5a
Since the surface temperature of 4c becomes close to the temperature of the primary sodium, the temperature fluctuations imparted by the lower communication pipe to the primary sodium flowing into the heat exchanger tube section of the intermediate heat exchanger are alleviated.

FIG. 3 is a sectional view of a main part of another embodiment of the present invention. In FIG. 2, the inner tube is constructed by aligning metal tube blocks 18a divided in the axial direction. In this embodiment, the temperature difference between the inner tube and the outer tube is large, and when a difference in thermal expansion occurs between the two, this can be effectively absorbed. Furthermore, in this embodiment, when the metal tube block 18a is coated with a heat insulating material, the heat insulation effect due to the double pipe structure is further increased, and the temperature of the downcomer part 14a and the lower connecting pipe 14c is almost the same as that of the primary sodium 3. Therefore, better results can be obtained.

[0017]

As is clear from the above, in the core decay heat removal device of the present invention, the temperature difference between the inner and outer surfaces of the downcomer portion and the lower connecting pipe is reduced, and thermal stress can be alleviated. In addition, forced displacement of the heat exchanger tubes due to the difference in thermal expansion between the downcomer portion, the lower connecting pipe, and the heat exchanger tube support is alleviated, making it easier to maintain the integrity of the heat exchanger tubes. Furthermore, since the temperature difference between the lower connecting pipe installed directly above the upper tube plate of the intermediate heat exchanger and the primary sodium is small, fluctuations in the temperature of the primary sodium flowing into the intermediate heat exchanger are alleviated. . Therefore, the integrity can be significantly improved not only in the PRACS heat transfer section but also in the intermediate heat exchanger.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of essential parts of an embodiment of the present invention.

FIG. 2 is a sectional view of the structural members of the embodiment.

FIG. 3 is a sectional view of a component of another embodiment.

FIG. 4 is a vertical sectional view of an intermediate heat exchanger and a decay heat removal device incorporated therein in a conventional tank-type fast breeder reactor.

[Explanation of symbols]

1...Roof slab 2...Main body shell 3...Primary sodium 4...Heat transfer tube bundle 5a...Upper tube sheet 5b...Lower tube sheet 5c...Opening 6...Bellows 7...Rising pipe
8... Down pipe 9... Biological shield 10... Inlet pipe 11...
Outlet pipe 12...Inlet ring header 13...Outlet ring header 14...PRACS heat transfer section 14a...Downcomer section 14b...Heat transfer tube 14c...Lower connecting pipe
14d...Upper connecting pipe 15...Secondary sodium 16...
PRACS secondary sodium 17...Drain pipe 18
…inner tube

Claims (1)

[Claims] 1. A cylindrical main body shell suspended and supported within the reactor vessel from a roof slab that closes an upper opening of the reactor vessel, a heat transfer section provided on the main body shell, and a primary sodium and 2. The heat transfer passes through an upper tube sheet that partitions a heat exchanger tube bundle portion that exchanges heat with sodium sodium, a lower tube sheet provided below the heat exchanger tube bundle, and a biological shield attached to the upper end of the main body body. A downcomer pipe and a riser pipe through which secondary sodium flows to exchange heat with the tube bundle; a heat transfer tube of the heat transfer section provided outside the riser pipe, a downcomer part, a lower communication pipe, and an upper communication pipe; and the upper communication pipe. and an inlet pipe and an outlet pipe that communicate with the downcomer part via a ring header and allow secondary sodium to flow therethrough, wherein inner pipes are provided in the downcomer part and the lower connecting pipe, and these have a double pipe structure. A core decay heat removal device characterized by: