A nitrogen purging maintenance system and method for the secondary side of a high-temperature gas-cooled reactor steam generator.

Abstract

This invention discloses a nitrogen purging and maintenance system and method for the secondary side of a high-temperature gas-cooled reactor steam generator, including a steam generator, a nitrogen storage tank, a first solenoid valve, an electric regulating valve, a second solenoid valve, a third solenoid valve, a fourth solenoid valve, a booster, a fifth solenoid valve, a sixth solenoid valve, a liquid storage tank, a feedwater discharge valve, and an exhaust valve. This system and method can realize nitrogen purging and maintenance of the secondary side of the steam generator, and has high safety.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power technology and relates to a nitrogen purging and maintenance system and method for the secondary side of a high-temperature gas-cooled reactor steam generator. Background Technology

[0002] The steam generator in a high-temperature gas-cooled reactor (HTGR) serves as a crucial link between the primary and secondary loop equipment in a nuclear power plant. It is characterized by unstable two-phase flow and complex, variable boiling heat transfer. According to historical accident statistics for nuclear power plants, steam generator heat transfer tube rupture accidents are the leading cause of unplanned shutdowns in pressurized water reactor nuclear power plants. Therefore, improving response measures to steam generator heat transfer tube rupture accidents is of paramount importance to the safety and reliability of the entire nuclear power plant.

[0003] During the period from the handover and commissioning of the steam generator to its formal operation, nitrogen purging maintenance measures are used to meet the pressure requirements of the steam generator and ensure that the heat transfer tubes on the secondary side of the steam generator are properly protected and maintained. The secondary side feedwater operating pressure of the high-temperature gas-cooled reactor steam generator currently under construction is 14 MPa, and the nitrogen purging maintenance pressure is 9 MPa. Manual nitrogen purging has at least the following drawbacks: First, the secondary side of the steam generator is located at the -15m level of the nuclear island building, the lowest level of the building, where asphyxiating gases are difficult to disperse, increasing the risk of asphyxiation for personnel in the working area; second, the pressure on the secondary side of the steam generator rises to a maximum of 9 MPa during nitrogen purging, requiring numerous temporary facilities and increasing the number of connection points. Considering that the damage caused by pressure leakage is far greater than that of a hydrostatic test, this increases the risk of high-pressure gas injury to personnel in the working area; third, because the nitrogen purging process requires continuous injection of the medium into the steam generator, and the cleanliness requirements for the heat transfer tubes are very high, this increases the risk of foreign matter being introduced into the steam generator. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a nitrogen purging maintenance system and method for the secondary side of a high-temperature gas-cooled reactor steam generator. This system and method can achieve nitrogen purging maintenance on the secondary side of the steam generator and has high safety.

[0005] To achieve the above objectives, this invention discloses a secondary side nitrogen purging and maintenance system for a high-temperature gas-cooled reactor steam generator, comprising a steam generator, a nitrogen storage tank, a first solenoid valve, an electric regulating valve, a second solenoid valve, a third solenoid valve, a fourth solenoid valve, a booster, a fifth solenoid valve, a sixth solenoid valve, a liquid storage tank, a feedwater discharge valve, and an exhaust valve.

[0006] The secondary heat transfer tube outlet of the steam generator is divided into two paths. One path is connected to the inlet of the exhaust valve, and the other path is connected to the inlet of the sixth solenoid valve. The outlet of the exhaust valve is connected to the atmospheric environment. The outlet of the sixth solenoid valve is connected to the nitrogen storage tank via the second solenoid valve, the electric regulating valve, and the first solenoid valve. The outlet of the nitrogen storage tank is connected to the inlet of the fourth solenoid valve. The outlet of the fourth solenoid valve is connected to the inlet of the booster compressor. The outlet of the booster compressor is connected to the inlet of the fifth solenoid valve. The outlet of the sixth solenoid valve is connected to the inlet of the third solenoid valve. The outlets of the third and fifth solenoid valves are connected together by a pipeline and then divided into two paths. One path is connected to the liquid storage tank via the feedwater discharge valve, and the other path is connected to the inlet of the secondary heat transfer tube of the steam generator.

[0007] The outlet of the exhaust valve is connected to a purity monitor.

[0008] The outlet of the secondary heat transfer tube of the steam generator is connected to a first pressure monitoring instrument.

[0009] The nitrogen storage tank is connected to a second pressure detector.

[0010] The first differential pressure switch is set to the difference between the first pressure monitor and the second pressure detector. After the switch is activated, the booster compressor is started.

[0011] The set value of the second signal switch is the reading of the first pressure monitor. After the switch is activated, the booster compressor stops.

[0012] The setting value of the third differential pressure switch is the difference between the first pressure monitor and the second pressure detector. After the switch is activated, the sixth solenoid valve is closed.

[0013] This invention discloses a method for nitrogen purging and maintenance of the secondary side of a high-temperature gas-cooled reactor steam generator, including depressurization and purging of the secondary side of the steam generator, nitrogen purging of the secondary side of the steam generator, and depressurization of the secondary side of the steam generator.

[0014] The process of depressurization and purging on the secondary side of the steam generator is as follows:

[0015] 11) When the steam generator is taken out of operation for maintenance, pressurize the nitrogen storage tank to high pressure, open the feed water discharge valve and the exhaust valve, discharge the water in the steam-water mixture in the secondary heat transfer tube of the steam generator into the storage tank through the feed water discharge valve, and discharge the steam in the steam-water mixture in the secondary heat transfer tube of the steam generator into the atmosphere through the exhaust valve.

[0016] 12) After the steam-water mixture is discharged, close the water supply discharge valve, open the first solenoid valve, the electric regulating valve, the second solenoid valve and the third solenoid valve, keep the fourth solenoid valve and the fifth solenoid valve closed, and pass the high-pressure nitrogen in the nitrogen storage tank into the heat transfer tube for pipeline purging. When the exhaust purity monitored by the purity monitor meets the requirements, close the exhaust valve.

[0017] The process of nitrogen charging on the secondary side of the steam generator is as follows:

[0018] 21) The secondary side pressure of the steam generator is monitored by the first pressure monitor and the pressure in the nitrogen storage tank is monitored by the second pressure detector. The electric regulating valve is set to automatic and controls the nitrogen charging rate, so that the pressure of the steam generator gradually increases. When the data measured by the first pressure monitor and the second pressure detector are close, the first differential pressure switch is activated and drives the booster to start. At the same time, the fourth and fifth solenoid valves are opened and the first, second and third solenoid valves are closed.

[0019] 22) When the secondary side pressure of the steam generator measured by the first pressure monitor reaches the target nitrogen charging pressure, the second signal switch is activated and the booster is stopped. The fourth and fifth solenoid valves are interlocked and closed, completing the nitrogen charging maintenance.

[0020] The present invention has the following beneficial effects:

[0021] The nitrogen purging and maintenance system and method for the secondary side of a high-temperature gas-cooled reactor steam generator described in this invention ensures gas source quality through pipeline purging before nitrogen purging, while simultaneously achieving remote automatic control during the purging process, effectively reducing the risks associated with manual operation. Specifically, before purging the secondary side of the high-temperature gas-cooled reactor steam generator, high-pressure nitrogen from the nitrogen storage tank is introduced into the heat transfer tubes for pipeline purging until the purity monitor detects that the exhaust purity meets the requirements, preventing the introduction of foreign matter from affecting the cleanliness of the heat transfer tubes on the secondary side of the steam generator. This invention automatically controls the nitrogen purging rate through an electric regulating valve and controls the start and stop of the booster compressor through a pressure switch, simplifying the manual operation process during nitrogen purging. In addition, this invention uses a solenoid valve to realize the switching between two nitrogen purging circuits and the nitrogen purging and depressurization circuits, enabling rapid and flexible switching of operating circuits and maintaining a relatively stable operating pressure. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the present invention.

[0023] Among them, 1 is a steam generator, 2 is a nitrogen storage tank, 3 is a first solenoid valve, 4 is an electric regulating valve, 5 is a second solenoid valve, 6 is a third solenoid valve, 7 is a fourth solenoid valve, 8 is a booster, 9 is a fifth solenoid valve, 10 is a sixth solenoid valve, 11 is a liquid storage tank, 12 is a water supply and discharge valve, 13 is an exhaust valve, 14 is a purity monitor, 15 is a first pressure monitor, 16 is a second pressure detector, 17 is a first differential pressure switch, 18 is a second signal switch, and 19 is a third differential pressure switch. Detailed Implementation

[0024] To enable those skilled in the art to better understand the present invention, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, not all embodiments, and are not intended to limit the scope of the present invention. Furthermore, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion regarding the concepts disclosed in the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0025] The accompanying drawings show structural schematic diagrams according to embodiments disclosed in this invention. These drawings are not drawn to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0026] refer to Figure 1 The high-temperature gas-cooled reactor steam generator secondary side nitrogen charging and maintenance system of the present invention includes a steam generator 1, a nitrogen storage tank 2, a first solenoid valve 3, an electric regulating valve 4, a second solenoid valve 5, a third solenoid valve 6, a fourth solenoid valve 7, a booster 8, a fifth solenoid valve 9, a sixth solenoid valve 10, a liquid storage tank 11, a water supply discharge valve 12, an exhaust valve 13, a purity monitor 14, a first pressure monitor 15, a second pressure detector 16, a first differential pressure switch 17, a second signal switch 18, and a third differential pressure switch 19.

[0027] The outlet of the secondary heat transfer tube of the steam generator 1 is divided into two paths. One path is connected to the inlet of the exhaust valve 13, and the other path is connected to the inlet of the sixth solenoid valve 10. The outlet of the exhaust valve 13 is connected to the atmospheric environment. The outlet of the sixth solenoid valve 10 is connected to the nitrogen storage tank 2 via the second solenoid valve 5, the electric regulating valve 4, and the first solenoid valve 3. The outlet of the nitrogen storage tank 2 is connected to the inlet of the fourth solenoid valve 7. The outlet of the fourth solenoid valve 7 is connected to the inlet of the booster 8. The outlet of the booster 8 is connected to the inlet of the fifth solenoid valve 9. The outlet of the sixth solenoid valve 10 is connected to the inlet of the third solenoid valve 6. The outlets of the third solenoid valve 6 and the fifth solenoid valve 9 are connected to each other through a pipe and then divided into two paths. One path is connected to the liquid storage tank 11 via the water discharge valve 12, and the other path is connected to the inlet of the secondary heat transfer tube of the steam generator 1.

[0028] In this embodiment, the outlet of the exhaust valve 13 is connected to a purity monitor 14 to monitor the exhaust purity; the outlet of the secondary heat transfer tube of the steam generator 1 is connected to a first pressure monitor 15 to monitor the outlet pressure of the steam generator 1; and the nitrogen storage tank 2 is connected to a second pressure detector 16 to monitor the pressure of the nitrogen storage tank 2.

[0029] In this embodiment, the set value of the first differential pressure switch 17 is the difference between the first pressure monitor 15 and the second pressure detector 16. After the switch is activated, the booster 8 is started. The set value of the second signal switch 18 is the reading of the first pressure monitor 15. After the switch is activated, the booster 8 is stopped. The set value of the third differential pressure switch 19 is the difference between the first pressure monitor 15 and the second pressure detector 16. After the switch is activated, the sixth solenoid valve 10 is closed.

[0030] The nitrogen purging maintenance method for the secondary side of a high-temperature gas-cooled reactor steam generator according to the present invention includes the following steps:

[0031] 1) Depressurization and purging of the secondary side of steam generator 1;

[0032] 11) When the steam generator 1 is taken out of operation for maintenance, pressurize the nitrogen storage tank 2 to high pressure, open the water discharge valve 12 and the exhaust valve 13, discharge the water in the steam-water mixture in the secondary heat transfer tube of the steam generator 1 into the storage tank 11 through the water discharge valve 12, and discharge the steam in the steam-water mixture in the secondary heat transfer tube of the steam generator 1 into the atmospheric environment through the exhaust valve 13.

[0033] 12) After the steam-water mixture is discharged, close the water supply discharge valve 12, open the first solenoid valve 3, the electric regulating valve 4, the second solenoid valve 5 and the third solenoid valve 6, keep the fourth solenoid valve 7 and the fifth solenoid valve 9 closed, and pass the high-pressure nitrogen in the nitrogen storage tank 2 into the heat transfer tube for pipeline purging. When the exhaust purity monitored by the purity monitor 14 meets the requirements, close the exhaust valve 13.

[0034] 2) Nitrogen purging on the secondary side of steam generator 1

[0035] 21) The secondary side pressure of the steam generator 1 is monitored by the first pressure monitor 15, and the pressure in the nitrogen storage tank 2 is monitored by the second pressure detector 16. The electric regulating valve 4 is set to automatic and controls the nitrogen charging rate, so that the pressure of the steam generator 1 gradually increases. When the data measured by the first pressure monitor 15 and the second pressure detector 16 are close, the first differential pressure switch 17 is activated and drives the booster 8 to start. At the same time, the fourth solenoid valve 7 and the fifth solenoid valve 9 are opened, and the first solenoid valve 3, the second solenoid valve 5 and the third solenoid valve 6 are closed.

[0036] 22) When the secondary side pressure of the steam generator 1 measured by the first pressure monitor 15 reaches the target nitrogen charging pressure, the second signal switch 18 is activated and the booster 8 is stopped. The fourth solenoid valve 7 and the fifth solenoid valve 9 are interlocked and closed to complete the nitrogen charging maintenance.

[0037] 3) Secondary side pressure relief of steam generator 1

[0038] When steam generator 1 is taken out of maintenance and resumes operation, the pressure in nitrogen storage tank 2 is reduced to a low pressure. The first solenoid valve 3, the second solenoid valve 5, and the sixth solenoid valve 10 are opened, while the third solenoid valve 6, the fourth solenoid valve 7, and the fifth solenoid valve 9 remain closed. Nitrogen in steam generator 1 is recovered into nitrogen storage tank 2. The electric regulating valve 4 is set to automatic and controls the pressure relief rate to meet the design requirements, so that the pressure in steam generator 1 gradually decreases. When the data measured by the first pressure monitor 15 and the second pressure detector 16 are close, the third differential pressure switch 19 is activated and the sixth solenoid valve 10 is closed. The exhaust valve 13 is opened and steam generator 1 is released to atmospheric pressure.

[0039] Example 1

[0040] This embodiment uses a currently under-construction high-temperature gas-cooled reactor (HTGR) nuclear power unit demonstration project as an example. The HTGR steam generator 1 is a vertical, DC-DC helical tube assembly structure. The working fluid inside the secondary side heat transfer tubes of steam generator 1 is a mixture of steam and water from the secondary loop, while the outer shell side contains helium from the primary loop. The primary loop operating pressure is 7 MPa, and the secondary loop operating pressure is 13.9 MPa. During the period from the handover and commissioning of steam generator 1 to its formal operation, nitrogen purging maintenance measures are implemented to ensure that the secondary side of steam generator 1 receives proper protection and maintenance as much as possible. The nitrogen purging maintenance method for HTGR steam generator 1 is as follows:

[0041] 1) Secondary side pressure relief and purging of steam generator 1

[0042] 11) When the steam generator 1 is taken out of operation for maintenance, pressurize the nitrogen storage tank 2 to 14MPa, open the feed water discharge valve 12 and the exhaust valve 13, discharge the water in the steam-water mixture in the secondary heat transfer tube of the steam generator 1 into the liquid storage tank 11 through the feed water discharge valve 12, and discharge the steam in the steam-water mixture in the secondary heat transfer tube to the atmosphere through the exhaust valve 13.

[0043] 22) After the steam-water mixture is discharged, close the water supply discharge valve 12, open the first solenoid valve 3, the electric regulating valve 4, the second solenoid valve 5 and the third solenoid valve 6, keep the fourth solenoid valve 7 and the fifth solenoid valve 9 closed, and pass the high-pressure nitrogen in the nitrogen storage tank 2 into the heat transfer tube for pipeline purging. Control the purging pressure to be about 1 MPa. When the purity monitor 14 detects that the nitrogen purity in the exhaust gas is >95%, close the exhaust valve 13.

[0044] 2) Nitrogen purging on the secondary side of steam generator 1

[0045] 21) The secondary side pressure of the steam generator 1 is monitored by the first pressure monitoring instrument 15, and the pressure in the nitrogen storage tank 2 is monitored by the second pressure detection instrument 16. The electric regulating valve 4 is put into automatic mode and controls the nitrogen charging rate so that the pressure increase rate of the steam generator 1 is ≤0.15MPa / min.

[0046] 22) When the pressure displayed by the first pressure monitor 15 reaches 5MPa, a pressure holding test is performed for 30 minutes, and all interface locations are checked for leaks and leaks are repaired.

[0047] 23) During the pressurization process, when the readings of the first pressure monitor 15 and the second pressure detector 16 are close (≤0.1MPa), the first differential pressure switch 17 is activated and drives the booster 8 to start. At the same time, the fourth solenoid valve 7 and the fifth solenoid valve 9 are interlocked and opened, and the first solenoid valve 3, the second solenoid valve 5 and the third solenoid valve 6 are quickly closed.

[0048] 24) When the first pressure monitor 15 detects that the secondary side pressure of the steam generator 1 reaches the target nitrogen charging pressure of 9.0 MPa, the second signal switch 18 is activated and the booster 8 is stopped. The fourth solenoid valve 7 and the fifth solenoid valve 9 are interlocked and closed. A 30-minute pressure holding test is performed. After checking for leaks at all interface locations and repairing any leaks, the nitrogen charging maintenance is completed.

[0049] 3) Secondary side pressure relief of steam generator 1

[0050] When steam generator 1 is taken out of maintenance and resumes operation, the pressure in nitrogen storage tank 2 is reduced to below 1 MPa. The first solenoid valve 3, the second solenoid valve 5, and the sixth solenoid valve 10 are opened, while the third solenoid valve 6, the fourth solenoid valve 7, and the fifth solenoid valve 9 remain closed. Nitrogen in steam generator 1 is recovered into nitrogen storage tank 2. The electric regulating valve 4 is set to automatic and the pressure relief rate is controlled to ≤0.3 MPa / min, so that the pressure in steam generator 1 is reduced slowly. When the readings of the first pressure monitor 15 and the second pressure detector 16 are close (≤0.1 MPa), the third differential pressure switch 19 is activated and the sixth solenoid valve 10 is closed. The exhaust valve 13 is opened and steam generator 1 is released to atmospheric pressure of 0.1 MPa.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator, characterized in that, Includes a steam generator (1), a nitrogen storage tank (2), a first solenoid valve (3), an electric regulating valve (4), a second solenoid valve (5), a third solenoid valve (6), a fourth solenoid valve (7), a booster (8), a fifth solenoid valve (9), a sixth solenoid valve (10), a liquid storage tank (11), a water supply and discharge valve (12), and an exhaust valve (13); The secondary heat transfer tube outlet of the steam generator (1) is divided into two paths. One path is connected to the inlet of the exhaust valve (13), and the other path is connected to the inlet of the sixth solenoid valve (10). The outlet of the exhaust valve (13) is connected to the atmospheric environment. The outlet of the sixth solenoid valve (10) is connected to the nitrogen storage tank (2) via the second solenoid valve (5), the electric regulating valve (4), and the first solenoid valve (3). The outlet of the nitrogen storage tank (2) is connected to the inlet of the fourth solenoid valve (7). The outlet of the solenoid valve (7) is connected to the inlet of the booster (8), the outlet of the booster (8) is connected to the inlet of the fifth solenoid valve (9), the outlet of the sixth solenoid valve (10) is connected to the inlet of the third solenoid valve (6), and the outlet of the third solenoid valve (6) and the outlet of the fifth solenoid valve (9) are connected to each other through a pipeline and then split into two paths. One path is connected to the liquid storage tank (11) through the water discharge valve (12), and the other path is connected to the inlet of the secondary heat transfer tube of the steam generator (1).

2. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The outlet of the exhaust valve (13) is connected to a purity monitor (14).

3. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The outlet of the secondary heat transfer tube of the steam generator (1) is connected to a first pressure monitoring instrument (15).

4. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The nitrogen storage tank (2) is connected to a second pressure detector (16).

5. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The first differential pressure switch (17) is set to the difference between the first pressure monitor (15) and the second pressure detector (16). After the switch is activated, the booster (8) is started.

6. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The setting value of the second signal switch (18) is the reading of the first pressure monitor (15), and the booster (8) stops after the switch is activated.

7. The nitrogen purging and maintenance system for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 1, characterized in that, The setting value of the third differential pressure switch (19) is the difference between the first pressure monitor (15) and the second pressure detector (16). After the switch is activated, the sixth solenoid valve (10) is closed.

8. A method for nitrogen purging maintenance of the secondary side of a high-temperature gas-cooled reactor steam generator, characterized in that, The high-temperature gas-cooled reactor steam generator secondary side nitrogen charging and maintenance system according to claim 1 includes secondary side depressurization and purging of steam generator (1), secondary side nitrogen charging of steam generator (1), and secondary side depressurization of steam generator (1).

9. The nitrogen purging maintenance method for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 8, characterized in that, The process of depressurization and purging on the secondary side of the steam generator (1) is as follows: 11) When the steam generator (1) is taken out of operation and maintenance begins, pressurize the nitrogen storage tank (2) to high pressure, open the water discharge valve (12) and the exhaust valve (13), discharge the water in the steam-water mixture in the secondary heat transfer tube of the steam generator (1) into the storage tank (11) through the water discharge valve (12), and discharge the steam in the steam-water mixture in the secondary heat transfer tube of the steam generator (1) into the atmospheric environment through the exhaust valve (13); 12) After the steam-water mixture is discharged, close the water supply discharge valve (12), open the first solenoid valve (3), the electric regulating valve (4), the second solenoid valve (5) and the third solenoid valve (6), keep the fourth solenoid valve (7) and the fifth solenoid valve (9) closed, and pass the high-pressure nitrogen in the nitrogen storage tank (2) into the heat transfer tube for pipeline purging. When the exhaust purity monitored by the purity monitor (14) meets the requirements, close the exhaust valve (13).

10. The nitrogen purging maintenance method for the secondary side of a high-temperature gas-cooled reactor steam generator according to claim 8, characterized in that, The process of nitrogen charging on the secondary side of the steam generator (1) is as follows: 21) The secondary side pressure of the steam generator (1) is monitored by the first pressure monitor (15), and the pressure in the nitrogen storage tank (2) is monitored by the second pressure detector (16). The electric regulating valve (4) is set to automatic and controls the nitrogen charging rate, so that the pressure of the steam generator (1) gradually increases. When the data measured by the first pressure monitor (15) and the second pressure detector (16) are close, the first differential pressure switch (17) is activated and drives the booster (8) to start. At the same time, the fourth solenoid valve (7) and the fifth solenoid valve (9) are opened, and the first solenoid valve (3), the second solenoid valve (5) and the third solenoid valve (6) are closed. 22) When the secondary side pressure of the steam generator (1) measured by the first pressure monitor (15) reaches the target nitrogen charging pressure, the second signal switch (18) is activated and the booster (8) is stopped. The fourth solenoid valve (7) and the fifth solenoid valve (9) are interlocked and closed to complete the nitrogen charging maintenance.

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