CN111256104A - Purification system and method for high-temperature gas cooled reactor nuclear power unit thermal equipment during starting period - Google Patents

Abstract

The invention relates to a water chemical working condition of a high-temperature gas cooled reactor nuclear power plant, in particular to a purification system and a purification method during the starting period of thermal equipment of a high-temperature gas cooled reactor nuclear power unit. The purification system of the present invention includes a purification loop during start-up; the thermal equipment comprises a steam generator and a condenser; the steam outlet of the steam generator is connected with the input end of a condenser through a steam turbine set, and the output end of the condenser is connected with the main water supply end of the steam generator through a water supply loop; a deaerator is arranged on the water supply loop; the purification loop comprises a primary door connected to the main water supply end of the steam generator during starting and three pipelines arranged behind the primary door; the three pipelines comprise a discharge pipeline, a first water return pipeline and a second water return pipeline; the tail end of the discharge pipeline is connected to a unit flash tank or a heat exchanger, the first water return pipeline is connected to the output end of the condenser, and the second water return pipeline is connected to the inlet of the deaerator. The invention ensures the quality of the main feed water entering the steam generator during the cold-state starting and the hot-state starting of the unit, and greatly reduces the corrosion and scaling risks.

Description

System and method for decontamination during startup of thermal equipment of high temperature gas-cooled reactor nuclear power plant

technical field

The invention relates to a water chemical working condition of a high-temperature gas-cooled reactor nuclear power plant, in particular to a purification system and method during startup of thermal equipment of a high-temperature gas-cooled reactor nuclear power unit.

Background technique

The characteristics of the steam generator of the high temperature gas-cooled reactor nuclear power unit: (1) the main water supply pipe side (secondary side), the helium gas shell side (primary side), compared with the pressurized water reactor nuclear power unit (pressurized water reactor cooling It is easier to deposit corrosion products and other impurities in the heat exchange tube of the steam generator of the high temperature gas-cooled reactor nuclear power unit, and it is also more prone to fouling and blocking, which will affect the safe operation of the unit and affect the heat transfer efficiency. (2) The same point as most PWR nuclear power units is that the steam generator material contains nickel-based alloys, which are sensitive to oxygen corrosion. When the unit starts and enters the hot flushing process, the dissolved oxygen content entering the steam generator is required. Not more than 100μg/L. After the unit is connected to the grid with load, the dissolved oxygen in the main feed water entering the steam generator is required to be not more than 3μg/L. (3) The high temperature gas-cooled reactor nuclear power unit has the characteristics of a once-through boiler. The feed water in the evaporator changes from water to saturated steam until it becomes superheated steam, and directly enters the steam turbine to do work. No blowdown, no superheater. (4) The inner diameter of the steam generator pipe is small, and corrosion or the deposition of corrosion products on the pipe wall will seriously affect the efficiency of the unit and increase the power consumption of the feed pump. (5) The steam generator has a special structure. When the leakage of the heat exchange tube occurs, the tube cannot be replaced or repaired, and the tube can only be stopped and blocked. (6) The unit parameters are high, and the steam parameters at the steam generator outlet are 13.90MPa and 571°C; the startup process of the high temperature gas-cooled reactor nuclear power unit is short, about 585 minutes for cold start and 511 minutes for warm start.

Therefore, strictly controlling the content of impurities such as dissolved oxygen and corrosion products entering the main feed water of the steam generator is one of the important tasks to ensure the safe and economical operation of high temperature gas-cooled reactor nuclear power plants.

In the technical field of water chemistry working conditions in nuclear power plants, the risk of corrosion and scaling of steam generators should be reduced from three stages of normal operation, decommissioning and maintenance, and start-up of the unit. Among them, the purification of thermal equipment during the start-up of the unit is a very important step. Before the main feed water enters the steam generator, it should be supervised, and if it is not qualified, it should be recycled or discharged. However, in the current thermal equipment, there is no return pipe section or discharge pipe section between the outlet of the high pressure heater and the inlet of the steam generator, so it should be technically upgraded.

SUMMARY OF THE INVENTION

In view of the problems existing in the prior art, the present invention provides a high-temperature air-cooling system that ensures the quality of the main feed water entering the heat exchange tube of the steam generator during the start-up process of the unit, and minimizes the risk of corrosion and scaling of the heat exchange tube of the steam generator. A system and method for decontamination during startup of a nuclear power plant thermal plant.

The present invention is achieved through the following technical solutions:

Purification system during start-up of thermal equipment of high temperature gas-cooled nuclear power plant, including purification circuit during start-up;

The thermal equipment includes a steam generator and a condenser; the steam outlet of the steam generator is connected to the input end of the condenser through the steam turbine unit, and the output end of the condenser is connected to the main water supply end of the steam generator through a water supply circuit; water supply A deaerator is set on the circuit;

The purification circuit during startup includes a primary door connected to the main water supply end of the steam generator, and a three-way pipeline arranged behind the primary door; the three-way pipeline includes a discharge pipeline, a first return water pipeline and a second return water pipeline; discharge The end of the pipeline is connected to the expansion vessel or heat exchanger of the unit, the first return water pipeline is connected to the output end of the condenser, and the second return water pipeline is connected to the inlet of the deaerator.

Further, the first return water pipeline is connected with the output end of the condenser through the temperature reducing and pressure reducing device and the connecting pipeline arranged in sequence.

Further, the first water return pipe is made of alloy steel or stainless steel containing more than 0.15% chromium, the thickness of the pipe wall is greater than 20mm, and the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator.

Further, the connecting pipeline is made of alloy steel or stainless steel with a chromium content of more than 0.15%, the wall thickness is 10 mm, and the inner diameter of the pipeline is not less than the inner diameter of the first return water pipeline.

Further, the second water return pipe is made of alloy steel or stainless steel with a chromium content of more than 0.15%, the thickness of the pipe wall is greater than 20mm, and the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator.

Further, the discharge pipe is made of alloy steel or stainless steel containing more than 0.15% chromium, the thickness of the pipe wall is greater than 20mm, and the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator.

Further, the three-way pipelines are connected by a three-way connection or are connected by three independent pipelines, and the three-way pipelines are respectively provided with secondary doors.

A method of decontamination during startup of thermal equipment of a high temperature gas-cooled reactor nuclear power plant, including,

During the start-up of the unit, during cold flushing, before the main feed water enters the steam generator, if the water quality of the main feed water does not meet the requirements, the main feed water inlet pipe of the steam generator is directly returned to the unit expansion vessel/heat exchanger through the discharge pipe, or through the discharge pipe. The first return water pipeline is recycled to the condenser for circular purification;

During hot flushing, before the main feed water enters the steam generator, if only the dissolved oxygen content of the main feed water is unqualified, it will be recycled from the main feed water inlet pipe of the steam generator to the inlet of the deaerator through the second return water pipeline for circulating deoxygenation to dissolved oxygen. After the content meets the requirements, it enters the steam generator; if the content of iron, silicon, sodium and dissolved oxygen in the main feed water is unqualified, according to the size of the content, choose to discharge directly from the main feed water inlet pipeline of the steam generator to the unit expansion container/heat exchange through the discharge pipeline It is recycled to the condenser through the first recovery pipeline, and purified by the condensate polishing device connected to the output end of the condenser.

Further, during cold flushing,

When the iron content of the feed water sample at the inlet sampling point of the steam generator is greater than 200μg/L, it shall be discharged to the expansion vessel or heat exchanger of the unit through the discharge pipeline;

When the iron content of the feed water sample at the inlet sampling point of the steam generator is between 100 μg/L and 200 μg/L, it is recovered to the condenser through the first return water pipeline to continue purification treatment;

When the iron content of the feed water sample at the inlet sampling point of the steam generator is less than 100 μg/L, enter the steam generator for cold flushing.

Further, during hot flushing,

When the iron content of the feed water sample at the inlet sampling point of the steam generator is greater than 200μg/L, it shall be temporarily discharged through the discharge pipeline;

When the iron content of the feed water sample at the inlet sampling point of the steam generator is between 50 μg/L and 200 μg/L, it will be recovered to the condenser through the first return water pipeline to continue purification treatment;

When the main feed water iron is less than 50μg/L and the dissolved oxygen content exceeds 100μg/L, it is recycled to the deaerator through the second return water pipeline, and the cyclic deoxidation treatment of chemical deoxidization and thermal deoxidation is carried out, so that the main water entering the steam generator The iron content of feed water is less than 50μg/L, and the dissolved oxygen content is less than 100μg/L.

Compared with the prior art, the present invention has the following beneficial technical effects:

The invention can effectively control the water quality when the unit is flushed in cold or hot state by adding the discharge pipe from the main feed water inlet pipe of the steam generator to the expansion vessel or heat exchanger of the unit, prevent unqualified water from entering the steam generator, and reduce steam The risk of corrosion and scaling of the generator; by adding the first return water pipe from the main feed water inlet pipe of the steam generator to the condenser, it can ensure that the unqualified main feed water is cleaned during the cold and hot flushing process of the unit startup. Cyclic purification treatment can not only ensure water quality, reduce the risk of corrosion and scaling of the steam generator, but also save water resources; by adding a second return water pipe from the main feed water inlet pipe of the steam generator to the inlet pipe of the deaerator, the Ensure that the dissolved oxygen in the main feed water entering the steam generator meets the requirements under various startup conditions, reducing the risk of corrosion and scaling of the steam generator, thereby improving the flushing quality of the unit in cold and hot states and ensuring that the unit is in a hot state. After flushing and circulating deoxygenation, the feedwater entering the steam generator is anaerobic or hypoxic (dissolved oxygen content is less than 20μg/L), iron content in the feedwater is less than 20μg/L, and other feedwater indicators conform to the feedwater quality during the operation of the high temperature gas-cooled reactor unit , which can minimize the risk of corrosion and scaling of the steam generator caused by the lower water quality in the initial stage of reactor startup than the operating water quality.

Preferably, the pressure and temperature of the return water can be adjusted to the required parameters by arranging a desuperheater and pressure reducer with the function of cooling and depressurizing on the first return pipe, and effectively keeping the pressure and temperature at the outlet of the pipe within a certain value. within the range.

Preferably, by using alloy steel or stainless steel with a chromium content of more than 0.15%, and the thickness of the pipe wall is greater than 20mm, the inner diameter of the discharge pipe, the first return pipe and the second return pipe is not less than the inner diameter of the main pipe connecting the steam generator. , reduce the pressure of the return pipe and reduce the corrosion of the return pipe to minimize the corrosion products entering the thermal system.

Preferably, by using alloy steel or stainless steel with a chromium content of more than 0.15%, the thickness of the pipe wall is 10mm, and the inner diameter of the connecting pipe is not less than the inner diameter of the first return pipe, so as to reduce the pressure of the return pipe and reduce the corrosion of the return pipe In order to minimize the corrosion products entering the thermal system.

The method of the invention has the advantages of simple design, easy operation, flexible pipeline connection, and can ensure that the iron content of the main feed water entering the steam generator is reduced to 50 μg/L or less during the hot flushing process, and the dissolved oxygen content is reduced To below 100μg/L, and can continue to meet higher feed water quality requirements, it can greatly reduce the risk of steam generator corrosion and scaling, which is of great significance to the safe and economic operation of high-temperature gas-cooled reactor nuclear power units.

Description of drawings

FIG. 1 is a schematic flow chart of the purification system according to the present invention.

In the figure: condenser 1, condensate pump 2, condensate polishing device 3, No. 3 low-pressure heater 4, No. 2 low-pressure heater 5, No. 1 low-pressure heater 6, deaerator 7, feed pump 8, high pressure Heater 9, steam generator 10, high pressure cylinder 11, low pressure cylinder 12, hydrazine addition point at the condensate polishing outlet 13, hydrazine addition point at the condensate polishing outlet 14, hydrazine dosing point in the water pipe of the deaerator, 15, Ammonia addition point 16 in the water pipe of the deaerator, hydrazine addition point at the outlet of the high pressure heater 17, steam generator inlet sampling point 18, helium blower 19, nuclear reactor 20, unit expansion vessel or heat exchanger 21, desuperheater 22 , The first return water pipeline 23 , the second return water pipeline 24 , the steam-water separator 25 , the radioactivity monitoring pool 26 , the discharge pipeline 27 , and the connecting pipeline 28 .

Detailed ways

The present invention will be further described in detail below in conjunction with specific embodiments, which are to explain rather than limit the present invention.

As shown in Figure 1, the secondary circuit system of a high temperature gas-cooled reactor nuclear power plant includes a condenser 1, a condensate pump 2, a condensate polishing device 3, a No. 3 low-pressure heater 4, a No. 2 low-pressure heater 5, a No. 6 low-pressure heater, deaerator 7, feed water pump 8, high-pressure heater 9, steam generator 10, high-pressure cylinder 11 and low-pressure cylinder 12, the primary loop is the nuclear reactor 20, helium blower 19 and steam generator 10 connected in sequence . As shown in FIG. 1 , the purification system during the startup of the thermal equipment of the high-temperature gas-cooled nuclear power plant includes a drainage pipe 27 arranged from the inlet pipe of the steam generator 10 to the expansion vessel or heat exchanger 21 of the unit, and a second drainage pipe 27 to the deaerator 7 The return water pipe 24 is to the first return water pipe 23 of the condenser 1 .

As a preferred embodiment of the present invention, the drainage pipe 27 from the inlet pipe of the steam generator 10 to the expansion vessel of the unit or the heat exchanger 21 should be alloy steel or stainless steel containing more than 0.15% chromium, and the thickness of the pipe wall should be greater than 20mm. The inner diameter is not smaller than the inner diameter of the main pipe connecting the steam generator 10 .

As a preferred embodiment of the present invention, the second return water pipe 24 from the inlet pipe of the steam generator 10 to the inlet of the deaerator 7 should be alloy steel or stainless steel containing more than 0.15% chromium, and the thickness of the pipe wall should be greater than 20mm. The inner diameter is not smaller than the inner diameter of the main pipe connecting the steam generator 10 .

As a preferred embodiment of the present invention, a desuperheating and pressure reducing device 22 should be provided on the inlet pipe of the steam generator 10 to the first return water pipe 23 of the condenser 1; The water pipe 23 should be alloy steel or stainless steel with a chromium content of more than 0.15%, the thickness of the pipe wall should be greater than 20mm, and the inner diameter of the pipe should not be less than the inner diameter of the main pipe connecting the steam generator 10; the connecting pipe located after the outlet of the desuperheater 22 28 should be alloy steel or stainless steel containing more than 0.15% chromium, the wall thickness should be about 10mm, and the inner diameter of the pipe should not be less than the inner diameter of the first return pipe 23.

As a preferred embodiment of the present invention, the inlet of the steam generator 10 is connected to the drainage pipe 27 of the expansion vessel of the unit or the heat exchanger 21 , to the second water return pipe 24 of the inlet of the deaerator 7 , to the second water return pipe 24 of the condenser 1 . The connection mode of the first return water pipeline 23 is the three-way connection mode or the respective independent pipeline connection mode.

The specific workflow is as follows:

1) During the cold start of the unit, the cold flushing process is as follows:

(1) The demineralized water is added to the condenser 1, and the condensed water adding ammonia pump is started to increase the pH of the system flushing water to 9.5-10.0; the condensed water pump 2 is started to establish recirculation. When the iron content at the outlet of the condensate pump 2 is greater than 400 μg/L, it is discharged from the outlet of the No. 1 low-pressure heater 6, and at the same time, water is supplied to the condenser 1 to maintain the water level and circulation.

(2) When the iron content of the effluent from the condensate pump 2 is less than 400 μg/L, water can be supplied to the deaerator 7 . After the water level of the deaerator 7 is normal, start the recirculation pump and cycle flushing; it is advisable to put into the auxiliary steam heating system at this time. A temporary discharge pipe is connected to the water discharge pipe from deaerator 7 to condenser 1, and the sewage with iron content greater than 200μg/L is discharged to the wastewater storage tank. After passing the test, it is recycled to condenser 1. At this time, it is put into condensed water for polishing treatment. Device 3, for water purification.

(3) When the iron content at the outlet of the deaerator 7 is less than 200 μg/L, the feed water pump 8 can be started to feed water to the high pressure heater 9 . When the iron content of the feed water sample at the inlet sampling point 18 of the steam generator 10 is greater than 200 μg/L, it should be discharged through the discharge pipe 27; When it is between L, it is recovered to the condenser 1 through the first return water pipeline 23 to continue the purification treatment; when the iron content of the feed water sample at the inlet sampling point 18 of the steam generator 10 is less than 100 μg/L, it can enter the steam generator 10 to carry out Cold rinse.

(4) Before the steam generator 10 enters the water, start adding hydrazine at the hydrazine dosing point 15 at the outlet of the deaerator 7 until the hydrazine concentration in the feed water is 200 μg/L to 500 μg/L. The water from the steam generator 10 enters the steam-water separator 25 of the start-stop system, and the unqualified flushing water is discharged from the drain pipe of the steam-water separator 25 to the discharge pipe of the radioactivity monitoring pool 26; when the iron content of the steam-water separator 25 is less than 50 μg/L , recycled to the condenser 1 through the steam-water separator 25 drain pipe to realize circulating flushing.

(5) When the iron content of the hydrophobic drainage of the steam-water separator 25 is less than 30 μg/L, the silica is less than 30 μg/L, and the hydrogen conductivity is less than 0.5 μS/cm, the cold flushing is completed.

2) During the cold start of the unit, the hot flushing process is as follows:

(1) After the cold flushing is qualified, set up condenser 1 → condensate pump 2 → No. 3 low-pressure heater 4 → No. 2 low-pressure heater 5 → No. 1 low-pressure heater 6 → Deaerator 7 → Condenser 1 In the first water cycle flushing process, the water discharged from the deaerator 7 is recycled to the condenser 1 through the existing pipeline. Start the auxiliary boiler, and use the auxiliary boiler steam to heat the deaerator 7 to about 170°C for hot flushing and thermal deaeration. When the iron content of the water from the deaerator 7 is greater than 100 μg/L, it should be discharged from the discharge pipe of the recovered water from the condenser 1.

(2) When the iron content of the effluent from the deaerator 7 is less than 100 μg/L, the water discharged from the deaerator 7 is recycled to the condenser 1 through the existing pipeline, and starts to flush into the high-pressure water supply system. At the same time, start the hydrazine dosing pump, and add hydrazine to the hydrazine dosing point 15 of the water pipe of the deaerator 7, until the hydrazine content in the feed water is between 200 μg/L and 500 μg/L, and carry out chemical deoxygenation.

(3) Maintenance condenser 1 → Condensate pump 2 → No. 3 low pressure heater 4 → No. 2 low pressure heater 5 → No. 1 low pressure heater 6 → Deaerator 7 → Feed water pump 8 → High pressure heater 9 → Condensate The second water cycle flushing process of the condenser 1 and start the boiler to heat steam, the outlet of the high pressure heater 9 is connected to the condenser 1 through the first return water pipe 23 , the desuperheater and pressure reducer 22 and the connecting pipe 28 . When the iron content of the feed water sample at the inlet sampling point 18 of the steam generator 10 is greater than 200 μg/L, it should be temporarily discharged through the discharge pipe 27; when the iron content of the feed water sample at the inlet sampling point 18 of the steam generator 10 is 50 μg/L～ When it is between 200μg/L, it should be recovered through the first return water pipeline 23 to the condenser 1 to continue purification treatment; when the main feed water iron is less than 50μg/L and the dissolved oxygen content exceeds 100μg/L, it should be recovered through the second return water pipeline 24. To the deaerator 7, the cyclic deoxidation treatment of chemical deoxidation and thermal deoxidation is performed to ensure that the iron content of the main feed water entering the steam generator 10 is less than 50 μg/L, and the dissolved oxygen content is less than 100 μg/L. When the iron content of the feed water sample at the inlet sampling point 18 of the steam generator 10 is less than 50 μg/L and the dissolved oxygen content is less than 100 μg/L, the water can be fed into the steam generator 10 and the start-stop reactor (steam-water separator 25 ) system for heating state water rinse. The flushing effect is improved by adjusting and changing the flushing flow through the recirculation of the feed pump 8 .

(4) The water from the steam generator 10 enters the steam-water separator 25 of the start-stop system, and the unqualified flushing water is discharged from the drain pipe of the steam-water separator 25 to the discharge pipe of the radioactivity monitoring pool 26; when the iron content of the steam-water separator 25 is less than At 30 μg/L, it is recovered to the condenser 1 through the drain pipe of the steam-water separator 25 to realize circulating flushing.

(5) Set up condenser 1 → condensate pump 2 → No. 3 low-pressure heater 4 → No. 2 low-pressure heater 5 → No. 1 low-pressure heater 6 → Deaerator 7 → Feed water pump 8 → High pressure heater 9 → Steam generation Condenser 10→start steam-water separator 25→condenser 1 large cycle, carry out hot cycle flushing, use condensate polishing device 3 to purify the water quality of the equipment, make the feed water quality meet the unit startup requirements, and complete the unit hot flushing.

(6) During the flushing process, the pH value of the feed water should be adjusted by adding ammonia at the ammonia adding point 14 at the condensate polishing outlet and at the ammonia adding point 16 in the lower water pipe of the deaerator to maintain the pH value of the flushing water between 9.5 and 10.0. Add hydrazine to the flushing water through the hydrazine dosing point 15 of the water pipe of the deaerator 7 to maintain 200 μg/L to 500 μg/L of hydrazine in the flushing water to ensure that the dissolved oxygen content of the main feed water entering the steam generator 10 is less than 100 μg /L.

The purification method of the present invention during the start-up of the thermal equipment of the high-temperature gas-cooled reactor nuclear power unit is shown in FIG. 1 . When the unit is started in a cold state, during the cold-state flushing process of the unit, when the iron content of the feed water at the inlet of the steam generator 10 is greater than 200 μg/L, it should be It is temporarily discharged through the drainage pipe 27 to the expansion vessel of the unit or the heat exchanger 21; when the iron content of the feed water at the inlet of the steam generator 10 is between 100 μg/L and 200 μg/L, it is recovered to the condensate through the first return water pipe 23 Continue the purification treatment in the steam generator 1; when the iron content of the feed water at the inlet of the steam generator 10 is less than 100 μg/L, it can enter the steam generator 10 for cold flushing;

During the hot flushing process of the unit, when the iron content of the feed water at the inlet of the steam generator 10 is greater than 200 μg/L, it should be temporarily discharged through the drainage pipe 27 to the expansion vessel of the unit or the heat exchanger 21; when the iron content of the feed water at the inlet of the steam generator 10 When it is between 50 μg/L and 200 μg/L, it is recycled to the condenser 1 through the first return water pipeline 23 to continue purification treatment; when the iron in the main feed water is less than 50 μg/L and the dissolved oxygen content exceeds 100 μg/L, it should pass through the second The return water pipeline 24 is recovered to the inlet of the deaerator 7, and the cyclic deoxidation treatment of chemical deoxidation and thermal deoxidation is carried out to ensure that the iron content of the main feed water entering the steam generator 10 is less than 50 μg/L, and the dissolved oxygen content is less than 100 μg/L.

The system and method for purifying the thermal equipment of the high temperature gas-cooled reactor nuclear power unit of the present invention during startup, by adding the inlet of the steam generator 10 to the drainage pipe 27 of the expansion vessel or heat exchanger 21 of the unit, and the second return water pipe to the inlet of the deaerator 7 24. The first return water pipeline 23 to the condenser 1 is a three-way pipeline. The first return water pipeline 23 includes a desuperheater and pressure reducer 22 to ensure that the unit enters the steam generator 10 during cold start and hot start. The quality of the main feed water greatly reduces the risk of corrosion and scaling of the steam generator 10 .

Claims (10)

1. A purification system during startup of thermal equipment of a high-temperature gas-cooled reactor nuclear power plant, characterized in that it includes a cleaning circuit during startup; The thermal equipment includes a steam generator (10) and a condenser (1); the steam outlet of the steam generator (10) is connected to the input end of the condenser (1) through the steam turbine unit, and the The output end is connected to the main water supply end of the steam generator (10) through the water supply circuit; a deaerator (7) is arranged on the water supply circuit; The purification circuit during the start-up includes a primary door connected to the main water supply end of the steam generator (10), and a three-way pipeline arranged behind the primary door; the three-way pipeline includes a discharge pipeline (27), a first return water pipeline (23) ) and the second water return pipe (24); the end of the discharge pipe (27) is connected to the expansion vessel of the unit or the heat exchanger (21), the first return pipe (23) is connected to the output end of the condenser (1), and the first water return pipe (23) is connected to the output end of the condenser (1). The secondary return water pipeline (24) is connected to the inlet of the deaerator (7). 2. The purification system during startup of the thermal equipment of a high temperature gas-cooled reactor nuclear power unit according to claim 1, characterized in that, the first water return pipe (23) is connected to the desuperheater (22) and the connection The pipe (28) is connected to the output of the condenser (1). 3. The purification system during startup of the thermal equipment of a high-temperature gas-cooled reactor nuclear power plant according to claim 1 or 2, characterized in that the first return water pipeline (23) is made of alloy steel or stainless steel containing more than 0.15% of chromium, The thickness of the pipe wall is greater than 20mm, and the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator (10). 4. The purification system during startup of the thermal equipment of a high temperature gas-cooled reactor nuclear power unit according to claim 2, wherein the connecting pipe (28) is made of alloy steel or stainless steel containing more than 0.15% of chromium, and the wall thickness is 10 mm, The inner diameter of the pipe is not less than the inner diameter of the first return water pipe (23). 5. The purification system according to claim 1, characterized in that, the second water return pipe (24) is made of alloy steel or stainless steel containing more than 0.15% of chromium, and the pipe The wall thickness is greater than 20mm, and the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator (10). 6. The purification system during startup of the thermal equipment of a high temperature gas-cooled reactor nuclear power unit according to claim 1, wherein the discharge pipe (27) is made of alloy steel or stainless steel with a chromium content greater than 0.15%, and the pipe wall thickness is greater than 20mm , the inner diameter of the pipe is not less than the inner diameter of the main pipe connecting the steam generator (10). 7. The purification system during startup of the thermal equipment of a high temperature gas-cooled reactor nuclear power plant according to claim 1, wherein the three-way pipeline is connected by a three-way connection or is connected by three independent pipelines, and the three-way pipeline is connected by three independent pipelines. There are secondary gates respectively on the top. 8. A purification method during startup of thermal equipment of a high temperature gas-cooled reactor nuclear power plant, characterized in that, based on the purification system according to any one of claims 1 to 7, comprising: During the start-up process of the unit, before the main feed water enters the steam generator (10) during cold flushing, if the water quality of the main feed water does not meet the requirements, the main feed water inlet pipe of the steam generator (10) is directly returned to the unit through the discharge pipe (27). The expansion vessel/heat exchanger (21), or recycled to the condenser (1) for recycling and purification through the first return water pipeline (23); During hot flushing, before the main feed water enters the steam generator (10), if only the dissolved oxygen content of the main feed water is unqualified, it is recovered from the main feed water inlet pipe of the steam generator (10) through the second return water pipe (24) to the deaerator. After the inlet of the device (7) is deoxygenated until the dissolved oxygen content meets the requirements, it enters the steam generator (10). ) from the main feed water inlet pipe of the steam generator (10) is directly discharged to the unit expansion vessel/heat exchanger (21) or recovered to the condenser (1) through the first recovery pipe (23). 1) The condensate polishing device (3) at the output end performs purification treatment. 9 . The purification method during startup of the thermal equipment of a high-temperature gas-cooled reactor nuclear power unit according to claim 8 , wherein, during cold flushing, When the iron content of the feed water sample at the inlet sampling point (18) of the steam generator (10) is greater than 200 μg/L, it is discharged to the unit expansion vessel or the heat exchanger (21) through the discharge pipe (27); When the iron content of the feed water sample at the inlet sampling point (18) of the steam generator (10) is between 100 μg/L and 200 μg/L, it is recovered to the condenser (1) through the first return water pipe (23) to continue purification deal with; When the iron content of the feed water sample at the inlet sampling point (18) of the steam generator (10) is less than 100 μg/L, it enters the steam generator (10) for cold flushing. 10 . The purification method during startup of the thermal equipment of a high temperature gas-cooled reactor nuclear power unit according to claim 8 , wherein, during hot flushing, When the iron content of the feed water sample at the inlet sampling point (18) of the steam generator (10) is greater than 200 μg/L, it is temporarily discharged through the discharge pipe (27); When the iron content of the feed water sample at the inlet sampling point (18) of the steam generator (10) is between 50 μg/L and 200 μg/L, it is recovered to the condenser (1) through the first return water pipe (23) to continue purification deal with; When the main feed water iron is less than 50 μg/L and the dissolved oxygen content exceeds 100 μg/L, it is recovered to the deaerator (7) through the second return water pipeline (24), and the cyclic deoxidation treatment of chemical deoxidation and thermal deoxidation is carried out, so that the The iron content of the main feed water entering the steam generator (10) is less than 50 μg/L, and the dissolved oxygen content is less than 100 μg/L.

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