Strategy of startup and deaeration after major repair of AP1000 nuclear power unit primary loop

Abstract

This invention provides a startup and deoxygenation strategy for the primary loop of AP1000 nuclear power units after a major overhaul. Addressing the unique startup and deoxygenation characteristics of the AP1000 nuclear power unit, it determines the RCS deoxygenation and pressurizer deoxygenation strategies and time control for the primary loop, the amount and timing of hydrazine addition, and integrates and optimizes the logical relationships of multiple stages, including RCS vacuum charging, dynamic and static venting, pressurizer deoxygenation, steam chamber establishment, main pump startup, and RCS deoxygenation, to shorten startup time and reduce the discharge of radioactive wastewater. Ultimately, a startup and deoxygenation strategy for the primary loop of AP1000 nuclear power units after a major overhaul is established, forming a standard procedure for primary loop startup and deoxygenation after a major overhaul, namely, starting the main pump after establishing the steam chamber using the pressurizer.

Description

Technical Field

[0001] This invention relates to the field of coolant deoxygenation technology, specifically to the startup and deoxygenation strategy after a major overhaul of the primary circuit of an AP1000 nuclear power unit. Background Technology

[0002] Traditional pressurized water reactor (PWR) nuclear power units typically have a common startup and deoxygenation process. The startup and deoxygenation process of a PWR nuclear power unit refers to the entire operational process from the initial stage of startup, where the reactor main coolant system (RCS) is online and filled with water to a solid water state, and the RCS is sealed, to the point where the RCS deoxygenates successfully and reaches a temperature above 121°C. Its main processes and key milestones are as follows:

[0003] 1) The main system completes online and starts the Chemical and Volumetric Control System (CVS) to perform RCS static filling to the water solid state;

[0004] 2) Use CVS for top charging and bottom venting to pressurize the water body and perform static venting of RCS. Note that due to the incompressibility of water, low temperature overpressure protection needs to be activated during water body operation to prevent system overpressure. In addition, static venting should be performed at multiple local high points of the system to ensure sufficient venting.

[0005] 3) The RCS pressure meets the net positive suction head (NPSH) requirement for the main pump startup, generally between 2.1MPa and 2.6MPa, and the main pump is jogged to dynamically exhaust air.

[0006] 4) After the RCS pressure meets the NPSH requirement, start the main pump. If the previous static and dynamic venting is not fully completed, the system pressure may drop below the NPSH requirement after the main pump starts.

[0007] 5) Adjust the cooling rate of the residual heat removal system (RNS), use the main pump circulation heating and core decay heat to raise the RCS temperature. When the RCS temperature rises above 60°C, wait for the steam cavity to be built before continuing to raise the temperature.

[0008] 6) While the RCS is heating up, the voltage regulator spray is turned off, the voltage regulator electric heater is fully activated to heat up, and hydrazine is added to the voltage regulator to remove oxygen when the temperature is above 70°C.

[0009] 7) The voltage regulator's electric heater accelerates the heating process. Higher temperatures are beneficial for improving the deoxygenation effect and speeding up the deoxygenation process. However, the voltage regulator temperature must not exceed 121°C before the deoxygenation of the voltage regulator is qualified (the oxygen content drops below 100 ppb).

[0010] 8) After the pressure regulator passes the deoxygenation test, continue to heat the pressure regulator to establish a steam chamber. During this process, the main coolant in the pressure regulator needs to be discharged through the CVS. After the RCS reaches 70°C, add chemicals to remove oxygen while heating up. The temperature should not exceed 121°C before the deoxygenation is qualified.

[0011] 9) Continue heating the RCS for deoxygenation. The higher the temperature, the higher the deoxygenation efficiency. However, please note the following: a) When the temperature reaches 93℃, the unit mode needs to be switched. It must be confirmed that the conditions fully meet the requirements of the technical specifications before continuing to heat up and switch the unit mode; b) Coordinate with the pressurizer to build up the steam chamber and ensure that the temperature difference between the pressurizer and the RCS hot leg does not exceed 178℃; c) Only after the RCS deoxygenation is qualified (oxygen content drops below 100ppb) can the RCS temperature be raised to above 121℃.

[0012] 10) Once the RCS deoxygenation is qualified and the RCS temperature rises to above 121℃, the start-up and deoxygenation process is complete.

[0013] The AP1000 series nuclear power units adopt a passive safety system design concept, which greatly reduces the number of equipment such as pumps, pipelines, valves, and fans in the power plant, thus reducing the complexity of the nuclear power plant system. Therefore, it differs from Generation II and Generation II pressurized water reactor nuclear power plants in many ways. Consequently, the methods and characteristics of the unit's startup and deoxygenation processes are quite different. These characteristics, differences, and their impacts result in the following features for the startup and deoxygenation of the AP1000 series nuclear power units:

[0014] 1) The AP1000 nuclear power unit has strict restrictions on the gases in the RCS system. If non-condensable gases are present in the system, they may cause corrosion and affect core heat transfer during normal operation, and will also seriously affect natural circulation, which is the main operating mode of the passive safety facilities in the event of an accident in the AP1000 nuclear power unit. The main equipment of the reactor main coolant system (RCS) and dedicated safety system (PXS) of the AP1000 nuclear power unit, including the heat transfer tubes of the steam generator (SG), the core makeup water tank (CMT), and the inlet and outlet pipelines of the passive residual heat removal heat exchanger (PRHR), are all connected to the hot leg and cold leg of the RCS. In order to maximize the removal of air from the system, a half-pipe operation mode is adopted to evacuate the RCS, and then charge it. After the charging is completed, pressurization and venting are carried out.

[0015] 2) The design of the canned motor pump simplifies the main coolant system, eliminating the need for continuous shaft seal cooling water. However, it adds a stator shielding sleeve. On the one hand, the equipment still requires cooling water to provide cooling for the stator shielding sleeve; on the other hand, the design does not allow the stator chamber pressure to exceed the coolant side pressure of 6.9 kPa, otherwise the stator shielding sleeve may be damaged. Therefore, before starting RCS vacuum charging, the stator chamber must be evacuated to a vacuum, and the vacuum in the stator chamber must be maintained greater than the RCS side vacuum during RCS vacuum charging until the vacuum is broken after RCS charging is completed before nitrogen can be introduced into the stator chamber.

[0016] 3) The purification module of the chemical and remediation system is located inside the containment, with the main pump providing the purification drive head. Since the shielded main pump does not require continuous shaft seal water injection, the chemical and remediation system is designed for intermittent operation. Simultaneously, the CVS design lacks a control tank and a boron recovery system, and the waste radioactive liquid system (WLS) discharge storage tank (EHT) is open to the atmosphere and not enclosed. These design features lead to the following problems: a) During unit startup, under water-based operating conditions, both system pressurization and pressure maintenance require the operation of the chemical and remediation system, with coordinated charging and discharging. However, since the CVS system cannot establish a closed-loop circulation, a large amount of radioactive wastewater will be generated; b) During unit startup... During the RCS deoxygenation operation, the system pressure still needs to be controlled by the CVS system's charging and discharging because it is still in the water entity condition. However, since the CVS system cannot establish a closed loop, the discharging medium is a coolant with low oxygen content and continuously decreasing, while the charging medium is boron-containing water with high oxygen content, which greatly prolongs the deoxygenation time. c) During the unit's downlink shutdown phase, when it is necessary to remove hydrogen from the main system, due to the lack of a control box, the limited number of times the exhaust valve on the top of the pressure regulator can be opened, and the small capacity of the radioactive waste gas system (WGS), hydrogen removal still requires 60 to 70 hours of main line operation after the unit enters mode 4, which greatly prolongs the overhaul period.

[0017] 4) Several factors are interdependent during unit startup: a) In water-body operation, at least one main pump must be running when the average temperature of the reactor coolant system exceeds 71°C; b) To ensure the net positive suction head (NPSH) of the main pump, the RCS pressure must be maintained at 2.1-2.6 MPa before starting the main pump; c) When starting the main pump in a water-body environment, the CVS pump needs to run continuously, with charging and discharging working in tandem to pressurize and maintain the RCS; During commissioning and startup, the core experiences little or no decay heat and will not generate radiation. The reactor core generates radioactive wastewater, but the RCS temperature rise can only be achieved by the main pump. During startup after refueling overhaul, the core decay heat can raise the RCS temperature, but the non-closed operation of the CVS will generate a large amount of radioactive wastewater. d) When the steam chamber is built and the main pump is started, the CVS system does not need to be used. During the commissioning and startup phase, the reactor core has no decay heat or low decay heat, and the RCS temperature rise can only be achieved by the main pump. The steam chamber cannot be built before the main pump is started. During startup after refueling overhaul, the core decay heat can raise the RCS temperature. Because the CVS does not need to be used, a large amount of radioactive wastewater will not be generated.

[0018] 5) The main pump of the AP1000 nuclear power unit is a variable frequency pump, which converts the 50Hz, 10kV input power to a 60Hz, 6.9kV output power, allowing the main pump to operate at variable speeds within the 3-60Hz range during startup. It is designed with five speed platforms: 0%, 23.6%, 50%, 88%, and 100%. Under the water-saturated conditions during unit startup, the main pump typically undergoes two rounds of jogging and static venting at 23.6% speed, followed by a final round of jogging and static venting at 50% speed. During the RCS (Regenerative Catalytic Reduction) warming phase, the main pump operates at approximately 3.3-5.0℃ / h at the 50% speed platform and approximately 27.8℃ / h at the 88% speed platform. However, the main pump speed can only exceed 50% when the total gas content of the RCS is less than 119cc / kg.

[0019] From the above description, it is easy to see that the startup and deoxygenation process of the AP1000 nuclear power unit is similar to that of a traditional pressurized water reactor power plant, but it has unique and significant characteristics, including vacuum charging and the ability to start the main pump using either a solid water system or a steam chamber. Therefore, the unique design and startup method of the AP1000 nuclear power unit means that it cannot simply replicate the existing experience of domestic and international nuclear power plants. It requires exploration, summarization, and research based on the unit's technical characteristics. In particular, the primary loop (reactor coolant system, RCS system) startup phase—vacuum charging—main pump startup—primary loop deoxygenation typically takes more than 40 hours, leaving considerable room for optimization.

[0020] Therefore, based on the characteristics of the AP1000 passive safety system and the variable frequency shielded main pump, two different starting methods for the main pump need to be considered: water-based main pump start-up and steam chamber start-up. During the unit startup phase, the RCS is first evacuated while the hot leg half-pipe is running, then the RCS is filled with water. After the RCS is full, the vacuum is broken. Then, the main pump is started using either water-based or steam chamber start-up, and the deoxygenation of the pressurizer and RCS is completed sequentially at different temperature platforms before the RCS temperature can be raised to above 121°C. This process is logically complex and subject to many constraints. Therefore, it is necessary to explore, summarize, and refine the research based on the technical characteristics of the unit. We need to summarize the practical experience of the two AP1000 nuclear power units under different operating conditions, such as hot functional testing (no radioactivity, no decay heat) and startup phase (with radioactivity, low decay heat), and refine a standard primary loop startup and deoxygenation strategy for subsequent cycles (with radioactivity, high decay heat) to form a standardized startup and deoxygenation process for the unit after overhaul. Under the premise of ensuring safety, we can shorten the unit startup time and overhaul period, while minimizing the discharge of radioactive wastewater. Summary of the Invention

[0021] In view of the problems existing in the prior art, this invention provides a startup and deoxygenation strategy for the primary loop of AP1000 nuclear power units after major overhaul. Targeting the unique startup and deoxygenation characteristics of AP1000 nuclear power units, the invention determines the RCS deoxygenation and pressurizer deoxygenation strategies and time control for the primary loop, the amount and timing of hydrazine addition, and integrates and optimizes the logical relationships of multiple stages such as RCS vacuum charging, dynamic and static venting, pressurizer deoxygenation, steam chamber establishment, main pump startup, and RCS deoxygenation to shorten startup time and reduce the discharge of radioactive wastewater. Finally, a startup and deoxygenation strategy for the primary loop of AP1000 nuclear power units after major overhaul is formulated, forming a standard procedure for primary loop startup and deoxygenation after unit overhaul, namely, starting the main pump after establishing the steam chamber using the pressurizer.

[0022] To achieve this objective, the present invention adopts the following technical solution:

[0023] The purpose of this invention is to provide a startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after a major overhaul, including the following steps:

[0024] S1. Determine initial conditions: The primary loop is at half-pipe liquid level, and the residual heat is discharged from the system to cool the primary loop;

[0025] S2. Evacuate the stator chamber of the main pump;

[0026] S3. The reactor coolant system is vacuum-charged until the pressurizer level reaches 90%.

[0027] S4. The reactor coolant system is filled with water into the water body and water body control is established;

[0028] S5, Static venting at the pressure boundary of the reactor coolant system;

[0029] S6. The pressurizer is heated to 105-120℃ to remove oxygen until it meets the requirements, and then heated to 220-230℃ and the pressure is increased to 2.1-2.6MPa to establish a steam chamber; the waste heat removal system is adjusted to remove waste heat, and the reactor coolant system is heated to 66-90℃ through decay heat.

[0030] S7. Start the main pump and maintain continuous operation at a speed of over 50%;

[0031] S8. When the reactor coolant system reaches 93°C, the AP1000 nuclear power unit enters mode 4.

[0032] S9. The reactor coolant system is heated to 105-120℃ and deoxygenated to meet the requirements;

[0033] S10. A purification loop is established using the chemical and volume control system. The reactor coolant system is heated to above 121°C, and the startup and deoxygenation process is completed.

[0034] It is worth noting that the strategy described in this invention refers to a method, process, or process method.

[0035] As a preferred technical solution of the present invention, step S1 includes: the AP1000 nuclear power unit is in mode 4, the reactor coolant system is completed online, the main system is closed, the core is in half-pipe operation, and the liquid level of the heat pipe section of the reactor coolant system is between 70-80%.

[0036] As a preferred technical solution of the present invention, step S2 includes: evacuating the stator chambers of the four main pumps to a pressure of less than 8 kPa; in addition, if necessary, the vacuum pumps can be started intermittently to maintain the pressure.

[0037] As a preferred technical solution of the present invention, step S3 includes steps S3a and S3b performed sequentially, as detailed below:

[0038] S3a. Open the automatic pressure relief valve in column A and use the ejector to evacuate the reactor coolant system. Pay attention to the operation of the residual heat removal pump to prevent pump cavitation and loss of core cooling. Continue until the appropriate reactor coolant system vacuum level is determined based on the reactor coolant system temperature, equipment cooling water system temperature, and residual heat removal flow rate.

[0039] S3b. The reactor coolant system is charged through the chemical and volume control system. The main operations during this period include shutting down the reactor coolant system to reduce the water load, the disappearance of the P-12 (pressurizer level below 16%) signal, charging and commissioning of the PRHR HX (passive residual heat exchanger), powering on the pressurizer electric heater, shutting down the vacuum pump and closing the A-series automatic pressure relief valves, engaging the pressure relief control mode, and engaging the cryogenic overpressure protection.

[0040] As a preferred technical solution of the present invention, step S4 includes: using the chemical and volume control system to charge and discharge the reactor coolant system to pressurize the water body, and putting into operation the low temperature overpressure protection during the water body phase.

[0041] It is worth noting that in step S4, due to the incompressibility of water, a low-temperature overpressure protection is required when the water is in a solid state to prevent the system from overpressured.

[0042] As a preferred technical solution of the present invention, step S5 includes: activating the pressure relief control mode, continuing to statically fill the reactor coolant system with water to a solid state and pressurizing it to 0.35 MPa, sampling the reactor coolant system and pressurizer, and venting the local high points.

[0043] It is worth noting that the local high points mentioned in this invention include auxiliary spray pipelines, ADS pipelines, reactor coolant system top cover, core makeup water tank, passive residual heat removal heat exchanger, main pump external heat exchanger, and related instruments.

[0044] As a preferred technical solution of the present invention, step S6 includes: shutting off the pressurizer spray, activating the electric heater to heat the pressurizer to between 105-120°C, adding hydrazine from the auxiliary spray pipeline for deoxygenation, controlling the temperature of the pressurizer between 105-120°C, which can improve the deoxygenation efficiency at high temperature, and continuing to heat to above 121°C after the oxygen content is qualified; continuing to heat the pressurizer to build a steam chamber after the deoxygenation is qualified, increasing the downward discharge, raising the temperature to 220-230°C, and the pressure to 2.1-2.6 MPa to build a steam chamber; adjusting the cooling rate of the residual heat discharge system, using the core decay heat to raise the temperature of the reactor coolant system to the range of 66-90°C, and adding hydrazine for deoxygenation.

[0045] Preferably, the amount of hydrazine added to the voltage regulator is 2-3 times the dissolved oxygen, such as 2 times, 2.1 times, 2.3 times, 2.5 times, 2.7 times, 2.8 times or 3 times, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0046] Preferably, the amount of hydrazine added to the reactor coolant system is 1.5 to 2 times the amount of dissolved oxygen, such as 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times, or 2 times, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0047] It is worth noting that hydrazine deoxygenation generally focuses on three temperature ranges: 66-82℃, below 93℃, and 110-120℃. The reaction rate of hydrazine deoxygenation is fastest and the deoxygenation effect is best within the 90-120℃ range. When starting the main pump after establishing the pressurizer steam chamber, hydrazine should be added to the pressurizer first. Since the radioactivity of the pressurizer is relatively lower than that of the reactor core, its decomposition-promoting effect is limited. The amount of hydrazine added can be 2-3 times the dissolved oxygen. After adding the chemical, the pressurizer should be heated and deoxygenated first. After the pressurizer deoxygenation is qualified, the pressurizer steam chamber is established to meet the main pump start-up conditions. After the main pump is started and the planned heating is completed, hydrazine should be added to the RCS to avoid excessive decomposition of the hydrazine under the influence of radioactivity if added prematurely. The amount of hydrazine added should be 1.5-2 times the dissolved oxygen.

[0048] Preferably, step S6 also includes the following simultaneous actions: if necessary, the main pump can be jogged during the time of steam chamber construction: establishing a closed-loop primary circuit, starting the chemical and volume control system's water supply pump and the combined water pressurization system to 2.1-2.6 MPa, and jogging the main pump to exhaust gas.

[0049] As a preferred technical solution of the present invention, step S7 includes: the reactor coolant system pressure meets the net positive suction head requirement of 2.1-2.6 MPa for the main pump start-up, the four main pumps are started to 23.6% speed, and then the speed is increased to 50% synchronously.

[0050] As a preferred technical solution of the present invention, step S8 includes: adjusting the cooling rate of the residual heat removal system, using the main pump circulation heating and core decay heat to raise the temperature of the reactor coolant system to above 90°C to continue deoxygenation, ensuring that the temperature difference between the liquid and vapor phases of the pressurizer is below the critical value, which is the minimum value corresponding to the presence of non-condensable gases due to a large temperature difference. At this time, it is necessary to vent the pressurizer steam chamber; when the temperature of the reactor coolant system exceeds 93°C, the AP1000 nuclear power unit enters mode 4, and it is necessary to check and confirm the mode conversion checklist in advance, close the containment vessel, and ensure that the secondary side feedwater quality of the SG is qualified.

[0051] As a preferred technical solution of the present invention, in step S9, if the oxygen content of the reactor coolant system drops below 100 ppb, it is considered to be deoxygenated to the qualified level.

[0052] It is worth noting that the reactor coolant system temperature rises to between 110-121°C to continue deoxygenation; higher temperatures are beneficial for improving deoxygenation efficiency and accelerating the deoxygenation process; however, the main system temperature must not exceed 121°C before the reactor coolant system passes deoxygenation qualification; only after the reactor coolant system passes deoxygenation qualification (oxygen content drops below 100 ppb) can the reactor coolant system temperature be raised to above 121°C, and the startup and deoxygenation process ends.

[0053] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0054] (1) The technical solution described in this invention adopts a comparative test method to test and study the primary loop startup and deoxygenation process of multiple AP1000 units under various operating conditions (hot test, low decay heat with fuel, high decay heat with fuel, etc.). The characteristics, applicable operating conditions and startup logic diagrams of starting the main pump under water body conditions and starting the main pump after the pressurizer establishes the steam chamber are compared and analyzed. The primary loop RCS deoxygenation and pressurizer deoxygenation strategies and time control are determined, as well as the amount and timing of adding hydrazine. The logical relationships of multiple links such as RCS vacuum charging, dynamic and static exhaust, pressurizer deoxygenation, steam chamber establishment, main pump startup, and RCS deoxygenation are integrated and optimized to shorten the startup time and reduce the discharge of radioactive wastewater. Finally, the primary loop startup and deoxygenation strategy of AP1000 nuclear power units is formulated, and the standard process of primary loop startup and deoxygenation after unit overhaul is formed, that is, the main pump is started after the pressurizer establishes the steam chamber.

[0055] (2) The technical solution described in this invention, through testing and research on the first loop startup and deoxygenation process of multiple AP1000 units under multiple operating conditions, logically sorts out the unique startup and deoxygenation characteristics of AP1000, analyzes and studies the applicability and advantages and disadvantages of two methods: starting the main pump with water body and starting the main pump after vacuum filling the steam chamber, analyzes and studies the RCS deoxygenation and pressure regulator deoxygenation strategies, and integrates and optimizes multiple links such as RCS vacuum filling, dynamic and static exhaust, steam chamber construction, main pump startup, RCS deoxygenation, and pressure regulator deoxygenation, determines the unit's first loop startup and deoxygenation strategy, establishes a standard process for unit startup and deoxygenation after major overhaul, and fundamentally solves the problem of how to implement first loop startup and deoxygenation after subsequent unit overhaul. Attached Figure Description

[0056] Figure 1 This is a flowchart of the startup and deoxygenation strategy for the AP1000 nuclear power unit after a major overhaul of the primary loop, as described in this invention.

[0057] Figure 2 This is the RCS deoxygenation trend diagram corresponding to the start-up of AP1000 nuclear power unit HY201 after overhaul as described in Embodiment 1 of the present invention;

[0058] Figure 3This is the PZR deoxygenation trend diagram corresponding to the start-up of AP1000 nuclear power unit HY201 after overhaul as described in Embodiment 1 of the present invention;

[0059] Figure 4 This is a flowchart of the startup and deoxygenation strategy of the AP1000 nuclear power unit described in Comparative Example 1. Detailed Implementation

[0060] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0061] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows:

[0062] Example 1

[0063] This embodiment provides a startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after a major overhaul, such as... Figure 1 As shown, it includes the following steps:

[0064] S1. Determine initial conditions: The AP1000 nuclear power unit is in mode 4, the reactor coolant system is online, the main system is closed, the core is in half-pipe operation, and the liquid level in the heat pipe section of the reactor coolant system is between 70-80%.

[0065] S2. Evacuate the stator chambers of the four main pumps until the pressure is less than 8 kPa;

[0066] S3. The reactor coolant system is vacuum-charged until the pressurizer level reaches 90%. Step S3 includes steps S3a and S3b performed sequentially:

[0067] S3a. Open the automatic pressure relief valve in column A and use the ejector to evacuate the reactor coolant system. Pay attention to the operation of the residual heat removal pump to prevent pump cavitation and loss of core cooling. Continue until the appropriate reactor coolant system vacuum level is determined based on the reactor coolant system temperature, equipment cooling water system temperature, and residual heat removal flow rate.

[0068] S3b. The reactor coolant system is charged through the chemical and volume control system. The main operations during this period include shutting down the reactor coolant system to reduce the water loading, the P-12 signal disappearing, PRHR HX being charged and put into operation, the pressurizer electric heater being powered on, the automatic pressure relief valve series valves of the A-row being shut down and vacuumed, the pressure relief control mode being activated, and the cryogenic overpressure protection being activated.

[0069] S4. The reactor coolant system is filled with water to the water body and water body control is established. The chemical and volume control system is used to pressurize the reactor coolant system by charging and discharging water, and low temperature overpressure protection is activated when the water body is in the water body.

[0070] S5. Static venting of the reactor coolant system pressure boundary, activation of the pressure relief control mode, continued static filling of the reactor coolant system to a water-solid state and pressurization to 0.35MPa, sampling of the reactor coolant system and pressurizer, and venting of local high points.

[0071] S6. Shut down the pressurizer spray, activate the electric heater to heat the pressurizer to between 105-120℃, add hydrazine from the auxiliary spray line for deoxygenation, the amount of hydrazine is 2-3 times the dissolved oxygen; after the pressurizer deoxygenation is qualified, continue to heat up to build the steam chamber, increase the downward discharge, raise the temperature to 220-230℃, raise the pressure to 2.1-2.6MPa to build the steam chamber; adjust the cooling rate of the residual heat removal system, use the core decay heat to raise the reactor coolant system to the range of 66-90℃, and add hydrazine for deoxygenation, the amount of hydrazine is 1.5-2 times the dissolved oxygen;

[0072] If necessary, the main pump can be started in jogs during the steam chamber construction period: establish a closed loop, start the chemical and volume control system water supply pump and the upper and lower discharge coordination water body pressurization to 2.1-2.6MPa, and use the main pump to jog the exhaust.

[0073] S7. Start the main pump and maintain it at a speed of more than 50% for continuous operation. The reactor coolant system pressure should meet the net positive suction head requirement of 2.1-2.6MPa for the main pump startup. Start the four main pumps to 23.6% speed, and then increase the speed to 50% synchronously.

[0074] S8. When the reactor coolant system reaches 93°C, the AP1000 nuclear power unit enters mode 4; the cooling rate of the residual heat removal system is adjusted, and the reactor coolant system is heated to above 90°C using the main pump circulation heating and core decay heat to continue deoxygenation, ensuring that the temperature difference between the liquid and vapor phases of the pressurizer is below the critical value; when the reactor coolant system reaches more than 93°C, the AP1000 nuclear power unit enters mode 4.

[0075] S9. The reactor coolant system is heated to 105-120℃ to remove oxygen until it is qualified. When the oxygen content of the reactor coolant system drops below 100ppb, it is considered to be qualified after deoxygenation.

[0076] S10. A purification loop is established using the chemical and volume control system. The reactor coolant system is heated to above 121°C, and the startup and deoxygenation process is completed.

[0077] The startup method of starting the main pump after the pressurizer builds up the steam chamber is only suitable for unit startup conditions with high core decay heat, such as the late stage of the first cycle and subsequent cycles. It has many advantages: 1) It does not require static or dynamic jogging of the main pump to vent, greatly shortening the startup time; 2) It does not require pressurization and pressure maintenance of the RCS water body through the CVS makeup water pump and the top charging and bottom discharging, and will not generate radioactive wastewater; 3) Since the pressurizer is in steam chamber operation, the primary loop pressure fluctuation is very small, and it usually does not cause pressure fluctuations, and will not cause the main pump or CVS pump to trip; 4) When the pressurizer builds up the steam chamber and starts the main pump, the primary loop temperature must be strictly controlled within a suitable range.

[0078] Analysis shows that for newly built AP / CAP series nuclear power units, in the later stages of the first fuel cycle and under high decay heat conditions in subsequent cycles, compared to starting the main pump using a pressurizer to build a steam chamber, the startup method not only shortens the unit startup and deoxygenation time but also avoids generating large amounts of wastewater, compared to starting the main pump using a water-based system. Therefore, the standard startup and deoxygenation procedure after a major overhaul of the unit adopts the following standard procedure for starting the main pump after establishing a steam chamber using a pressurizer.

[0079] Beneficial effects:

[0080] The AP1000 nuclear power technology is unprecedented both domestically and internationally, requiring complete independent development. Coupled with the complexity of the unit startup process and numerous limiting factors, this study involved testing and research on the primary loop startup and deoxygenation processes of multiple AP1000 units under various operating conditions. This resulted in the rational optimization of the primary loop deoxygenation, pressurizer chamber establishment, main pump startup, and RCS exhaust operations during the startup of units after overhauls. A standard strategy for the primary loop startup and deoxygenation of the AP series nuclear power units was established. Subsequently, the standard process was verified during the overhauls of two units (HY101 / HY201) and achieved the expected results. Currently, this technological achievement possesses leading domestic and international capabilities. The RCS deoxygenation trend diagram for the startup of HY201 after the overhaul is shown below. Figure 2 As shown in the figure, the PZR deoxygenation trend chart corresponding to the start-up of HY201 after overhaul is as follows: Figure 3 As shown. The main implementation effects are as follows:

[0081] (1) Economic benefits

[0082] After the initial overhaul verification of the two units, the startup time for HY101 / 201 was 39.5 / 36.5 hours respectively. Excluding the waiting time due to equipment defects and other reasons, the actual startup and deoxygenation time was less than 22 / 20 hours respectively. This reduced the startup time of the two units in this stage from 40 hours during the startup test to less than 22 / 20 hours. This also shortened the main startup time of the units by about 18 hours each time a refueling overhaul is carried out, increasing revenue by about 9 million yuan.

[0083] (2) Project promotion

[0084] Since the primary loop design of AP / CAP series nuclear power plants is basically the same, the research results on the primary loop start-up and deoxygenation strategy of AP1000 nuclear power units have universality and promotion value for AP / CAP series nuclear power plants. They are applicable to the hot commissioning, start-up and commissioning, and post-overhaul start-up phase of this series of nuclear power plants. The AP / CAP series nuclear power plants include at least Shandong Haiyang, Zhejiang Sanmen, State Nuclear Power Demonstration Plant, State Nuclear Power Zhanjiang, and Guangxi Bailong nuclear power plants.

[0085] (3) Environmental benefits

[0086] Starting the main pump after establishing the steam chamber using a pressure regulator, compared to starting the main pump in the water body, eliminates the need for prior pressurization and pressure maintenance of the RCS water body via a CVS makeup water pump and top-fill / bottom-release method, thus preventing the discharge of radioactive wastewater. It also reduces condensate venting during static and dynamic jogging of the main pump, contributing to a reduction in radioactive wastewater emissions. Therefore, it offers certain environmental benefits, achieving ALARA (reasonably feasible, as low as possible) from a radioactive emission perspective.

[0087] (4) Safety benefits

[0088] Since the standard procedure for starting the unit and deoxygenating after a major overhaul has been established by using a pressure regulator to establish a steam chamber before starting the main pump, the occurrence of unexpected equipment tripping events caused by primary loop pressure fluctuations under water conditions has been greatly reduced. This has also reduced the workload of reactor operators in adjusting the primary loop pressure, thus providing certain benefits for the safe operation of the unit.

[0089] (5) Other benefits

[0090] This technological achievement can provide a standard strategy for the start-up and deoxygenation of subsequent AP or CAP series units, and the research results can also be used as training materials for AP1000 operators.

[0091] Optimization analysis of subsequent loop startup methods

[0092] For newly built AP / CAP series nuclear power units, the system experiences high decay heat and radioactivity in the later stages of the first fuel cycle and subsequent cycles, with CVS values ​​representing the operating values ​​for the next cycle. Therefore, in this stage, in addition to considering the impact of temperature and hydrazine concentration on deoxygenation, the promoting effect of radioactivity on hydrazine deoxygenation must also be considered. Compared to the water-based main pump start-up method, starting the main pump after the pressurizer builds the steam chamber is more suitable. Based on theoretical analysis and experience from unit overhauls in the early stages of the first cycle, the main areas for optimization include:

[0093] (1) Static venting and dynamic jogging of the main pump are no longer required, which can greatly shorten the start-up time. By adopting the method of starting the main pump after the pressure regulator builds the steam chamber, the RCS / pressure regulator can be directly entered into the heating and hydrazine deoxygenation stage after the vacuum filling is completed. Static venting can be arranged simultaneously in the early stage of heating, which simplifies the static venting and the 23.6% jogging venting of the two main pumps. This reduces the number of main pump jogging and starting times and reduces the main line occupation time by 2-6 hours. In some cases, the main pump can still be jogging venting after the material replacement overhaul. These cases include: a) the main pump body has been emptied and then filled with water, such as the stator chamber inspection, which requires venting of the main pump heat exchanger; b) the main pump has been scheduled for a major overhaul, such as the replacement of the inverter isolation transformer, or the major overhaul of the inverter's electrical or mechanical components. The main pump should be jogged as soon as possible for trial operation as a post-overhaul test.

[0094] (2) It does not require the CVS water supply pump and the top-fill and bottom-release to pressurize and maintain the pressure of the RCS water body, and will not generate radioactive wastewater;

[0095] (3) The timing of hydrazine addition can be optimized to allow deoxygenation and heating of the RCS and pressurizer to proceed in parallel, shortening the deoxygenation time. After the RCS is closed, heating begins. Before the main pump starts, the RCS temperature is controlled at 66-90℃. During this period, hydrazine is added to simultaneously deoxygenate the RCS and pressurizer. All electric heaters in the pressurizer are activated to begin heating. Before the oxygen content in the pressurizer drops below 100ppb, the temperature is maintained between 110-120℃ to improve deoxygenation efficiency. When the oxygen content in the pressurizer is within acceptable limits, heating and pressurization continue to build the steam chamber and discharge excess coolant until the pressurizer temperature reaches approximately 228℃, the RCS pressure reaches 2.1-2.6MPa, and the pressurizer's narrow-range liquid level drops to approximately 55%, completing the steam chamber construction. Then, the main pump is started at 50% speed to increase the RCS temperature and improve deoxygenation efficiency. Compared with starting the main pump after the steam chamber is built, the method of starting the main pump after the steam chamber is built has two effects on the start-up time: First, it is constrained by the condition that "when the average RCS temperature is greater than 71℃ under the operation of the steam chamber, at least one main pump is required to be running", so the temperature rise of the RCS is forced to be divided into two stages, which prolongs the start-up time; Second, hydrazine deoxygenation and temperature rise are carried out in parallel for the RCS and the pressure regulator. Deoxygenation is basically completed during the temperature rise process. In addition, the temperature rise relies on the operation and decay heat of the main pump, and the relative temperature rise rate is also relatively high (reaching 7-8℃ / h), which greatly shortens the start-up and deoxygenation process.

[0096] Comparative Example 1

[0097] This comparative example provides another startup and deoxygenation strategy for AP1000 nuclear power units, such as... Figure 4 As shown, it includes the following steps:

[0098] T1. Determine the initial conditions: The primary loop is at half-pipe liquid level, and the residual heat is discharged to cool the primary loop;

[0099] T2. Evacuate the stator chamber of the main pump;

[0100] T3. The reactor coolant system is vacuum-charged until the pressurizer level reaches 90%.

[0101] T4. The reactor coolant system is filled with water into the water body and water body control is established.

[0102] T5, Static venting at the pressure boundary of the reactor coolant system;

[0103] T6. Establish a closed loop;

[0104] T7, with the chemical and volume control system, pressurizes to 2.1-2.6 MPa through a combination of charging and discharging.

[0105] T8, the two wheels of the main pump are jogged at 23.6% speed for venting, the main pump is started at 50% speed for dynamic venting under water conditions, and four main pumps are started to maintain continuous operation at more than 50% speed;

[0106] T9. The main pump circulation heating loop is heated to 66-90℃, and N2H4 is added to PZR and RCS.

[0107] T10 and PZR are heated to about 110℃ for deoxygenation until qualified. PZR is heated to 220-230℃ and pressure is 2.1-2.6MPa to establish the steam chamber. The CVS water supply pump is shut down. RCS is heated to 93℃ and the unit enters mode 4. RCS is heated to about 110℃ for deoxygenation until qualified.

[0108] T11 and CVS establish a purification loop, and RCS is heated to above 121℃.

[0109] This comparative example describes the starting method of the main pump using a water-based system. It is applicable to new AP / CAP series units under conditions of no or low decay heat, such as during hot functional testing, startup, and the initial stage of the first cycle. Its main features are as follows: a) Ensure adequate venting, typically using two rounds of static venting at 23.6% speed and one round of dynamic venting at 50% speed; b) First, pressurize the water system through top-charge and bottom-release, then start the main pump to raise the system temperature, followed by deaeration and steam chamber construction; c) Requires the CVS system makeup water pump and the non-closed-loop top-charge and bottom-release system to operate for a relatively long time, generating a significant amount of wastewater; d) Minimize interference with the primary circuit pressure boundary to prevent pressure fluctuations from causing the main pump or CVS pump to trip.

[0110] In summary, the technical solution of this invention, through testing and research on the first loop startup and deoxygenation process of multiple AP1000 units under various operating conditions, logically analyzes the unique startup and deoxygenation characteristics of AP1000, examines the applicability and advantages and disadvantages of two methods: starting the main pump with water body and starting the main pump after vacuum charging to build the steam chamber, analyzes and studies the RCS deoxygenation and pressure regulator deoxygenation strategies, and integrates and optimizes multiple links such as RCS vacuum charging, dynamic and static exhaust, steam chamber building, main pump startup, RCS deoxygenation, and pressure regulator deoxygenation. This has determined the unit's first loop startup and deoxygenation strategy, established a standard procedure for unit startup and deoxygenation after major overhaul, and fundamentally solved the problem of how to implement first loop startup and deoxygenation after subsequent unit overhauls.

[0111] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0112] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0113] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0114] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after a major overhaul, characterized in that: Includes the following steps: S1. Determine initial conditions: The primary loop is at half-pipe level, and the residual heat removal system cools the primary loop; among them, the AP1000 nuclear power unit is in mode 4, the reactor coolant system is online, the main system is closed, the core is in half-pipe operation, and the reactor coolant system heat pipe section liquid level is between 70-80%. S2. Evacuate the stator chamber of the main pump; S3. The reactor coolant system is vacuum-charged until the pressurizer level reaches 90%. S4. The reactor coolant system is filled with water into the water body and water body control is established; S5, Static venting at the pressure boundary of the reactor coolant system; S6. The pressurizer is heated to 105-120℃ to remove oxygen until it meets the requirements, and then heated to 220-230℃ and the pressure is increased to 2.1-2.6MPa to establish a steam chamber; the waste heat removal system is adjusted to remove waste heat, and the reactor coolant system is heated to 66-90℃ through decay heat. S7. Start the main pump and maintain continuous operation at a speed of over 50%; S8. When the reactor coolant system reaches 93°C, the AP1000 nuclear power unit enters mode 4. S9. The reactor coolant system is heated to 105-120℃ and deoxygenated to meet the requirements; S10. A purification loop is established using the chemical and volume control system. The reactor coolant system is heated to above 121°C, and the startup and deoxygenation process is completed.

2. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul as described in claim 1, characterized in that, Step S2 includes: evacuating the stator chambers of the four main pumps to a pressure of less than 8 kPa.

3. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul, as described in claim 1, is characterized in that... Step S3 includes steps S3a and S3b performed sequentially, as detailed below: S3a. Open the automatic pressure relief valve in column A and use the ejector to evacuate the reactor coolant system. Pay attention to the operation of the residual heat removal pump to prevent pump cavitation and loss of core cooling. Continue until the appropriate reactor coolant system vacuum level is determined based on the reactor coolant system temperature, equipment cooling water system temperature, and residual heat removal flow rate. S3b. The reactor coolant system is charged through the chemical and volume control system. The main operations during this period include shutting down the reactor coolant system to reduce the water load, the P-12 signal disappearing, PRHR HX being charged and put into operation, powering on the pressurizer electric heater, shutting down the vacuum pump and closing the A-row automatic pressure relief valve series valves, engaging the pressure relief control mode, and engaging the cryogenic overpressure protection.

4. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul, as described in claim 1, is characterized in that... Step S4 includes: using the chemical and volume control system to pressurize the reactor coolant system by charging and discharging water, and engaging cryogenic overpressure protection during the water phase.

5. The startup and deoxygenation strategy for the primary loop of an AP1000 nuclear power unit after major overhaul, as described in claim 1, is characterized in that... Step S5 includes: activating the pressure relief control mode, continuing to statically fill the reactor coolant system with water to a solid state and pressurizing it to 0.35 MPa, sampling the reactor coolant system and pressurizer, and venting the local high points.

6. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul, as described in claim 1, is characterized in that... Step S6 includes: shutting off the pressurizer spray, activating the electric heater to heat the pressurizer to between 105-120°C, and adding hydrazine from the auxiliary spray line for deoxygenation; after the pressurizer has passed deoxygenation, continue to heat up to build the steam chamber, increase the discharge, raise the temperature to 220-230°C, and raise the pressure to 2.1-2.6 MPa to build the steam chamber; adjust the cooling rate of the residual heat removal system, use the core decay heat to heat the reactor coolant system to the range of 66-90°C, and add hydrazine for deoxygenation.

7. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul, as described in claim 6, is characterized in that... The amount of hydrazine added to the voltage regulator is 2-3 times the amount of dissolved oxygen.

8. The AP1000 nuclear power unit primary loop start-up and deoxygenation strategy according to claim 6, characterized in that, The amount of hydrazine added to the reactor coolant system is 1.5 to 2 times the amount of dissolved oxygen.

9. The startup and deoxygenation strategy for the primary loop of an AP1000 nuclear power unit after major overhaul, as described in claim 6, is characterized in that... Step S6 also includes the following simultaneous actions: establishing a closed-loop circulation, starting the water supply pump of the chemical and volume control system and pressurizing the water body to 2.1-2.6 MPa by filling and draining the water, and using the main pump to jog the air.

10. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul according to claim 1, characterized in that, Step S7 includes: the reactor coolant system pressure meets the net positive suction head requirement of 2.1-2.6 MPa for the main pump startup, the four main pumps are started to 23.6% speed, and then the speed is increased to 50% synchronously.

11. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul according to claim 1, characterized in that, Step S8 includes: adjusting the cooling rate of the residual heat removal system, using the main pump circulation heating and core decay heat to raise the temperature of the reactor coolant system to above 90°C to continue deoxygenation, ensuring that the temperature difference between the liquid and vapor phases of the pressurizer is below the critical value; when the temperature of the reactor coolant system exceeds 93°C, the AP1000 nuclear power unit enters mode 4.

12. The startup and deoxygenation strategy for the primary circuit of an AP1000 nuclear power unit after major overhaul according to claim 1, characterized in that, In step S9, if the oxygen content of the reactor coolant system drops below 100 ppb, it is considered to have been deoxygenated to the required standard.

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