Half-tube operation method during overhaul period of nuclear power unit

Abstract

The invention relates to the field of nuclear power unit overhaul, in particular to a half-tube operation method during nuclear power unit overhaul, which comprises the following steps of: 1, shutting down a reactor and opening a loop of the reactor; step 2, keeping continuous cooling of the primary loop; step 3, dewatering the primary loop of the reactor; 4, draining the primary loop of the reactor to a half-tube operation liquid level, and synchronously carrying out pressure vessel bolt stretching and uncovering preparation work; 5, the actual liquid level of the first loop is confirmed; 6, taking risk management and control measures; step 7, completing the plugging of the cold section and the hot section of the primary loop of the reactor; step 8, quitting the half-tube operation; and 9, unloading the reactor core. According to the method, during the overhaul period of the nuclear power unit, the descending work of the unit, such as pressure vessel uncovering, reactor core unloading and steam generator heat transfer tube inspection, is changed from the original three processes of series connection to parallel connection, the start time of the steam generator heat transfer tube inspection work is greatly advanced, and the overhaul main or secondary critical path time can be effectively shortened.

Description

Technical Field

[0001] This invention relates to the field of nuclear power unit overhaul, and in particular to a method for half-pipe operation during nuclear power unit overhaul. Background Technology

[0002] Nuclear power plant overhauls are large-scale maintenance activities conducted after a period of operation to ensure the safe continued operation of the nuclear power plant. These overhauls are carried out according to the aging characteristics of the equipment and the requirements of the technical specifications, and are carried out on a planned shutdown basis. As a periodic task, nuclear power plant overhauls face numerous quality challenges, including tight schedules, heavy workloads, high first-time success rates, and frequent personnel changes. Key tasks involved in nuclear power plant overhauls include tightening pressure vessel bolts and lifting the pressure vessel top cover, core unloading, and inspection of steam generator heat transfer tubes. In existing technologies, these tasks are critical path tasks during the overhaul, with long working hours that affect the overall overhaul duration. Summary of the Invention

[0003] This invention provides a method for operating half-pipes during a nuclear power unit overhaul, which addresses the problem of extended overhaul time caused by unreasonable implementation of overhaul work in the prior art.

[0004] The technical solution of the present invention is as follows: This invention proposes a method for semi-pipe operation during a nuclear power unit overhaul, the method comprising: Step 1: Shut down the reactor and open the primary loop of the reactor; Step 2: The waste heat removal system normally cools the reactor primary loop, maintaining continuous cooling of the primary loop; Step 3: Drain the reactor primary circuit to bring the water level in the reactor primary circuit below the pressure vessel flange face; Step 4: Drain the reactor primary loop to half-pipe operating level, and simultaneously carry out pressure vessel bolt tensioning and opening preparation work; Step 5: Confirm the actual liquid level in the primary circuit, and confirm that it meets the liquid level requirements of the manhole on the primary side of the steam generator and the plugs in the hot and cold sections of the primary circuit; Step Six: Implement risk management measures for the containment closure capability during partial operation; Step 7: Implement reactive control and risk management measures for semi-pipeline operation; Step 8: Implement risk management measures to control the waste heat discharge capacity during half-pipe operation; Step 9: Implement power availability risk management measures for partial-pipe operation; Step 10: Implement risk management measures to control the water volume in the primary loop during half-pipe operation; Step 11: Complete the sealing plates for the cold and hot sections of the reactor primary loop. After the sealing plates are completed, the inspection of the steam generator heat transfer tubes can begin. Step 12: Fill the reactor primary loop with water to below the pressure vessel flange face, and then discontinue half-pipe operation; Step 13: Open the reactor pressure vessel; Step Fourteen: Fill the refueling tank with water; Step 15: Unload the reactor core.

[0005] In some embodiments, step one specifically includes: Step 1.1: Reduce the power output of the nuclear power unit; Step 1.2: Reactor shutdown; Step 1.3: Cooling and depressurizing the reactor primary loop; Step 1.4: Open the reactor primary circuit boundary.

[0006] In some embodiments, step three, lowering the reactor primary circuit water level to below the pressure vessel flange face, specifically means 0.3 meters below the pressure vessel flange face; and step twelve, filling the reactor primary circuit with water to below the pressure vessel flange face, specifically means 0.3 meters below the pressure vessel flange face.

[0007] In some embodiments, the liquid level in the half-pipe during step four is 70%-80% of the liquid level in the primary loop hot section.

[0008] In some embodiments, step six involves implementing risk management measures for the containment shutdown capability during half-pipe operation, specifically including: Step 6.1: Ensure that the containment equipment gates are closed or in an emergency-closeable state; Step 6.2: Ensure that at least one of the containment personnel gates is closed; Step 6.3: Ensure that the mechanical or electrical penetrations of the containment are in a closable position.

[0009] In some embodiments, the risk management measure for half-pipe operation reactivity control in step seven is to implement isolation of the primary loop non-borized water source.

[0010] In some embodiments, step eight involves implementing risk management measures for controlling the waste heat discharge capacity during half-pipe operation, specifically including: Step 8.1: Keep the reactor's two-stage equipment cooling water and two-stage seawater systems available, and ensure that the relevant instruments characterizing residual heat removal capacity and core undercooling are available; Step 8.2: Adjust the normal operating flow rate of the waste heat discharge pump to ensure the net positive suction head of the waste heat discharge pump; Step 8.3: Adjust the temperature control valve of the normal waste heat discharge pump to control the average temperature of the primary circuit to be below 71℃; In some embodiments, step nine adopts partial pipeline operation power availability risk management measures, specifically including: keeping the backup diesel generator available, keeping the priority power supply or auxiliary power supply of the power supply bus corresponding to the waste heat discharge pump, equipment cooling water pump, and seawater pump available, suspending the power switching operation of the main transformer, plant transformer, and backup transformer, and ensuring that there is backup power support after the equipment power is unexpectedly lost.

[0011] In some embodiments, step ten involves implementing risk management measures for controlling the primary loop water volume during half-pipe operation, specifically including: Step 10.1: Ensure that the liquid level in the primary loop is precisely controlled at the half-pipe operating level; Step 10.2: Keep multiple primary loop water replenishment paths available so that water can be replenished quickly when the primary loop liquid level drops abnormally; Step 10.3: Control the work that causes changes in the primary coolant charge in the planning and control personnel entering and leaving the containment. Step 10.4: Conduct primary loop coolant leakage monitoring.

[0012] The implementation of this invention has the following beneficial effects: This invention proposes a method for semi-pipe operation during nuclear power unit overhauls. This method transforms the downlink operations during overhauls—pressure vessel opening, core unloading, and steam generator heat transfer tube inspection—from a series of three processes into a primarily parallel process. The start time of the steam generator heat transfer tube inspection is significantly advanced, effectively shortening the time of the main or secondary critical paths during the overhaul and contributing to improved overhaul performance. Furthermore, this invention clarifies the semi-pipe operation logic and process, effectively controlling the risks associated with semi-pipe operation. Attached Figure Description

[0013] Figure 1 This is a flowchart of a method for operating a half-pipe during a major overhaul of a nuclear power unit, as proposed in an embodiment of the present invention. Figure 2 This is a risk management diagram for a semi-pipe operation method during a major overhaul of a nuclear power unit, as proposed in an embodiment of the present invention. Detailed Implementation

[0014] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] During the current overhaul of nuclear power units, the general procedures involved, in chronological order, are as follows: reducing the power output of the nuclear power unit; shutting down the reactor; cooling and depressurizing the reactor primary loop; opening the reactor primary loop boundary; draining the reactor primary loop to below the pressure vessel flange; opening the pressure vessel cover; filling the reactor refueling pool to full; unloading the reactor core; draining the refueling pool from the high span; draining the primary loop to half-pipe level; opening the primary manhole of the steam generator; and inspecting the heat transfer tubes of the steam generator.

[0016] Among the above procedures, pressure vessel opening, core unloading, and steam generator heat transfer tube inspection are critical path operations during major overhauls, and these operations are lengthy. Taking a 1,000 MW nuclear power unit as an example, the approximate times for these three procedures are: pressure vessel opening (20-30 hours), core unloading (30-40 hours), and steam generator heat transfer tube inspection (120-200 hours). The inspection time varies depending on the nuclear power unit model and the method of group or random inspection of the heat transfer tubes. Generally, the larger the power of the nuclear power unit and the more heat transfer tubes are in the steam generator, the longer the inspection time will be.

[0017] like Figures 1 to 2 As shown, this invention proposes a method for semi-pipe operation during a nuclear power unit overhaul, the method comprising: Step 1: Shut down the reactor and open the primary loop.

[0018] Step 1.1: Reduce the power output of the nuclear power unit; Step 1.2: Reactor shutdown; Step 1.3: The pressure in the reactor primary loop is reduced to atmospheric pressure, and the temperature is reduced to below 71°C; Step 1.4: Open the reactor primary circuit boundary.

[0019] Step 2: The residual heat removal system normally cools the reactor primary loop, maintaining continuous cooling of the primary loop.

[0020] Step 3: Drain the reactor primary circuit to bring the water level in the primary circuit down to 0.3 meters below the pressure vessel flange.

[0021] Step 4: Drain the reactor primary circuit to the half-pipe operating level, and simultaneously carry out pressure vessel bolt tightening and opening preparation work. Half-pipe operation is an operation mode in which the primary circuit is drained to the level below the primary cold and hot section plugs when the core has spent fuel assemblies. The half-pipe operating level is 70%-80% of the primary hot section level. On the one hand, the level must be ensured to be below the primary cold and hot section plug levels, and on the other hand, the primary circuit level should not be too low to avoid insufficient net positive suction head of the normal residual heat discharge pump, which may cause cavitation.

[0022] Step 5: Install a temporary Pitot tube and confirm the actual liquid level in the primary circuit, ensuring that it meets the liquid level requirements of the manhole on the primary side of the steam generator and the plugs in the hot and cold sections of the primary circuit. Step Six: Implement risk management measures for the containment closure capability during half-pipe operation; maintain the emergency closure capability of the equipment gates of the containment during half-pipe operation, ensure that at least one of the personnel gates of the containment is closed, and ensure that the mechanical or electrical penetrations of the containment are capable of closure, so as to ensure the integrity of the third radioactive barrier in the worst-case scenario and ensure that radioactive materials do not leak out.

[0023] Step 6.1: Ensure that the containment equipment gates are closed or in an emergency-closeable state; Step 6.2: Ensure that at least one of the containment personnel gates is closed; Step 6.3: Ensure that the mechanical or electrical penetrations of the containment are in a closable position.

[0024] Step 7: Implement reactivity control risk management measures for semi-pipe operation, specifically including confirming the isolation of non-borized water sources, avoiding accidental dilution in the primary loop, ensuring sufficient shutdown depth and availability of relevant source range neutron detectors.

[0025] Step 8: Implement risk management measures to control the residual heat discharge capacity during half-pipe operation. Taking a 1000 MW nuclear power unit as an example, the water volume in the pressure vessel during half-pipe operation is about 80 m³. If the liquid level reaches half-pipe 3-4 days after shutdown, the core decay heat is about 12 MW. If the core completely loses cooling and water replenishment during half-pipe operation, the primary coolant in the pressure vessel will reach saturation temperature in about 30 minutes. Therefore, it is necessary to implement risk management measures to control the residual heat discharge capacity.

[0026] Step 8.1: Keep the reactor's two-stage equipment cooling water and two-stage seawater systems available, and ensure that the relevant instruments characterizing residual heat removal capacity and core undercooling are available; Step 8.2: Adjust the normal operating flow rate of the waste heat discharge pump to ensure the net positive suction head of the waste heat discharge pump and avoid pump cavitation; Step 8.3: Adjust the temperature control valve of the normal waste heat discharge pump to control the average temperature of the primary circuit to be below 71℃; Step Nine: Implement power availability risk management measures for partial operation, including: keeping the backup diesel generator available, keeping the priority power supply or auxiliary power supply of the corresponding power supply bus for the waste heat discharge pump, equipment cooling water pump, and seawater pump available, suspending the power switching operation of the main transformer, plant transformer, and backup transformer, and ensuring that there is backup power support in case of accidental power loss of related equipment.

[0027] Step 10: Implement risk management measures to control the water volume in the primary loop during half-pipe operation; Step 10.1: Ensure the primary coolant level is precisely controlled at the half-pipe operating level. Control the primary coolant level using the primary hot section level gauge. Consider using a temporary Pitot tube to further verify the actual primary coolant level using the interconnect principle. During half-pipe operation, the primary coolant level must be precisely controlled. The primary coolant level should not be too low, otherwise insufficient positive suction head of the normal residual heat discharge pump may cause pump cavitation, potentially leading to core cooling failure. Simultaneously, the primary coolant level should not be too high; the primary coolant condensate level must be lower than the steam generator primary side manhole level and the primary coolant cold / hot section plug level, otherwise overflow may occur. During half-pipe operation, the primary coolant level must be precisely controlled to prevent accidental injection of other water sources into the primary coolant or abnormal rise in the primary coolant level due to temperature increase and expansion, which could lead to water leakage or radioactive contamination of personnel. Step 10.2: Keep multiple primary loop water replenishment paths available so that water can be replenished quickly when the primary loop liquid level drops abnormally; Step 10.3: Control the planning of work that may cause changes in the primary coolant charge, stagger the window period for the above work, and control personnel entering and leaving the containment. Step 10.4: Conduct primary loop coolant leakage monitoring, strengthen on-site inspections, and enhance monitoring of key parameters such as primary loop coolant level and containment sump coolant level.

[0028] Step 11: Complete the sealing plates for the cold and hot sections of the reactor primary loop. After the sealing plates are completed, the inspection of the steam generator heat transfer tubes can begin. Step 12: Fill the reactor primary loop with water to 0.3 meters below the pressure vessel flange, and then discontinue half-pipe operation; Step 13: Open the reactor pressure vessel; Step Fourteen: Fill the refueling tank with water; Step 15: Unload the reactor core.

[0029] In this patented operating method, the pressure vessel opening is performed simultaneously with the primary circuit liquid level being lowered from 0.3 meters below the pressure vessel flange to half-pipe level. The primary circuit dredging to half-pipe level itself does not occupy the critical path; that is, the main workload of opening the pressure vessel is in preparatory work such as bolt tensioning. The action of lifting the pressure vessel top cover is performed when the primary circuit is filled with water to 0.3 meters below the pressure vessel flange, thus shortening the process time. Secondly, because the primary circuit hot and cold sections are blocked at the half-pipe level, the steam generator heat transfer tube inspection can be performed simultaneously with the refueling pool filling to full and the core unloading, significantly advancing its start time. Taking a 1,000 kW nuclear power unit as an example, according to the general method of unit descent, if the primary circuit dredging to 0.3 meters below the pressure vessel flange is taken as the zero point, then the steam generator heat transfer tube inspection can be carried out after approximately 100 hours; according to the half-pipe operation method, the steam generator heat transfer tube inspection can be carried out after approximately 16 hours.

[0030] like Figure 2 As shown, the half-pipe operation scheme mentioned in this plan saves the critical path during overhaul. However, compared with the general process, when the primary coolant level drops to the half-pipe level, the reactor core does not have spent fuel assemblies, and there is no need to consider the continuous removal of core decay heat, resulting in relatively lower risk. During the half-pipe operation, due to the primary coolant level being drained to a lower level while the reactor core has spent fuel assemblies, risk management needs to be implemented in the following aspects during its approximately 13-hour operation: primary coolant level control, residual heat removal capacity control, power availability, reactivity control, and containment shutdown capability. Specific risk management measures are detailed in steps six through ten.

[0031] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A method for operating a partially-operated nuclear power unit during a major overhaul, characterized in that, The method includes: Step 1: Shut down the reactor and open the primary loop of the reactor; Step 2: The waste heat removal system normally cools the reactor primary loop, maintaining continuous cooling of the primary loop; Step 3: Drain the reactor primary circuit to bring the water level in the reactor primary circuit below the pressure vessel flange face; Step 4: Drain the reactor primary loop to half-pipe operating level, and simultaneously carry out pressure vessel bolt tensioning and opening preparation work; Step 5: Confirm the actual liquid level in the primary circuit, and confirm that it meets the liquid level requirements of the manhole on the primary side of the steam generator and the plugs in the hot and cold sections of the primary circuit; Step Six: Implement risk management measures for the containment closure capability during partial operation; Step 7: Implement reactive control and risk management measures for semi-pipeline operation; Step 8: Implement risk management measures to control the waste heat discharge capacity during half-pipe operation; Step 9: Implement power availability risk management measures for partial-pipe operation; Step 10: Implement risk management measures to control the water volume in the primary loop during half-pipe operation; Step 11: Complete the sealing plates for the cold and hot sections of the reactor primary loop. After the sealing plates are completed, the inspection of the steam generator heat transfer tubes can begin. Step 12: Fill the reactor primary loop with water to below the pressure vessel flange face, and then discontinue half-pipe operation; Step 13: Open the reactor pressure vessel; Step Fourteen: Fill the refueling tank with water; Step 15: Unload the reactor core.

2. The method for semi-pipe operation during a nuclear power unit overhaul according to claim 1, characterized in that, Step one specifically includes: Step 1.1: Reduce the power output of the nuclear power unit; Step 1.2: Reactor shutdown; Step 1.3: Cooling and depressurizing the reactor primary loop; Step 1.4: Open the reactor primary circuit boundary.

3. A method for semi-pipe operation during a nuclear power unit overhaul according to claim 2, characterized in that, In step three, the water level in the reactor primary circuit is brought down to 0.3 meters below the pressure vessel flange face; in step twelve, the water level in the reactor primary circuit is brought down to 0.3 meters below the pressure vessel flange face.

4. A method for operating a partially-operated nuclear power unit during a major overhaul, as described in claim 3, is characterized in that... In step four, the liquid level in the half-pipe is 70%-80% of the liquid level in the primary loop hot section.

5. A method for operating a nuclear power unit in half-pipe mode during a major overhaul, as described in claim 4, is characterized in that... The risk management measures for the partial-pipeline operation containment closure capability adopted in step six specifically include: Step 6.1: Ensure that the containment equipment gates are closed or in an emergency-closeable state; Step 6.2: Ensure that at least one of the containment personnel gates is closed; Step 6.3: Ensure that the mechanical or electrical penetrations of the containment are in a closable position.

6. A method for semi-pipe operation during a nuclear power unit overhaul according to claim 5, characterized in that, In step seven, the risk management measure for the reactive control of the semi-pipe operation is to implement the isolation of the primary loop non-borized water source.

7. A method for semi-pipe operation during a nuclear power unit overhaul according to claim 6, characterized in that, Step eight involves implementing risk management measures to control the waste heat discharge capacity during half-pipe operation, specifically including: Step 8.1: Keep the reactor's two-stage equipment cooling water and two-stage seawater systems available, and ensure that the relevant instruments characterizing residual heat removal capacity and core undercooling are available; Step 8.2: Adjust the normal operating flow rate of the waste heat discharge pump to ensure the net positive suction head of the waste heat discharge pump; Step 8.3: Adjust the temperature control valve of the normal waste heat discharge pump to control the average temperature of the primary circuit to be below 71℃; According to claim 7, a method for semi-pipe operation during a nuclear power unit overhaul is characterized in that step nine adopts risk control measures for the availability of power supply during semi-pipe operation, specifically including: keeping the standby diesel generator available, keeping the priority power supply or auxiliary power supply of the corresponding power supply bus of the waste heat discharge pump, equipment cooling water pump, and seawater pump available, suspending the power switching operation of the main transformer, plant transformer, and standby transformer, and ensuring that there is backup power support after the equipment power supply is unexpectedly lost.

8. A method for semi-pipe operation during a nuclear power unit overhaul according to claim 8, characterized in that, Step ten involves implementing risk management measures for controlling the water volume in the first loop during half-pipe operation, specifically including: Step 10.1: Ensure the primary loop liquid level is precisely controlled at the half-pipe operating level; Step 10.2: Keep multiple primary loop water replenishment paths available so that water can be replenished quickly when the primary loop liquid level drops abnormally; Step 10.3: Control the work that causes changes in the primary coolant charge in the planning and control personnel entering and leaving the containment. Step 10.4: Conduct primary loop coolant leakage monitoring.

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