Response method for tripping of single main feed pump under full-power working condition of nuclear power unit

Abstract

The invention relates to the technical field of nuclear power control, in particular to a response method for tripping of a single main feed pump under the full-power working condition of a nuclear power unit. The method comprises the steps that during power operation, operation parameters and states of subsystems and equipment of a power plant are continuously monitored, and when it is monitored that a reactor operates at high power and a single main water feeding pump of a main water feeding system trips, a steam turbine load shedding signal is triggered, and meanwhile a rod falling signal is generated; hard wiring is added between the control rod cabinet and the rod controller, and after a rod falling signal is received, pin claw coils of the corresponding pre-selected control rod group are all de-energized, so that the pre-selected control rod group quickly falls into the reactor bottom under the action of gravity, and the reactor power is reduced to PL. According to the method, the power of the steam turbine is rapidly reduced, and the predetermined control rod group is rapidly inserted into the reactor core, so that the nuclear power is rapidly reduced, the power balance of the first loop and the second loop is maintained, the SG liquid level is not too low, and shutdown is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of nuclear power control, and in particular to a response method for the tripping of a single main feedwater pump under full-power operating conditions of a nuclear power unit. Background Art

[0002] The main feedwater pump (MFWP) in a nuclear power plant is a crucial component of the secondary steam-water cycle. Nuclear power plants typically utilize multiple MFWPs operating in parallel to boost the secondary feedwater pressure and deliver it to the nuclear island steam generators (SGs) to maintain the reactor hot well fluid level. With improved equipment reliability and to reduce power plant manufacturing costs, some nuclear power plants lack backup MFWPs. For example, a currently operating passive advanced pressurized water reactor (PWR) (Type A) has 3 × 33.3% capacity MFWPs, with no backup pumps.

[0003] Although main feedwater pumps (MFWPs) boast high operational reliability, the possibility of a single MFWP tripping due to factors such as malfunctioning protection signals and human error must still be considered during actual operation. During normal power operation, a single MFWP trip can cause a significant drop in feedwater flow and a rapid decrease in the SG level. If timely action is not taken, the reactor could shut down due to low SG levels, potentially triggering the passive core cooling system and triggering a primary circuit overcooling safety injection signal, resulting in serious consequences. Therefore, nuclear power unit designs incorporate automatic response measures in the event of a single MFWP trip.

[0004] Taking the A-type unit as an example, when the reactor power is higher than 70% FP, if a main feedwater pump trip transient occurs, according to the unit logic design, the turbine will shed load at a rate of 100% / min to 70% of the rated load. During the load shedding process, the power of the primary and secondary circuits will be mismatched, and the temperature difference between the primary and secondary circuits will exceed the control band of the control rods and be inserted downward, thereby reducing the nuclear power. The steam bypass discharge system will open according to the temperature difference to discharge the core heat, the SG water level will drop, and the main feedwater regulating valve will automatically open to increase the SG water supply.

[0005] According to the design and simulation test results of Reactor Type A, if a single main feedwater pump trips at 100% power at the beginning of its lifecycle, the minimum SG NR (narrow range) liquid level will be around 26% to 27%, only 5% above the automatic shutdown setting. As the lifecycle increases, the minimum liquid level will further decrease to around 24% before gradually recovering, ultimately reaching around 31%. Actual operating experience shows that the actual contraction of the SG liquid level under transient conditions is more severe than the simulation. Based on experience with similar units, manual shutdown at SG NR level of 26% causes the passive residual heat removal heat exchanger to operate due to the contraction of the SG water level. Forced circulation by the main pump causes the primary cold-leg temperature to reach a value lower than the set value, resulting in the false triggering of the safety injection.

[0006] When the main feedwater pump of the reactor trips under high-power conditions, the main reason why the SG water level is challenged is that after the turbine quickly reduces the load to the target value, the control rod is limited in the insertion speed, and the nuclear power cannot be reduced to near the target value in a short period of time, resulting in a large temperature difference between the first and second circuits, causing the steam bypass valve to open, causing steam to be discharged to the condenser, causing the SG water level to drop. The main feedwater regulating valve is fully open about 5 seconds after the start of the transient, and the unit loses further automatic adjustment means.

[0007] Therefore, it is necessary to study and improve the response logic of similar units to avoid unplanned shutdown events and more serious conditions when a single main feedwater pump trips under full power conditions. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a response method for the tripping of a single main feedwater pump of a nuclear power unit under full-power conditions. In addition to quickly reducing the power of the turbine, the method also rapidly reduces the nuclear power by quickly inserting a predetermined control rod group into the reactor core, maintaining the power balance between the primary and secondary circuits, and preventing the SG liquid level from being too low, thereby avoiding a shutdown.

[0009] The present invention provides a response method for a single main feedwater pump tripping under full power conditions of a nuclear power unit, comprising the following steps:

[0010] Step 1: During power operation, the operating parameters and status of each subsystem and equipment of the power plant are continuously monitored. When it is detected that the reactor is operating at high power and a single main feedwater pump in the main feedwater system has tripped, a turbine load rejection signal is triggered and a rod drop signal is generated simultaneously;

[0011] Step 2: Add hard wiring between the control rod cabinet and the rod controller. After receiving the rod drop signal, all the pin coils of the corresponding pre-selected control rod group are de-energized, causing the pre-selected control rod group to fall quickly to the bottom of the reactor under the action of gravity, reducing the reactor power to P L .

[0012] In a specific embodiment of the present invention, in step 1,

[0013] The generated turbine load rejection signal causes the turbine control system to exit the load tracking mode, close or reduce the opening of the steam inlet regulating valves of different cylinders of the turbine, reduce the amount of steam entering the turbine, and enable the turbine generator to run at the rate R m Reduce turbine power to F L ;

[0014] F L is a fixed value, Where N is the number of main feed water pumps running at full power.

[0015] In one embodiment of the present invention, the F 裕量Usually 3% to 5% FP is taken.

[0016] In a specific embodiment of the present invention, the cylinder includes a high-pressure cylinder, a medium-pressure cylinder and a low-pressure cylinder.

[0017] In a specific embodiment of the present invention, the opening of the steam inlet regulating valve of any one of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder is closed or reduced; or the steam inlet regulating valves of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder are closed or reduced at the same time; or the opening of the steam inlet regulating valves of any two of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder are closed or reduced.

[0018] In one embodiment of the present invention, the R m It is 100%-150% FP / min.

[0019] In a specific embodiment of the present invention, in step 2, the conditions for preselecting the control rod group are:

[0020] The integral value is Δρ R =-(F H -F L +ΔF)×α P ;

[0021] The sequence of movements is adjusted regularly so that fuel consumption and wear levels are similar.

[0022] In one embodiment of the present invention, the ΔF is 0-15% FP.

[0023] In one embodiment of the present invention, the preselected control rod group is low-value control rods.

[0024] In a specific embodiment of the present invention, the preselected control rod group is located at the top of the stack.

[0025] Compared to the prior art, the present invention's response method for a single main feedwater pump tripping in a nuclear power unit operating at full power adjusts the post-trip response logic when a main feedwater pump trips, rapidly reducing both turbine power and nuclear power, maintaining power balance between the primary and secondary circuits, and preventing excessively low steam generator liquid levels, thereby avoiding reactor shutdown. Simulations have shown that when a single main feedwater pump trips at full power, the steam generator liquid level will not fall below 42%, leaving a significant margin from the trip threshold that triggers a low steam generator liquid level. The reactor's minimum power margin decreases only slightly, maintaining a relatively flat core distribution, and allowing the reactor to continue operating safely. This effectively avoids automatic emergency shutdowns and the activation of dedicated safety features. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram showing the automatic response logic after the main feedwater pump trips. DETAILED DESCRIPTION

[0027] In order to further understand the present invention, embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than for limiting the present invention.

[0028] An embodiment of the present invention discloses a method for responding to a trip of a single main feedwater pump of a nuclear power unit under full power operating conditions, comprising the following steps:

[0029] Step 1: During power operation, the operating parameters and status of each subsystem and equipment of the power plant are continuously monitored. When it is detected that the reactor is operating at high power and a single main feedwater pump in the main feedwater system has tripped, a turbine load rejection signal is triggered and a rod drop signal is generated simultaneously;

[0030] The generated turbine load rejection signal causes the turbine control system to exit the load tracking mode, close or reduce the opening of the steam inlet regulating valves of different cylinders of the turbine, reduce the amount of steam entering the turbine, and enable the turbine generator to run at the rate R m Reduce turbine power to F L ;

[0031] F L It is a fixed value, which mainly considers the water supply capacity of the remaining main water supply pump group after the loss of one main water supply pump. Where N is the number of main feed water pumps running at full power, F 裕量 Usually 3% to 5% FP is taken.

[0032] The cylinders of the steam turbine include high-pressure cylinder, medium-pressure cylinder and low-pressure cylinder;

[0033] The steam inlet regulating valve opening of any one of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder can be closed or reduced, or the steam inlet regulating valves of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder can be closed or reduced at the same time; the steam inlet regulating valve opening of any two of the high-pressure cylinder, the medium-pressure cylinder and the low-pressure cylinder can also be closed or reduced;

[0034] During the process of a main feedwater pump trip triggering a rapid turbine load shedding and a rapid load reduction in a circuit, in addition to meeting the requirements of P H -P L ≈F H -F L , the load reduction rate of the first and second circuits should be the same as far as possible, that is, the time for dropping the rod This will allow the first and second circuits to reach a new equilibrium state at the same time. m The selection of R needs to consider the turbine control system, the response capability of the turbine regulating valve, and the control capability of the steam generator liquid level in transient state. m It is 100%-150% FP / min.

[0035] As the steam demand in the secondary circuit decreases, the steam generator liquid level control system will immediately close the opening of the steam generator main feed water regulating valve to maintain the stability of the SG liquid level and maintain the water-steam balance, thereby avoiding the risk of shutdown caused by excessively low SG water level in the transient state of a single feed water pump tripping.

[0036] Step 2: Add hard wiring between the control rod cabinet and the rod controller. After receiving the rod drop signal, all the pin coils of the corresponding pre-selected control rod group are de-energized, causing the pre-selected control rod group to fall quickly to the bottom of the reactor under the action of gravity, reducing the reactor power to P L .

[0037] During the process of dropping the preselected control rods, the overall reactivity of the reactor does not change and the reactor is in a critical state.

[0038] The conditions for preselecting the control rod are:

[0039] First, choose the integral value Δρ R =-(F H -F L +ΔF)×α P The control rod group falls into the core, which meets the requirements of the present invention. ΔF is preferably 0-15% FP.

[0040] Second, the sequence of actions of the preselected control rod groups is adjusted periodically so that the burnup and wear levels are similar;

[0041] Specifically, the control rod group operation sequence is divided into rod sequence 1 and rod sequence 2. For rod sequence 1, the rod lifting order is D → C → B → A; for rod sequence 2, the rod lifting order is A → B → C → D. During high-power operation, the rods that are lifted first are the first to reach the top of the stack. That is, for rod sequence 1, group D is the first to reach the top of the stack, and for rod sequence 2, group A is the first to reach the top of the stack. During high-power operation, to maintain sufficient load tracking capability of the control rods, group D or group A must be at the top of the stack, and their integral value is fixed. The positions of the other three groups of rods may vary depending on fuel burnup and changes in core boron concentration, and their integral values ​​are also uncertain. To reduce control difficulty and improve control system reliability, the rod group at the top of the stack is preferred as the preselected rod group.

[0042] The present invention makes changes in hardware, shortens the time for the control rod group to operate, and accelerates the speed of reactor power reduction.

[0043] During normal control rod movement, the standard rod controller issues movement commands, energizing and de-energizing the control rod's various pin coils in a fixed sequence, thereby causing the control rod to move. The speed of control rod movement is limited by the rod control system's response time and the rod's physical structure. Normal rod movement typically does not exceed 80 steps per minute, while the rod's full extension position reaches over 240 steps. Therefore, a normal insertion and full extension of the control rod requires at least three minutes to reach the reactor bottom. However, by adding a hardwired connection between the control rod cabinet and the rod controller, the present invention de-energizes the corresponding control rod pin coils when a rapid power reduction signal is triggered, allowing the control rod to quickly fall to the reactor bottom under the influence of gravity, typically within a few seconds.

[0044] In order to further understand the present invention, the response method for tripping of a single main feedwater pump under full power conditions of a nuclear power unit provided by the present invention is described in detail below in conjunction with the embodiments. The protection scope of the present invention is not limited by the following embodiments.

[0045] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0046] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A response method for a single main feedwater pump tripping under full power conditions of a nuclear power unit, characterized in that: The following steps are involved: Step 1: During power operation, the operating parameters and status of each subsystem and equipment of the power plant are continuously monitored. When it is detected that the reactor is operating at high power and a single main feedwater pump in the main feedwater system has tripped, a turbine load rejection signal is triggered and a rod drop signal is generated simultaneously; Step 2: Add hard wiring between the control rod cabinet and the rod controller. After receiving the rod drop signal, all the pin coils of the corresponding pre-selected control rod group are de-energized, causing the pre-selected control rod group to fall quickly to the bottom of the reactor under the action of gravity, reducing the reactor power to P L .

2. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 1, characterized in that: In the step 1, The generated turbine load rejection signal causes the turbine control system to exit the load tracking mode, close or reduce the opening of the steam inlet regulating valves of different cylinders of the turbine, reduce the amount of steam entering the turbine, and enable the turbine generator to run at the rate R m Reduce turbine power to F L ; F L is a fixed value, Where N is the number of main feed water pumps running at full power.

3. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 2, characterized in that: The F 裕量 Usually 3% to 5% FP is taken.

4. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 2, characterized in that: The cylinders include a high-pressure cylinder, a medium-pressure cylinder and a low-pressure cylinder.

5. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 3, characterized in that: Close or reduce the opening of the steam inlet regulating valve of any one of the high-pressure cylinder, medium-pressure cylinder and low-pressure cylinder; or close or reduce the opening of the steam inlet regulating valve of the high-pressure cylinder, medium-pressure cylinder and low-pressure cylinder at the same time; or close or reduce the opening of the steam inlet regulating valve of any two of the high-pressure cylinder, medium-pressure cylinder and low-pressure cylinder.

6. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 2, characterized in that: The R m It is 100%-150% FP / min.

7. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 1, characterized in that: In step 2, the conditions for preselecting the control rod group are: The integral value is Δρ R =-(F H -F L +ΔF)×α P ; The sequence of movements is adjusted regularly so that fuel consumption and wear levels are similar.

8. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 7, characterized in that: The ΔF is 0-15% FP.

9. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 7, characterized in that: The preselected control rod group is low-value control rods.

10. The method for responding to a trip of a single main feedwater pump under full power conditions of a nuclear power unit according to claim 9, characterized in that: The preselected control rod group is located at the top of the stack.

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