JPH0241716B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is capable of tripping or running back the reactor recirculation pump and providing the necessary The present invention relates to a power control device for a boiling water reactor that enables continuous low-power operation of the reactor by urgently inserting all or part of the selected control rods in accordance with the situation.

In general, boiling water nuclear power plants with large-capacity turbine bypass valves are designed to continue isolated operation in the event of an off-site power system accident or turbine trip. At the same time, the reactor recirculation pump is tripped or runback, pre-selected control rods are urgently inserted into the reactor core to reduce reactor power, and the turbine bypass valve is suddenly opened to reduce the steam generated from the reactor. By releasing the reactor to the condenser, continuous low-power operation of the reactor is possible.

When operating a nuclear reactor continuously at low power in this way,
The turbine is supplied with a steam flow rate of approximately 10% of its rated capacity to cover the power required within the power plant.
In this state of low steam flow, the feedwater heater, which heats the feedwater extracted from the turbine, cannot perform its original function, and the water cooled by the condenser is supplied to the reactor without being heated. will be done. If this happens, the cooling water temperature at the core inlet will drop, the void ratio inside the reactor will decrease, and the core thermal output will gradually increase, making it impossible to continue low-power operation.
In order to prevent this problem, a pre-selected control rod is urgently inserted. This will be explained with reference to FIG. Figure 1 shows the change in core thermal output over time when the reactor is shifted to low-power operation.The vertical axis shows the core thermal output as a percentage of the rated value, and the horizontal axis shows the passage of time. There is. Curve A in Figure 1 shows the case where the reactor recirculation pump is run back from the initial state of 100% core thermal output (relative to rated) and core flow rate 100% (relative to rated), and curve B ₁ is Core thermal output 100
%, from the initial state of core flow rate 100%, curve B ₂ is from the initial state of core thermal power 80% and core flow rate 100%,
Curve _B3 shows the case where the reactor recirculation pump is runback from the initial state of 60% core thermal output and 100% core flow rate, and selective control rods are inserted. Note that the number of inserted selection control rods for curves B ₁ to _{B 3} is the same.
A comparison of these curves reveals the following points.

As shown in curve A, when only the reactor recirculation pump is runback and no selective control rods are inserted, the core thermal output gradually increases.
Although it becomes difficult to continue low-power operation, if the selective insertion rods are inserted, the increase in core thermal output can be suppressed to a low level as shown by curves B ₁ to B ₃ , making it possible to continue low-power operation.

A comparison of curves B ₁ to _{B 3} shows that when the number of selected control rods is the same, the reactor settling output differs significantly depending on the initial state. If the reactor settling power takes on a wide range of values, it becomes difficult to stably control the reactor power. Therefore, it is desirable to maintain the reactor settling output within the range regardless of the initial state.

On the other hand, if the power is increased too much during partial power operation of the nuclear reactor, the stability of the core will be impaired and the condenser will not be able to perform the bypass steam processing function. Furthermore, in order to shorten and smooth the time required to restore the initial state from the partial power operating state, it is desirable to maintain the reactor settling power at a high level of about 50%.

The present invention has been made based on the above considerations, and its purpose is to provide a power control system for a boiling water reactor that shifts the reactor to partial power operation when a cause for reducing the reactor output occurs. , it is possible to stably control the reactor output after transitioning to partial power operation, cover the necessary power within the power plant, and shorten and smooth the time when restoring from partial power operation to the initial state. It's about doing.

The configuration of the present invention will be explained below based on the embodiment shown in FIG.

Steam generated in a reactor core 2 within a reactor pressure vessel 1 passes through a main steam pipe 3 and is supplied to a turbine 5 via a main steam control valve 4 to drive the turbine 5. The rotation of the turbine 5 drives a generator 6 to generate electricity, and this electrical energy is sent to a power transmission system 9 via a main transformer 7 and a main circuit breaker 8 . On the other hand, the steam that drove the turbine 5 is condensed into water in the condenser 10, and is sent to the feed water heater 11 by a condensate pump (not shown), where it is heated by the steam extracted from the turbine 5, and the water is It is recycled into the reactor pressure vessel 1 by the pump 12.

On the other hand, a turbine bypass pipe 13 branched from the middle of the main steam pipe 3 communicates with the condenser 10, and a part of the bypass pipe 13 bypasses the turbine 5 and is connected to the condenser 10. A turbine bypass valve 14 is provided to vent steam. In addition, 15 in the figure is a reactor recirculation pump that recirculates the coolant in the reactor pressure vessel 1 through the reactor core 2, and 16... are control rods (not shown) for the reactor core 2.
This is a control rod drive mechanism that controls the reactivity within the reactor core 2 by inserting or withdrawing the rods. Reference numeral 17 in the figure is a turbine/pressure control circuit that controls the rotational speed of the turbine 5, the pressure of main steam, etc., and this circuit includes a control section 18 and an unbalance relay 19. Then, the signal from the generator 6 is input to the unbalance relay 19, and when a failure in the power transmission system 9 or any other cause that requires a reduction in the reactor output occurs, this is used as an output reduction request signal to the unbalance relay 19.
The main steam control valve 4 is suddenly closed by the output from the control unit 18, and the turbine 5 is
At the same time, the turbine bypass valve 14 is suddenly opened to release excess steam to the condenser 10. In addition, the control section 1
8 is sent to the feedwater pump control circuit 20 to stop some of the plurality of feedwater pumps 12 in response to the partial power operation of the nuclear reactor, thereby controlling the feedwater flow rate. Further, the signal from the control section 18 is sent to the output control circuit 21 together with the valve opening signal from the turbine bypass valve 14. Further, the reactor pressure vessel 1 is provided with a reactor status monitoring circuit 22, and this monitoring circuit 22 sends reactor status signals corresponding to the reactor status, such as the thermal output of the reactor core 2, to the output control circuit 21. I'm trying to send it out.
Then, the signals from the output control circuit 21 are sent to the selection control insertion circuit 23, the emergency stop control rod insertion circuit 24, and the recirculation pump control circuit 25, respectively.
The control rod drive mechanism 16 is controlled via the control rod insertion circuit 23 or the emergency stop control rod insertion circuit 24, and the reactor recirculation pump 15 is controlled via the recirculation pump control circuit 25, thereby controlling the reactor. is configured to shift to partial power operation. Further, the reactor pressure vessel 1 is provided with a reactor water level gauge 26 for measuring the water level inside the vessel 1.
It is configured to send a signal from the control rod to the emergency stop control rod insertion circuit 24.

In the boiling water nuclear power plant configured as described above, for example, if some kind of failure occurs in the power transmission system 9 or the generator 6, the main circuit breaker 8 and the main transformer 7 are opened, and the turbine/pressure control circuit 17 is disconnected. A load cutoff signal as an output reduction request signal is input to the balance relay 19. The above signal is the control unit 1
8 to the main steam control valve 4 and the turbine bypass valve 14, the main steam control valve 4 is temporarily closed quickly to prevent the turbine 5 from overspeeding, and the turbine bypass valve 14 is suddenly opened. Therefore, the steam sent from the reactor pressure vessel 1 to the turbine 5 is quickly shut off by the main steam control valve 4 and is sent to the condenser 10 via the turbine bypass valve 14. On the other hand, the power control circuit 21 receives the core thermal output and the core flow rate as reactor status signals, and trips or runs back the reactor recirculation pump 15 via the recirculation pump control circuit 25, and simultaneously generates a load shedding signal. It determines whether it is necessary to insert selected control rods selected in advance according to the reactor state at the time of the operation, and if insertion is necessary, it further determines the number of selected control rods to be inserted and selects the signal. The control rods are sent to the control rod insertion circuit 23, and the required number of control rods are urgently inserted into the reactor core 2, thereby reducing the reactor output. Note that if the open signal of the turbine bypass valve 14 cannot be confirmed here, the output control circuit 21 sends a signal to the emergency stop control rod insertion circuit 24 to make an emergency stop of all the control rods in the reactor core 2. Shut down the reactor. Further, the water supply pump control circuit 20 includes the turbine/pressure control circuit 1
In response to the signal from 7, multiple water supply pumps 12...
stop some of the When the rotation speed of the turbine 5 becomes stable, the opening degrees of the main steam control valve 4 and the turbine bypass valve 14 are automatically adjusted by the turbine/pressure control circuit 17, and the reactor is shifted to partial power operation.

For example, when the water level in the reactor pressure vessel 1 drops to a set value, the reactor water level gauge 26 detects this and sends a low water level signal to the emergency shutdown control rod insertion circuit 24. All control rods are urgently inserted into the reactor core 2 to stop the reactor operation and ensure the safety of the reactor.

In this embodiment, the reactor steam output and the rotation speed of the reactor recirculation pump 15 are used as the reactor status signal for operating the output control circuit 21. However, the reactor status monitoring circuit 22
The output of the reactor may be used as the reactor status signal.

Next, the operation of the above embodiment will be explained with reference to the nuclear reactor operating characteristic diagram shown in FIG.

The vertical axis of FIG. 3 shows the core thermal output as a percentage of the rated value, and the horizontal axis shows the core flow rate as a percentage of the rated value. In the figure, curves S ₁ to _{S 2} are for natural circulation, and curves MP ₁ to _{S 2} are for the initial state of thermal output 105% and core flow rate 100% (MP ₁ point).
This is the flow rate control curve when the core flow rate is decreased as follows. In addition, curves MP ₂ to _{C 1} indicate the minimum flow rate operation of the reactor recirculation pump;
C ₀ to _{C 1} are flow rate control curves when the core flow rate is decreased with the initial state (C ₀ point) being 100% thermal output and 100% core flow rate. Further curve C ₀ ~ _{CV 2}
indicates the rated operation of the reactor recirculation pump, and the curves CV ₁ to _{CV 2} are cavitation lines. If the heat output falls below the cavitation line, cavitation of the jet pump will occur and it must be operated above this cavitation line. i.e. curve
The area between MP ₁ to _{S 2} and cavitation lines CV ₁ to _{CV 2} is the permissible operating area for the reactor, and control rod insertion control and reactor recirculation pump control must be within this area. It is necessary to control the flow rate.

Therefore, for example, if the turbine bypass valve 14 suddenly opens and the reactor recirculation pump 15 shifts to minimum flow operation with point _C0 as the initial state, the relationship between core thermal output and core flow rate moves to point _C1 . , After that, when the core inlet temperature decreases, the thermal output increases and attempts to move beyond the operation permissible region to the _C2 point. However, with the insertion of the selective control rod, the thermal output decreases and settles at point _C3 .

Furthermore, if the number of selective control rods to be inserted is constant regardless of the initial state, the following problems will occur. For example, if the initial state is at point D ₀ and the same number of selected control rods are inserted as when the initial state is at point C ₀ , the relationship between thermal output and core flow rate will be from point D ₀ to curve C ₀ ~ Point _D1 is reached along the flow control curve that is almost parallel to _C1 , and the thermal output decreases further and settles at point _D3 , which is below the operational tolerance area, and at such a power level, the thermal output Since the steam output is smaller, it is not possible to supply the necessary electricity within the power plant (in other words, it is not possible to secure a steam flow rate of 10% of the rated value), and there is no hope for recovery of the reactor output. . For this reason, it is necessary to vary the number of selective control rods inserted depending on the initial state so that partial power operation is performed at a power level that is convenient for restoring reactor power as much as possible. It is.

Therefore, in the output control circuit 21, the necessity of insertion of selected control rods, the number of insertions, etc. are determined as follows. First, the core thermal output P ₀ and the core flow rate (or the rotation speed of the reactor recirculation pump 15, the same applies hereinafter) in the initial state of the reactor.
Input W ₀ as the reactor status signal and calculate b=Pe−aW ₀ . However, a is a constant, which corresponds to the slope of the flow rate control curves such as MP ₁ to S ₂ and C ₀ to _{C 1} in FIG. 3, and is known to be approximately linearly approximated. Then, using this value of b, a predetermined value c is set in anticipation of an increase in core thermal output due to a decrease in core inlet temperature after the reactor recirculation pump 15 is runback, and when b≦c, In this case, the selective control rods are not inserted, and the reactor recirculation pump 15 is simply turned back to the minimum flow rate operation. In addition, if b>c, the reactor recirculation pump 15 is shifted to the minimum flow rate operation, and selective control rods are inserted, and the number of inserted control rods is increased in stages according to the value of b. I decided to. The classification of the number of inserted tubes is illustrated on the reactor operating characteristics diagram in FIG. 4. This defines the allowable reactor operating range between the flow control curve C ₀ ~ C ₁ and the cavitation line CV ₁ ~ CV ₂ ,
Five regions "I" to "V" are defined by the selected control rod reactivity division lines l ₁ , l ₂ , _{l 3} and the selected control rod insertion determination line l ₄ which are almost parallel to the flow rate control curve C ₀ to C 1 above. divided into
From the top to the bottom, when the initial state is in the "I" region, the number of selected control rods to be inserted is 16,
Hereinafter, in the areas "", "", "", and "", there will be 12, 8, 4, and 0 lines, respectively. That is, assuming that the core thermal output is P and the core flow rate is W, the selected control rod reactivity division lines l ₁ , l ₂ , and l ₃ are respectively d ₁ =P-aW _d2 =P-aW _d3 =P- The selection control rod insertion determination line _l4 can be approximated as c=P-aW. Therefore, if the value of b is b > d ₁ , the number of selective control rods inserted is 16, and if the value of b is d ₁ ≧ b > d ₂ , the number of selective control rods to be inserted is 12. , if the value of b is d ₂ ≧ b > d ₃ , the number of selected control rods to be inserted is 8, and if the value of b is d ₃ ≧ b > c, the number of selected control rods to be inserted is 4, If the value of b is b≦c, do not insert the selection control rod. Note that the classification of the number of insertions is not limited to the one shown in Figure 4; for example, when the reactor output is controlled within the range of maximum 50% rating and minimum 10% rating when the turbine bypass valve suddenly opens, it can be reduced by having the classification of the number of insertions. It's okay.

Furthermore, in addition to varying the number of selective control rods inserted, the insertion positions of the selective control rods may be changed, or the amount of insertion of each selective control rod may be varied. Furthermore, although the arrangement of the selection control rods may be set as appropriate, examples of arrangement patterns of the selection control rods are shown in FIGS. 5 and 6.

In FIGS. 5 and 6, 27... indicates a unit cell, 28... indicates control rods, and the control rods surrounded by circles are selected control rods selected in advance. Control rods with numbers indicate those inserted into the reactor core 2 during operation, and the numbers are the number of notches representing the amount of insertion. Here, if the selected control rod is selected from among the control rods at the outermost circumference as shown in Figure 5,
The control rods at the outermost periphery have a low reactivity value per rod, so it is easy to make minute output adjustments, and the control rods have the advantage of being able to minimize changes in output due to withdrawal, etc. By selecting the selected control rods from among the control rods in The furnace can be put into partial power operation.

By the way, when a nuclear reactor is operating at high output, the main steam control valve 4 is suddenly closed, the turbine bypass valve 14 is suddenly opened, and the reactor recirculation pump 1 is suddenly closed due to the generation of a load cutoff signal.
When the minimum flow rate operation and the insertion of selective control rods in step 5 are performed, the core thermal output rapidly decreases. However, in the unlikely event that the turbine bypass valve 14 fails to open, the pressure in the reactor core will rise rapidly, and the minimum critical fuel power ratio (MCPR), which is a thermal limit for the fuel, will become severe. In such a case, the output control circuit 21 confirms that the turbine bypass valve 14 does not open, and then sends a scram signal to the emergency stop control rod insertion circuit 24 to emergencyly insert all control rods into the reactor core 2. However, since a time delay is required to confirm the opening operation of the turbine bypass valve 14, the thermal restriction of the fuel becomes stricter by the amount of the delay time. Therefore, during high output operation where the output level in the initial state is 80% or more of the rated output, when the output control circuit 21 receives a load cutoff signal from the turbine/pressure control circuit 17, it simultaneously sends a trip signal to the recirculation pump control circuit 25. An insertion signal is sent to the selective control rod insertion circuit 23 to trip the reactor recirculation pump 15 and rapidly reduce the recirculation flow rate. This is to improve the decrease in the minimum critical fuel output ratio (MCPR) when the turbine bypass valve 14 is not operating, and to improve the transient characteristics.

In addition, if the reactor recirculation pump 15 is stopped for a long time, the loop temperature of the recirculation pump 15 will rapidly drop, causing thermal stress problems, and low power operation due to natural circulation will reduce the core thermal output. Since this may result in a lack of stability, the reactor recirculation pump 15 is automatically shifted to the minimum flow rate operation after, for example, about 6 minutes after the turbine bypass valve 14 is suddenly opened and the reactor core is almost stabilized. This year,
This ensures the integrity of the reactor recirculation pump 15 and maintains stability during low power operation of the reactor.

Furthermore, if the reactor power is, for example, 20% or less of the rated power, even if the power increases due to a drop in the core inlet temperature, the power will be suppressed to a level that does not impair the stability of the reactor core, allowing reactor recirculation. pump 15
There is no need to run back or trip. If the recirculation pump 15 is runback or tripped, the number of steps required to increase the reactor output will be increased when the failure is corrected. Therefore, by configuring the output control circuit 21 so as not to send a signal to the recirculation pump control circuit 25 and the selective control rod insertion circuit 23 even if the output control circuit 21 receives a load cutoff signal at a low output level, it is possible to control the reactor at a low output level. Complicated procedures for pressure build-up can be avoided.

As described above based on the embodiments, the boiling water reactor power control device according to the present invention provides power to the reactor core by inserting selective control rods when a cause for reducing reactor power occurs. By changing the negative reactivity according to the reactor status at that time, the reactor output after transition to partial power operation can be stably maintained within a narrow range of low output, and the required power within the power plant can be covered. Excellent effects can be obtained, such as shortening and smoothing the time required to restore the initial state from partial output operation.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the temporal change in core thermal output during minimum flow operation of the reactor recirculation pump, Fig. 2 is a schematic configuration diagram showing an embodiment of the present invention, and Fig. 3
4 and 4 are reactor operating characteristics diagrams for explaining the effects of the present invention, and FIGS. 5 and 6 are schematic diagrams of the reactor core showing examples of arrangement patterns of selective control rods in one embodiment of the present invention. . DESCRIPTION OF SYMBOLS 1...Reactor pressure vessel, 2...Reactor core, 4...Main steam control valve, 5...Turbine, 14...Turbine bypass valve, 15...Reactor recirculation pump, 16
... Control rod drive mechanism, 17 ... Turbine/pressure control circuit, 21 ... Output control circuit, 22 ... Reactor current status monitoring circuit, 23 ... Selective control rod insertion circuit, 2
5...Recirculation pump control circuit.

Claims (1)

[Claims] 1 When a cause that requires a reduction in reactor power occurs, the turbine bypass valve is opened to release steam to the condenser, and the pre-selected control rods are inserted into the reactor core and the reactor core flow rate is adjusted to restart the reactor. In a power control system for a boiling water reactor that is to be transitioned to partial power operation, the selected control rods are inserted into multiple regions divided by selected control rod reactivity division lines that are approximately parallel to the core flow control curve. The negative reactivity given is set in advance to decrease stepwise from the upper part to the lower part of the region, and is selected below the selected control rod reactivity division line and approximately parallel to the core flow control curve. Set a control rod insertion determination line, receive a reactor status signal corresponding to the reactor core thermal output and core flow rate when the above causes occur, and determine whether this reactor status signal falls within any of the above ranges. An output control device for a boiling water nuclear reactor, comprising an output control means for determining the negative reactivity given to the reactor core by inserting the selected control rod.