JPH0688898A - Reactor power control equipment - Google Patents

Abstract

PURPOSE:To obtain reactor power control equipment which reduces a burden of an operator and improves the rate of operation of a reactor, by a construction wherein a sudden change of reactivity is suppressed and an unnecessary automatic stop of the reactor is avoided by executing automatic insertion of selected control rods in stages on the occasion of stoppage of a primary loop recirculation pump. CONSTITUTION:In reactor power control equipment which executes power regulation on the basis of a recirculating flow rate of a coolant and the amount of insertion of a control rod, a recirculation pump stoppage detecting means 10 and a selected control rod insertion area determining means 18 which determines the area of insertion of a selected control rod on the basis of a reactor power and the recirculating flow rate, are provided. Moreover, a selected control rod insertion control means 21 which outputs a selected control rod insertion signal 22 on the basis of the logical product of output signals of the recirculation pump stoppage detecting means 10 and the control rod insertion area determining means 18, control rod insertion setting means 29 and 30 which set selected control rods so that they are divided in a number of stages, and a means 23 for inserting the control rods in stages which inserts in stages the control rods set by the control rod insertion setting means 29 and 30, according to the selected control rod insertion signal 22, are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a boiling water reactor in which at least one of the reactor recirculation pumps is stopped and the reactor stability is deteriorated. The present invention relates to a reactor output control device that improves operation and continues operation.

[0002]

2. Description of the Related Art Generally, the output of a boiling water reactor can be controlled by adjusting the insertion position of control rods and the core flow rate of the coolant by adjusting the reactor recirculation pump speed. The characteristics are shown in FIG.

The reactor power-core flow rate characteristic diagram of FIG. 5 shows the reactor power on the vertical axis and the core flow rate on the horizontal axis. Here, the reactor recirculation pump speed is kept constant and control is performed. When the rod is pulled out, the reactor power changes according to the curve 1. However, when the control rod position is kept constant and the reactor recirculation pump speed is increased to increase the core flow rate, the reactor power follows the curve 2. To increase.

Therefore, the operating state of the boiling water reactor is shown in FIG. 5 for the two parameters of the core flow rate and the reactor power.
It can be shown by plotting above. by the way,
The boiling water reactor cannot be operated satisfactorily at any point in FIG. 5, and has various restrictions, one of which is core stability.

The core stability is the convergence of the oscillatory movement of the neutron flux, which is an index of the reactor power. Even if the neutron flux changes due to some disturbance, it converges with an attenuation coefficient of 1 or less. If so, there is no problem as a nuclear reactor, and it is said that the smaller the damping coefficient, the better the core stability. On the other hand, when the attenuation coefficient exceeds 1, in principle, the peak value of the neutron flux gradually increases while exhibiting oscillation.

[0006] This core stability is qualitatively
In the low core flow rate region, it tends to be worse, and in FIG.
In the shaded area 3, the core stability is not good. Therefore, if one reactor recirculation pump is stopped while operating at the operating point 4 on the curve 2 of FIG. 5 at the beginning, the core flow rate will be significantly reduced due to this, and this will be shown at the operating point 4a. There is a risk of rushing into such a shaded region 3 having poor stability.

In such a case, if the neutron flux signal is left as it is, the peak value of the neutron flux signal increases while exhibiting oscillation, and the neutron flux high reactor automatic shutdown function of the protection device installed in the boiling water reactor is set. When the value is reached, the reactor will shut down automatically. However, at this time, if some control rods are quickly inserted by an operation operation, the operation moves to the operation point 4b along the curve 1 and the reactor output decreases, but the core stability becomes good and the reactor Automatic stop can be avoided. Therefore, conventionally, the control rod is inserted by the operation of the operator.

[0008]

SUMMARY OF THE INVENTION When a reactor recirculation pump stop event suddenly occurs during operation and the reactor is operated in a region where core stability is poor, it is necessary to avoid this. When the control rod is inserted by the operation of the operator, an abnormality may occur, and the operation time is restricted. Therefore, a large burden is imposed on the operator, and if the operation is insufficient, the reactor is automatically stopped and the operating rate of the reactor is reduced.

Conventionally, as a countermeasure against this, there has been a method of automatically inserting the predetermined selection control rods all at once. However, since a rapid change in reactivity is given to the core, the result is a void or feed water control amount. May result in an unstable state of the water level control of the reactor, and the protection device may be activated to automatically shut down the reactor.

Further, as a method of solving these problems, it is conceivable to operate at an operating point where core stability does not deteriorate even if one reactor recirculation pump is stopped. The method was impractical because it was severely restricted in reactor operation and efficient core operation was not possible.

The object of the present invention is to automatically insert selective control rods in stages when the reactor recirculation pump is stopped, without causing a sudden change in reactivity to the core. An object of the present invention is to provide a reactor output control device capable of avoiding unnecessary automatic shutdown of the reactor, reducing the burden on the operator, and improving the operating rate of the reactor.

[0012]

In a reactor power control device that adjusts the power output of a reactor according to a coolant recirculation flow rate and a control rod insertion amount, a recirculation pump stop detecting a stop of a reactor recirculation pump. Detection means, selection control rod insertion area determination means for determining the insertion area of the selection control rod based on the reactor output and recirculation flow rate, and outputs from the recirculation pump stop detection means and the selection control rod insertion area determination means Selection control rod insertion control means for calculating a logical product of signals and outputting a selection control rod insertion signal, control rod insertion setting means for dividing and setting the selection control rod in multiple stages, and the selection control rod insertion signal And a control rod stepwise insertion means for stepwise inserting the control rod set by the control rod insertion setting means.

[0013]

[Operation] The stop of at least one reactor recirculation pump is detected by the recirculation pump stop detection means, and
The reduction of the flow rate of the recirculation drive water at this time, and the determination of the insertion area of the selection control rod due to the decrease of the reactor stability by the selection control rod insertion area determination means based on the reduction of the recirculation drive water flow rate and the reactor output I do.

Further, from the logical product of the stop signal of the reactor recirculation pump and the determination signal of the selective control rod insertion region,
The control rod group previously divided and set by the control rod insertion setting means is inserted into the nuclear reactor in multiple stages in response to an insertion command from the control rod stepwise insertion means. As a result, a rapid change in reactor output is suppressed, an emergency automatic shutdown of the reactor is avoided, and stable operation is continued.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. In this embodiment, the automatic insertion of the selection control rod is shown in two stages. As shown in the reactor power control system diagram of FIG. 1, two A and B2 system reactor recirculation pumps 6a and 6b are installed in the reactor pressure vessel 5, and the A system reactor recirculation pump 6a is installed. And the B system reactor recirculation pump 6b
It is driven by an A system recirculation pump motor 7a and a B system recirculation pump motor 7b, respectively.

Each recirculation pump motor 7a, 7b is
It is designed to be driven by three-phase AC from the A-system variable frequency power supply device 8a and the B-system variable frequency power supply device 8b of the A and B2 systems, and the power supply line has a recirculation pump stop detection means A System recirculation pump motor voltage detection device 9
A and B system recirculation pump motor voltage detection devices 9b are provided respectively. The voltage signals from the recirculation pump motor voltage detection devices 9a and 9b are input to the A system recirculation pump stop detection device 10a and the B system recirculation pump stop detection device 10b, respectively. .

That is, each reactor recirculation pump 6a, 6
In order to detect the stop of b, the recirculation pump motors 7a, 7
The method of detecting the voltage shortage of b is adopted, and when the voltage signal from each recirculation pump motor voltage detection device 9a, 9b becomes below the set voltage, the reactor recirculation pump 6a,
6b has stopped, and each of the recirculation pump stop detection devices 10a and 10b outputs an A system recirculation pump stop signal 11a.
And a B system recirculation pump stop signal 11b is output.

Further, each reactor recirculation pump 6a, 6b
In the discharge side line of the A system recirculation drive water flow rate detector 12a
And B system recirculation drive water flow rate detector 12b is installed,
The A-system recirculation drive water flow rate signal 13a and the B-system recirculation drive water flow rate signal 13b, which are detection signals from the respective recirculation drive water flow rate detectors 12a and 12b, are input to the adder 14.

On the other hand, a neutron detector 15 is arranged in the reactor pressure vessel 5, and the detection signal thereof is input to the core thermal output measuring device 16, and the measurement result in the core thermal output measuring device 16 is The core heat output signal 17 is input to the selection control rod insertion area determination circuit 18, which is the selection control rod insertion area determination means.

The recirculation drive water flow rate signal 19 output from the adder 14 is also input to the selection control rod insertion area determination circuit 18, and is output from the selection control rod insertion area determination circuit 18. Signal 20 and recirculation pump stop signal 11a from each recirculation pump stop detection device 10a, 10b,
Both 11b are input to the logic circuit 21.

The selection control rod insertion signal 22 output from the logic circuit 21 is input to the control rod insertion circuit 23, which is the control rod stepwise insertion means composed of the control rod insertion setting means, to the reactor pressure vessel 5. A first stage control rod setting insertion command 24 and a second stage control rod setting insertion command 25 for stepwise inserting the selected control rods into the installed control rod driving mechanism (not shown)
Is configured to output.

FIG. 2 is a block circuit diagram showing the main part of the reactor output control system. Two recirculation pump stoppage detectors 10a and 10b of A and B systems, which are reactor recirculation pump stoppage detecting means.
A system recirculation pump stop signal 11a and B system recirculation pump stop signal 11b outputted from the control system, and a selection control rod insertion region output from a selection control rod insertion region determination circuit 18 which is a selection control rod insertion region determination means. The determination signal 20 is input to the logic circuit 21.

The logic circuit 21 has an OR circuit 26 for inputting the A system recirculation pump stop signal 11a or the B system recirculation pump stop signal 11b and outputting a signal 27, the signal 27 and the selection control rod. When the insertion area judgment signal 20 is input,
The AND control circuit 28 outputs the selection control rod insertion signal 22.

Further, by inputting the selection control rod insertion signal 22, the first stage control rod setting insertion command 24 and the second stage control rod setting insertion command
A control rod insertion circuit 23, which is a control rod stepwise insertion means for outputting 25, is composed of a first stage control rod setting insertion circuit 29 and a second stage control rod setting insertion circuit 30 which are control rod insertion setting means. . The first-stage control rod setting and inserting circuit 29 and the second-stage control rod setting and inserting circuit 30 in the control rod inserting circuit 23 can independently set the control rod and set the insertion delay time. I have to.

Therefore, the control rod insertion circuit 23 receives the selection control rod insertion signal 22 to change from the first stage control rod setting insertion circuit 29 and the second stage control rod setting insertion circuit 30 to different control rods. On the other hand, the first stage control rod setting insertion command 24 and the second stage control rod setting insertion command 25 are output with a certain time difference,
The control rod group that receives these commands is urgently inserted in stages.

Further, the selection control rod insertion area determination circuit 18 uses the recirculation drive water flow rate signal 19 from the adder 14 and the core heat output signal.
The operating point is determined on the basis of 17 and the recirculation drive water flow rate is W0
In the following, when the condition that the reactor output is P0 or more is satisfied and it is determined that the operating point is in the selection control rod insertion area within the shaded area 31, the selection control rod insertion area determination signal 20 is output.

Next, the operation of the above configuration will be described. When the two reactor recirculation pumps 7a and 7b are normally operated, the core flow rate of the coolant in the reactor pressure vessel 5 is sufficient, and the control rod (not shown) is pulled out,
The reactor is operating at normal power.

At this time, each recirculation pump stoppage detection device 10
The recirculation pump stop signals 11a and 11b are not output from a and 10b, and the respective recirculation drive water flow rate signals 13a and 13b are
Since both are sufficient values, the recirculation drive water flow rate signal 19 added by the adder 14 is also large. The stability of the reactor in this state is good, and the operating point is outside the shaded area 31 shown in FIG. The selection control rod insertion area determination signal 20 from the selection control rod insertion area determination circuit 18 is not output. Therefore, at this time, it is assumed that the operation is performed at the operating point 4 on the output-flow rate characteristic diagram shown in FIG.

Here, it is assumed that, of the two A and B system reactor recirculation pumps 6a and 6b, the A system reactor recirculation pump 6a is stopped due to a failure of the A system variable frequency power supply device 8a. . When the voltage to the A-system recirculation pump motor 7a drops or is lost due to the failure of the A-system variable frequency power supply device 8a, etc., the A-system reactor recirculation pump 6
For a, the speed decreases or stops, and the recirculation drive water flow rate in the A system decreases.

The decrease or loss of the voltage supplied to the A-system recirculation pump motor 7a is detected by the A-system recirculation pump motor voltage detector 9a and transmitted to the A-system recirculation pump stop detector 10a. . When this voltage signal falls below the set voltage, the system A recirculation pump stop detection device
The unit 10a determines that the A-system reactor recirculation pump 6a has stopped, and outputs the A-system recirculation pump stop signal 11a to the logic circuit 21.

Furthermore, in the reactor pressure vessel 5, the recirculation flow rate of the coolant is reduced due to the reduction of the recirculation drive water flow rate accompanying the stoppage of the A-system reactor recirculation pump 6a, and the reactor output is reduced. The reactor moves to the operating point 4a in FIG. 5 and core stability deteriorates. This decrease in the A system recirculation drive water flow rate is detected by the A system recirculation drive water flow rate detector 12a and is transmitted to the adder 14 as the A system recirculation drive water flow rate signal 13a so that the B system recirculation of the normal flow rate is performed. It is added to the drive water flow rate signal 13b.

However, the recirculation drive water flow rate signal 19 output from the adder 14 to the selection control rod insertion area determination circuit 18
Is small only in the B-system recirculation drive water flow rate signal 13b, which is the recirculation drive water flow rate for one B-system reactor recirculation pump 6b. In addition, the reduced power output of the reactor is
Detected by the core heat output measuring device 16
It is transmitted to the selection control rod insertion area determination circuit 18 as 17.

The selection control rod insertion area determination circuit 18 makes the determination according to FIG. 3 from the small recirculation drive water flow rate signal 19 and the reduced core heat output signal 17, and as a result, the recirculation drive water flow rate is determined. It is determined that the condition that the reactor power is W0 or less and the reactor power is P0 or more is satisfied, and the operating point is in the shaded area 31, and the selection control rod insertion area determination signal 20 is set to the logic circuit 21.
Output to.

In the logic circuit 21, the signal 27 from the OR circuit 26 is input to the AND circuit 28 by the previously input A system recirculation pump stop signal 11a, and the selection control rod insertion area determination signal 20 is further input. Since it is input to the AND circuit 28, the logical product is calculated and the logical circuit 21 is used to input the control rod insertion circuit.
The selection control rod insertion signal 22 is output to 23.

The selection control rod insertion signal 22 is the control rod insertion circuit 23.
Inside the first stage control rod setting insertion circuit 29 and the second stage control rod setting insertion circuit 30, the first stage control rod setting insertion circuit 29 is input.
And the second-stage control rod setting and inserting circuit 30 outputs the first-stage control rod setting and inserting command 24 and the second-stage control rod setting and inserting command 25 to the different control rods selected in advance with a time difference. .

As a result, the unillustrated control rod group in the reactor pressure vessel 5 is urgently inserted into the core in a stepwise manner. As a result, the reactor power is the reactor power-core flow rate shown in FIG. From the operating point 4a having poor core stability on the characteristic diagram, the operating point 4b having good core stability is gradually changed along the curve 1. By this power control, a sudden increase in negative reactivity in the reactor and a rapid decrease in voids,
Also, fluctuations in reactor water level and disturbances in the water supply system are suppressed.

It should be noted that when the operating state moves from the position of the operating point 4a to the position of the operating point 4b shown in FIG. 5, the core heat output signal 17 decreases as the reactor output decreases, and as shown in FIG. It will be below the reactor power P0. As a result,
The determination that the selection control rod insertion area determination circuit 18 determines that it is within the shaded area 31 disappears, and the selection control rod insertion signal 22 output to the control rod insertion circuit 23 is reset.

Usually, in a reactor under normal operation, an abrupt increase in negative reactivity due to the insertion of selective control rods causes an abrupt decrease in voids, causing fluctuations in the reactor water level and disturbances in the water supply system. Become. Due to this disturbance, the water supply system becomes temporarily unstable and the fluctuation of the reactor water level is further expanded as a result.

That is, the reactor recirculation pumps 6a, 6b
At the time of trip, the core flow rate decreases, the void increases, the reactor water level rises, the feedwater flow rate decreases, and the reactivity decreases due to the insertion of selective control rods, the void decreases, the reactor water level decreases, the feedwater flow rate increases, Behavior such as rise of reactor water level occurs. However, such fluctuations in the reactor water level are controlled in several stages with a certain time difference compared to inserting all the control rods required to get out of the unstable operating region at once. It is greatly relaxed by inserting a rod.

In the above-described embodiment, the selection control rods are inserted two times.
It is divided into stages, and if one example is shown, the first stage selection control rod insertion command 24 is instantaneously output by the input of the selection control rod insertion signal 22 to perform the control rod insertion, and the next second stage selection control rod is inserted. The insertion command 25 can be preset based on the analysis result of the reactor water level so as to insert the control rod in the process of rising the reactor water level.

According to this method, the number of control rods to be inserted first is smaller than in the case of simultaneous insertion of all of them, so that the amount of decrease in the reactor water level due to the rapid decrease of voids is small, and further, the remaining control is performed. By inserting the rod in the process of rising the reactor water level, it becomes possible to suppress the rise of the reactor water level due to the reduction of voids.

On the other hand, when one of the two A-system reactor recirculation pumps 6a trips, the large fluctuation of the reactor water level is
It takes about 60 seconds to converge, and it takes about 2 minutes from when the core operating point enters the unstable region to when neutron flux oscillation actually starts. It is known from the result of analysis. Therefore,
If the selective control rods are inserted in multiple stages within 60 seconds, the reactor stability will not be adversely affected.

In the above embodiment, the insertion of the selection control rod is described in two stages.
It is easy to make more than the stages, and the circuit configuration is the first stage control rod setting insertion circuit 2 which constitutes the control rod insertion circuit 23.
9. The number of second stage control rod setting and inserting circuits 30 should be increased.

The case where the reactor recirculation pumps 6a and 6b are stopped by detecting the voltage to the recirculation pump motors 7a and 7b by the recirculation pump motor voltage detection devices 10a and 10b has been described. A current detection device, a power detection device, or a combination thereof may be used. The case where the A-system reactor recirculation pump 6a is stopped has been described above.
Needless to say, the same action and effect can be obtained in the case of b stop.

The reactor power control system diagram of FIG. 4 shows another embodiment of the present invention. The reactor recirculation pump 6a,
In detecting the stoppage of 6b, a shortage of the recirculation drive water flow rate of each of the A and B systems is detected. That is, the recirculation pump water flow rate detectors 12a, 12 are connected to the recirculation pump stop detection means.
Using b, this detection signal is added to the A system recirculation drive water flow rate signal 13a and the B system recirculation drive water flow rate signal 13b.
A system recirculation pump stop detection device 10
The configuration is also such that the a and B system recirculation pump stop detection devices 10b are input.

When each of the flow rate values of the respective recirculation driving water becomes equal to or less than the set flow rate, the respective reactor recirculation pumps 6a, 6b
Is judged to have stopped, and the A-system recirculation pump stop signal 11 is sent from each of the recirculation pump stop detection devices 10a and 10b.
A or B system recirculation pump stop signal 11b is output.

Further, the flow rate signals such as the recirculation drive water flow rate signals 13a and 13b generally contain noise,
Since the signal may fluctuate, each recirculation drive water flow rate detector 12a, 12b and the recirculation pump stop detection device may be used to reliably detect the pump stop according to the signal characteristics.
Filters 32a and 32b are provided between 10a and 10b, and on-delay timers 33a and 33b are provided between the recirculation pump stop detection devices 10a and 10b and the logic circuit 21.

The other construction is the same as that of the above embodiment except that the recirculation pump motor voltage detecting devices 9a and 9b are not necessary, and the same operation and effect can be obtained.
Further, instead of obtaining the core heat output signal 17 from the neutron detector 15 and the core heat output measuring device 16 which are neutron flux measurement systems, a feed water flow rate signal (not shown) of the feed water to the reactor pressure vessel 5 may be adopted. good.

In the above-mentioned one embodiment and other embodiments,
Although the reactor recirculation pumps 6a and 6b installed in the loop reactor recirculation system of the A and B2 systems have been described as an example, the present invention is also applicable to a reactor recirculation pump with a built-in reactor. is there. Normally, about 10 reactor internal recirculation pumps are installed in one reactor. Therefore, when the present invention is applied to a recirculation pump with a built-in nuclear reactor, it is necessary to provide the recirculation pump stop detection devices 10a and 10b shown in FIG. Further, instead of the OR circuit 26 in the logic circuit 21 shown in FIG. 2, n units (an integer of 10 ≧ n ≧ 1;
Optimally, it is preferable to use the logic in which the signal 27 is output when the recirculation pump stop signal of n = 2) or more is input.

Further, in the case of the reactor built-in type recirculation pump, the flowmeters installed before and after each pump head may be used to detect the flow rate of the recirculation driving water. It is advisable to detect the rotational speed of the pump and use it as a flow rate signal. Further, the control circuit of the present invention may be programmed by using a microcomputer. Further, when the control rod insertion operation is provided with a hydraulic scram system and an electric motor driven insertion system, for example, It is also possible to use a circuit in which the emergency insertion of the selective control rod by hydraulic pressure is used as the first stage, and the control rod insertion by the electric motor drive is performed as the second stage.

[0051]

As described above, according to the present invention, even when a part of the reactor recirculation pump is stopped and the reactor moves to an operating point where the core stability is poor, it is preselected. The control rods are automatically inserted in multiple stages, and the reactor operation can be stably transferred to an operating point with good core stability, enabling continuous operation of the plant. Therefore, there is an effect of reducing the burden on the operator and improving the safety and reliability of the nuclear reactor, the plant operating rate, and the economical efficiency.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a reactor power control system according to an embodiment of the present invention.

FIG. 2 is a block circuit diagram of essential parts of a reactor power control system according to an embodiment of the present invention.

FIG. 3 is a determination characteristic diagram of a selection control rod insertion area according to an embodiment of the present invention.

FIG. 4 is a structural diagram of a reactor power control system of another embodiment according to the present invention.

FIG. 5 is a reactor power-core flow rate characteristic diagram during reactor operation.

[Explanation of symbols]

1, 2 ... Operating curve, 3 ... Diagonal area (poor stability), 4,
4a, 4b ... Operating points, 5 ... Reactor pressure vessel, 6a ... A system reactor recirculation pump, 6b ... B system reactor recirculation pump,
7a ... A system recirculation pump motor, 7b ... B system recirculation pump motor, 8a ... A system variable frequency power supply device, 8b ... B system variable frequency power supply device, 9a ... A system recirculation pump motor voltage detection device, 9b ... B system recirculation pump motor voltage detection device, 10a ... A system recirculation pump stop detection device, 10b ... B system recirculation pump stop detection device, 11a ... A system recirculation pump stop signal, 11b ... B system recirculation pump stop signal Signal, 12a ... A system recirculation drive water flow rate detector, 12b ... B system recirculation drive water flow rate detector, 13a ... A system recirculation drive water flow rate signal, 13b ... System recirculation drive water flow rate signal, 14 ... Addition Vessel, 15 ... Neutron detector,
16 ... Core heat output measuring device, 17 ... Core heat output signal, 18 ... Selection control rod insertion area determination circuit, 19 ... Recirculation drive water flow rate signal, 20 ... Selection control rod insertion area determination signal, 21 ... Logic circuit,
22 ... Selection control rod insertion signal, 23 ... Control rod insertion circuit, 24 ... First stage control rod setting insertion command, 25 ... Second stage control rod setting insertion command, 26 ... OR circuit, 27 ... Signal, 28 ... AND circuit , 29 ... First-stage control rod setting / inserting circuit, 30 ... Second-stage control rod setting / inserting circuit, 31 ... Diagonal region (insertion region determination), 32a, 32b ... Filter, 33a, 33b ... On-delay timer.

Claims (1)

[Claims] 1. A reactor output control device for adjusting a reactor output according to a coolant recirculation flow rate and a control rod insertion amount, and recirculation pump stop detection means for detecting stop of a reactor recirculation pump, Selection control rod insertion area determination means for determining the insertion area of the selection control rod based on the reactor output and the recirculation flow rate, and the logical product of the output signals from the recirculation pump stop detection means and the selection control rod insertion area determination means By selecting the control rod insertion setting means for outputting the selection control rod insertion signal, the control rod insertion setting means for dividing and setting the selection control rod in multiple stages, and the control rod insertion setting means for the selection control rod insertion signal. And a control rod stepwise insertion means for stepwise inserting the set control rods.