JPH06109892A - Reactor power controller - Google Patents

Abstract

(57) [Summary] [Purpose] In the event of a sudden decrease in core flow rate, the reactor power is quickly reduced to secure a thermal margin for the fuel rods in the reactor.
(EN) Provided is a reactor output control device which does not erroneously scramble against fluctuations in a power transmission system and does not lead to reactor shutdown due to excessive negative reactivity. [Structure] Detecting means 13 for detecting a cause event of a recirculation pump stop at the time of sudden decrease in core flow rate, consisting of a calculation section 27 and a reference data storage section 28, and comparing core flow rate 30 and core power distribution data 34 with reference data. Logic means 26 for calculating the negative reactivity and its reactivity distribution necessary to secure the thermal margin of the reactor at the time of sudden decrease in core flow rate, and a negative reactivity based on the negative reactivity distribution by the signal of the logic means 26. The control rod selection means 36 selects the optimum number and insertion pattern of control rods that should give the reactivity of 1 and reduces the reactor output without losing the thermal margin of the fuel rod when the core flow rate suddenly decreases. And

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plant protection system for a boiling water type LWR nuclear power plant, and more particularly to a reactor power control system for maintaining a thermal margin of fuel rods in a reactor even when a recirculation pump is stopped.

[0002]

2. Description of the Related Art In an improved boiling water type LWR nuclear power plant that employs a recirculation pump with a built-in reactor pressure vessel, an external power loss event occurs when the internal power supply that supplies power to the main auxiliary system of the plant is lost. There is a generator load cutoff caused by a ground fault of the entire transmission line or a short circuit.

This generator load cutoff is an event in which power transmission becomes impossible in the power transmission system and the load on the generator is lost. The loss of load on the generator causes the load of the generator and the energy of the steam sent to the steam turbine to which the generator is connected to be in an unbalanced state.

As a result, the energy difference between the load and the output is given to the steam turbine as kinetic energy, so that the rotation speed of the steam turbine rapidly increases. When the rotation speed of the steam turbine increases, excessive centrifugal force is applied to the turbine shaft and turbine blades, which may eventually damage the steam turbine.In such a case, the turbine steam control valve should be operated quickly. It is designed to protect the steam turbine by shutting it off and shutting off the steam sent to the steam turbine to prevent the steam turbine from increasing in speed.

On the other hand, by closing the turbine steam control valve,
Since the steam generated in the reactor remains in the reactor pressure vessel as it is and the reactor pressure rises, the turbine bypass valve is rapidly opened at the same time as the turbine control valve is quickly closed.
The steam generated in the reactor is sent directly to the condenser for condensation processing.

Conventionally, boiling water type light water reactor plants are classified into two types according to the processing capacity of generated steam. One is a plant called a partial capacity turbine bypass machine, which is equipped with a turbine bypass valve having a capacity capable of bypassing the steam turbine and condensing with a condenser for only a part of the rated steam flow rate. In this type of plant, if the generator load is cut off, it is not possible to process all the steam generated in the reactor as it is, so immediately reduce the reactor output by the reactor scrum and stop the reactor. There is. This type is the improved boiling water type LWR nuclear power plant currently under construction.

The other type is a nuclear power plant called a full-capacity turbine bypass machine, which is a turbine bypass valve having a capacity capable of condensing all steam generated in a nuclear reactor in a condenser by bypassing a steam turbine. Since it is installed, even if the load on the generator is cut off, it is designed to avoid the reactor scram and shift to in-house isolated operation in the plant.

This is because the generator load cutoff is due to an abnormality in the power transmission system, not due to an abnormality in the equipment on the nuclear power plant side. Therefore, if excess steam can be treated, the reactor If there is no need for an emergency stop on the side of the plant, and if it is possible to wait for the restoration of the transmission line accident while the plant is operating independently, it will be possible to start power transmission immediately after the restoration of the accident. This is because it will help strengthen the power supply and improve the reliability of securing the on-site power supply.

In this full-capacity turbine bypass machine, if the turbine bypass valve is not opened when the generator load is cut off, the opening of the turbine bypass valve is checked and then the reactor is scrammed. The full boiling turbine bypass machine is planned to be used in the improved boiling water type LWR nuclear power plant to be constructed in the future.

The mechanism when the generator load is cut off in the full capacity turbine bypass machine will be described. Generally, the power supply system of a boiling water nuclear power plant is as shown in the single line connection diagram of FIG. That is, the electric power generated by the generator 1 is boosted by the main transformer 2 and then
Power is transmitted to the demanded place by the power transmission line 3 of the line.

The internal power source supplies electric power to the internal transformers 5, 5 by the phase separation bus bar 4, and then the recirculation pump consisting of a pump motor directly connected to the recirculation pump in the pressure vessel via the normal high voltage bus lines 6, 6. It is supplied to the auxiliary machinery system 8 including the units 7, 7.

When a short circuit occurs in the two lines of the power transmission line 3 or a ground fault occurs, the line breaker 9 or the main breaker 10 is cut off and the generator load is cut off. In the plant interlock of the full capacity turbine bypass machine at this time, the load and the output of the generator 1 connected to the steam turbine 11 become unbalanced, as shown in the system main part configuration diagram of FIG.
The rotation speed of the steam turbine 11 tries to increase.

However, when the output / load unbalance detection circuit 12 detects the loss of load, the pressure control device 13 is activated by the steam turbine.
In order to prevent overspeed of 11, the turbine steam control valve rapid closing signal 14 is output to rapidly close the turbine steam control valve 15. At the same time, the turbine bypass valve rapid opening signal 16 is issued to rapidly open the turbine bypass valve 17 to send the surplus steam to the condenser 18 and prevent the pressure in the reactor pressure vessel 19 from rising.

In this way, in the full-capacity turbine bypass machine, the operation shifts to the independent operation in the plant without scramming the reactor, but at this time, the electric power of the auxiliary system 8 in the plant is temporarily surplus in the steam turbine 11. It is ensured by performing the turbine blowdown operation with steam, and after the transient event is settled, the turbine steam control valve 15 is slightly opened to generate the electric power required for the islanding operation.

However, if the turbine blowdown operation fails, the power to the auxiliary machinery system 8 is lost, resulting in an external power loss event. Assuming that the power source in the plant is lost in the improved boiling water type light water reactor, the recirculation pump units 7 and 7 have a small rotational inertia such as the recirculation pump in the pressure vessel, and a static pump (not shown) using a thyristor inverter. By adopting the motor control power supply device, the recirculation pump units 7, 7 are stopped at the same time as the power supply is lost, and the core flow rate of the coolant is rapidly reduced.

As a result, the cooling capacity of the core 20 is rapidly reduced, so that it is conceivable that the fuel rods (not shown) in the core 20 transiently reach a boiling transition and the thermal margin of the fuel rods is lost. Therefore, in the improved boiling water type light water reactor of the partial capacity turbine bypass machine currently designed,
When the load of the generator is cut off, the following measures are adopted to ensure a thermal margin in the fuel rods in the reactor that is affected by the sudden decrease in the core flow rate at the time of loss of external power, so that no trouble will occur due to the event of loss of external power. I am designing a plant.

First, when the turbine control valve 15 is suddenly closed due to the load interruption of the generator, the control rod is immediately inserted by the reactor scrum, and the recirculation pump unit 7,
A large amount of voids are generated by stopping a part of No. 7, and the furnace output is reduced by inputting a negative reactivity.

On the other hand, a part of the recirculation pump unit 7,
As for the power source of No. 7, a motor generator having a large inertia is adopted, and the recirculation pump unit 7 is operated after the external power source is lost for several seconds until the heat output of the fuel rods in the reactor is sufficiently reduced by the scrum. , 7 to rotate the recirculation pump in the pressure vessel to maintain the core flow rate.

[0019]

In designing a boiling water type light water reactor, from the viewpoint of the soundness of the fuel rods in the reactor, even when the external power source is lost, the fuel rods in the reactor transiently reach a boiling transition and Although it is required to never lose the thermal margin, in the full-capacity turbine bypass machine of the improved boiling water type light water reactor, the power source at the plant is cut off by the short circuit of all lines of the transmission line 3, etc. In the case of loss of power, the recirculation pump units 7 and 7 will have some power connected to the motor generator for a few seconds, but eventually all of them will be stopped and the core flow rate will be reduced. Will decrease rapidly.

At this time, in the current design, the reactor itself does not generate the scram because the plant itself shifts to the independent operation in the plant. Therefore, this causes the thermal margin of the fuel rods in the reactor to be lost, making it difficult to meet the above requirements. Therefore, in the event of a loss of external power supply, a method of scramming the reactor by detecting the voltage drop of the power supply in the plant can be considered. This is a boiling water type LWR nuclear power plant that employs some overseas pressure vessel recirculation pumps. It is also used in plants.

However, in the event of a momentary power failure, etc., the internal power source is temporarily lost, but the core flow rate is maintained in order to recover again.
Therefore, it is necessary to avoid the reactor scram for the event that the reactor output does not need to be reduced each time because the thermal margin is secured, but the scram due to the low voltage is used as the thermal margin when the external power source is lost. If it is used to secure the above, there is a possibility of erroneous scrum due to the voltage drop.

As a countermeasure against this, an event that temporarily loses internal power supply such as an instantaneous power failure and an event that external power supply is lost are distinguished by the time of voltage drop. However, there is a problem in that there is a high possibility that an accidental scrum will occur due to a temporary voltage drop that is longer than the momentary blackout caused by the system disturbance.

The full-capacity turbine bypass machine shifts to the on-site islanding operation without scramming with the generator load cut off,
Since the power transmission is required and designed to be able to be retransmitted immediately when it becomes possible, it is necessary to make the design strong against the fluctuation of the transmission system. Therefore, the interlock of directly detecting the voltage drop and scramming the reactor is not very desirable in the full capacity turbine bypass machine from the viewpoint of avoiding erroneous scrum.

Further, when the external power supply is lost due to the load cutoff of the generator, a large amount of main steam is directly sent to the condenser 18 via the turbine bypass valve 17 when shifting to the independent operation in the plant. From the viewpoint of protecting the condenser in such cases, selective insertion of control rods has been performed conventionally. However, the conventional control rod selective insertion system is for reducing the output of the entire core from the viewpoint of condenser protection, and is not the control rod selective insertion from the viewpoint of thermal protection of in-reactor fuel rods, and There is a delay in the response time of the furnace output with respect to the insertion of the control rod.

For this reason, in the conventional control rod selective insertion system, as shown in (b) (transient) of the furnace power distribution chart of FIG. 8, the power distribution shown by the solid line 21 and the thermal margin shown by the dotted line 22 are provided. Due to the limit, the output cannot be sufficiently reduced in the fuel rod portion at the peak portion in the center of the furnace, and the thermal margin is lost in the shaded portion 23. The chain line 24 shows the distribution of the input reactivity, FIG. 8A shows the initial state, and FIG. 8C shows the settling time.

Further, as shown in (b) (transient time) of the furnace power distribution chart of FIG. 9, in order to secure a thermal margin when the external power source is lost, a large input reactivity distribution such as the chain reactivity distribution 24 is shown. When a negative reactivity is input, as a result, the power distribution shown by the solid line 21 decreases, but it falls too much around the furnace. Therefore, when shifting from this state to the in-house isolated operation with the original load off of the generator, the output distribution is as shown by the solid line 21 in Fig. 9 (c) (during settling), and the reactor will stop. was there.

The object of the present invention is to secure the thermal margin of the fuel rods in the reactor by reducing the reactor power quickly in the event of a rapid decrease in the core flow rate, and to prevent the fluctuation of the transmission system. It is an object of the present invention to provide a reactor power control device that does not cause an accidental scrum and does not reach a reactor shutdown state due to excessive negative reactivity.

[0028]

[Means for Solving the Problems] Detection means for detecting an event causing a stop of a recirculation pump when the core flow rate of a boiling water type light water reactor suddenly decreases, and a separately introduced core flow rate and reactor power distribution consisting of a calculation section and a reference data storage section. Of the negative reactivity required to secure the thermal margin of the reactor at the time of sudden decrease in the core flow rate by comparing the data of No. 1 and the reference data, and the logic means for calculating the reactivity distribution, and the negative means by the signal of the logic means. Select the optimum number and insertion pattern of control rods that should give a negative reactivity based on the reactivity distribution, and lower the reactor power so that the thermal margin of the fuel rods in the reactor is not lost when the core flow rate decreases sharply. It is characterized by comprising a control rod selecting means.

[0029]

The event causing the rapid decrease in the core flow rate is directly or indirectly detected by the detecting means, and the load cutoff signal is output to the logic means. In this logic means, the core flow rate data and core power distribution data of the reactor, which have been input separately, and the reference data in the reference data storage section are compared and calculated in the calculation section, in order to secure a thermal margin when the core flow rate sharply decreases. And the data of the negative reactivity and the reactivity distribution that do not reach the reactor shutdown state are calculated, and this control rod selection data is output to the control rod selection means.

The control rod selecting means outputs a control rod selecting signal to the control rod driving means and inserts the control rods of the selected patterns into predetermined positions, respectively, to lower the core output and the in-core fuel. Securing the thermal margin of the rod to avoid unnecessary scrum and reactor shutdown due to voltage drop due to momentary power failure.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. It should be noted that the same components as those in the above-described conventional technique are designated by the same reference numerals and detailed description thereof will be omitted. FIG. 1 is a block configuration diagram of a main part of a reactor power control device, and particularly shows an interlock configuration of selective insertion of control rods when a recirculation pump is stopped.

When the external power source is lost, the recirculation pump units 7, 7 are stopped as described above, the core flow rate is rapidly reduced, and the thermal margin of the core fuel rods is lost. Therefore,
By interlocking the plant protection system, a part of the control rods is selectively inserted when the external power source is lost, and the reactor output is quickly reduced to secure the thermal margin of the fuel rods in the reactor.

Here, as an example, an external power loss event occurs due to generator load cutoff, so the recirculation pump stop 20
The load cutoff signal 25 from the pressure control device 13 was used as a means for detecting the cause event of the above.However, the signal of the generator load cutoff is generally the rapid closing signal 14 of the turbine steam control valve 15 for each plant.
It is introduced from.

The load cutoff signal 25 is output to the logic means 26. The logic means 26 is composed of an arithmetic unit 27 and a reference data storage unit 28 as shown in the block diagram of FIG.
27 separately inputs the core flow rate data 30 output from the core flow rate detection means 29, and the core output distribution data 34 obtained by calculating the core output data 32 from the plurality of core output detection means 31 by the output distribution calculation means 33, Compared with the data in the reference data storage unit 28 and the correlation equation, the negative reactivity and reactivity distribution necessary to secure a thermal margin at the time of sudden decrease in the core flow rate and to prevent the reactor from shutting down. Calculate the data,
The control rod selection data 35 is output to the control rod selection means 36.

The correlation equation in the reference data storage unit 28 is
What is the minimum core flow rate required to secure the thermal margin of the fuel rod with respect to the surface heat output of a certain fuel rod, or conversely, to secure the thermal margin at a certain core flow rate Represents the limit to which the heat output must be reduced, and is a correlation formula between the heat output and the core flow rate. It should be noted that the larger the heat output, the more the core flow rate is required.

The control rod selecting means 36 is necessary to secure the thermal margin of the fuel rods in the reactor even when the core flow rate is reduced, and has a negative reactivity so that the reactor is not shut down. The control rod insertion pattern is selected based on the control rod selection data 35 from the logic means 26, and the control rod selection insertion signal 37 is sent to the control rod drive means 38. In response to the control rod selection insertion signal 37, the control rod driving means 38 inserts the control rod into the core 20.

Next, the operation of the above configuration will be described. The state of the reactor power response according to the present invention is shown by the reactor power distribution diagram of FIG. In a normal operating state, as shown in the initial state of FIG. 3 (a), the furnace output of the power distribution indicated by the solid line 21 is large, but the limit of the thermal margin shown by the dotted line 22 is such that the power distribution of the solid line 21 becomes larger. It is higher. Therefore, the fuel rods in the core 20 are operated with a sufficient thermal margin.

Next, when the generator load is cut off for some reason in the plant, the recirculation pump units 7, 7
Is stopped and the flow rate of the coolant in the core 20 is rapidly reduced. When the turbine steam control valve 15 is rapidly closed by outputting the turbine steam control valve rapid closing signal 14 from the pressure control device 13, a load cutoff signal 25 is output to the logic means 26.

The logic means 26 uses the core flow rate data 30 at this time.
And the core output data 32, and the reference data of the reference data storage unit 28, in the calculation unit 27, in order to secure a thermal margin at the time of this sudden decrease in core flow rate, and also to prevent the reactor from shutting down. Data of reactivity and reactivity distribution are calculated, and this control rod selection data 35 is output to the control rod selection means 36.

The control rod selection means 36 uses the control rod selection insertion signal 37.
Is sent to the control rod driving means 38 to selectively insert the control rod into the core 20. By this selective insertion of the control rods, as shown in the transitional period of FIG.
The limit of the thermal margin shown by the dotted line 22 with respect to the power distribution of No. 1 exceeds this, and the thermal margin of the fuel rod in the reactor is secured. Therefore, even at the time of settling, the reactor is operated with a sufficient thermal margin as shown in FIG.

Although there are other events that cause the rapid closing of the turbine steam control valve 15 in FIG. 7 described above, these events are all scrum events except for the shift to the isolated island operation due to load shedding. There is no problem if the control rod selection insertion signal is input. Also, when shifting to independent operation in the plant by load shedding, a large amount of main steam is sent directly to the condenser 18, and the control rods are selectively inserted from the viewpoint of condenser protection. There is no problem even if the selective insertion of is simultaneously performed.

However, in reality, since the control rod insertion from the viewpoint of protecting the condenser and the control rod insertion for securing the thermal margin are duplicated, the reactor output will decrease too much, so that When the present invention is adopted, in selecting the insertion control rod, first, the control rod for securing the thermal margin of the fuel rod is selected, and the control rod for the condenser protection is selected based on the data, Alternatively, in the control rod selection means 36 of the present invention,
It is desirable to select control rods in consideration of condenser protection.

In an event that does not require selective insertion of control rods due to a temporary voltage drop of the on-site power supply due to an instantaneous power failure event and the securement of the core flow rate, the turbine steam control valve 15 may not close rapidly. Since it does not exist, it does not lead to erroneous insertion of the control rod and unnecessary reduction of output. Further, the rapid closing signal 14 of the turbine steam control valve 15 is multiplexed and has extremely high reliability. Therefore, it is considered that the probability that the control rod is inserted erroneously due to the erroneous signal of the rapid closing of the turbine steam control valve 15 is extremely low.

Furthermore, by selecting a control rod with a negative reactivity that can secure the thermal margin of the fuel rod and does not lead to the reactor shutdown state, even if it malfunctions, the output is temporarily output. Decrease, but the reactor will not be shut down,
It becomes easy to immediately return to the steady state.

The block diagram of FIG. 4 shows another embodiment of the present invention, in which the detecting means for detecting the event causing the stop of the recirculation pump is used in the above-mentioned one embodiment at the time of shifting to the independent operation in the office by the load interruption of the generator. It is provided with an event detector for preventing the selective insertion of the control rod.

This event detection unit is provided with a voltmeter 39 for detecting a voltage drop on the service high voltage busbar 6 which is supplying electric power from the generator 1 to the auxiliary equipment system 8 including the recirculation pump units 7 and 7. With the provision of the voltage drop signal 40, the generator load cutoff is detected by the load cutoff signal 25 obtained from the rapid closing signal 14 of the turbine steam control valve 15 as the cause event of the recirculation pump stop in the same manner as in the above embodiment. The detection signal 42 is taken out from the pressure control device 13 and is output to the logic means 26 via the AND circuit 41, and the other configurations are the same as those in the above-mentioned one embodiment.

Therefore, the detection signal 42 is sent to the logic means 26 on condition that both the load cutoff signal 25 and the voltage drop signal 40 of the high voltage busbar 6 from the voltmeter 39 are established, and the control rod is selectively inserted. carry out. According to the other embodiment, since the voltage to the recirculation pump units 7 and 7 by the service high voltage bus bar 6 is established when the operation shifts to the in-house isolated operation due to the load interruption of the generator, the control rod selective insertion according to the present invention is performed. Is not done.

Further, a voltage drop of the on-site power supply is detected by the voltmeter 39 due to a momentary power failure event, and the voltage drop signal 40 is output.
In addition, in the event that the selective control rod insertion is not required to secure the core flow rate, the turbine steam control valve 15 does not close suddenly, so the load control signal from the pressure control device 13
25 is not output, and therefore the detection signal from the AND circuit 41
Since 42 is not output to the logic means 26, the control rod selection insertion is not performed, and therefore erroneous insertion of the control rod and unnecessary reduction of the output do not occur.

The block diagram of FIG. 5 shows another embodiment of the present invention. As a detecting means for detecting the event causing the stop of the recirculation pump, the detection of the loss of the external power source is used for the regular high voltage bus 6
The voltage drop signal 40 from the voltmeter 39 installed at the
Is provided to deliver the detection signal 42 from the timed relay 43 to the logic means 26. The part after the logic means 26 has the same structure as that of the above-mentioned embodiment.

According to this and other embodiments, it is conceivable that the control rod selection insertion may malfunction like a low voltage scrum due to momentary power failure and loss of external power source. However, since the control rods are selected so as not to reach the reactor shutdown state, the steady state can be immediately returned after the malfunction.

The device for detecting the power loss in the station according to the present invention is not limited to the voltmeter 39, and the load cutoff signal 25
Is not limited to the quick closing signal 14 of the turbine control valve 15. Further, the core flow rate data 30 and the core power distribution data 34 are not limited to raw data from a core flow meter, a neutron flux monitor, etc. (not shown), but data obtained in advance by analysis or the like may be used.

Although the external power supply loss event has been taken as an example of the event causing the recirculation pump to stop, the present invention is not limited to this, and an accident may occur in which the cooling system of the recirculation pump fails and the backup system does not start. Even if it occurs, a plurality of recirculation pumps may be stopped at the same time, the core flow rate may be rapidly reduced, and the thermal margin of the fuel rod may be lost. Even in such a case, it can be applied by changing the detecting means of the cause event in the present invention from the detecting means of the power loss to the detecting means of the failure signal of the recirculation pump cooling system.

[0053]

As described above, according to the present invention, in the event of a sudden decrease in the core flow rate when the recirculation pump is stopped due to loss of external power source, etc., by immediately selecting and inserting an appropriate control rod, the thermal operation of the fuel rod in the reactor can be improved. In addition to securing a sufficient margin, the power source in the plant is temporarily lost due to a momentary power failure, etc., but because the core flow rate is maintained and a thermal margin is secured in order to recover, there is no need to reduce the reactor output. On the other hand, it becomes possible to avoid the insertion of the control rod and prevent an unnecessary decrease in output.

Further, since the interlock in the present invention is not the interlock of the reactor scrum, even if it malfunctions, it does not bring the reactor into the stopped state,
It also has the effect of making the plant strong against fluctuations in the power transmission system.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a main part of a reactor power control device according to an embodiment of the present invention.

FIG. 2 is a block configuration diagram of logic means of an embodiment according to the present invention.

FIG. 3 is a furnace power distribution diagram of one example according to the present invention, (a initial state), (b transient period), (c settling time).

FIG. 4 is a block configuration diagram of a main part of a reactor power control system of another embodiment according to the present invention.

FIG. 5 is a block configuration diagram of a main part of a reactor power control device according to another embodiment of the present invention.

FIG. 6 is a single wire connection diagram of a power supply system of a nuclear power plant.

FIG. 7 is a configuration diagram of a main part of a protection system of a nuclear power plant.

FIG. 8 is a reactor power distribution diagram of a conventional reactor power control device.

FIG. 9 is a reactor power distribution diagram of another conventional reactor power control device.

[Explanation of symbols]

13 ... Pressure control device, 26 ... Logic means, 27 ... Calculation part, 28 ... Reference data storage part, 30 ... Core flow rate data, 34 ... Core power distribution data, 36 ... Control rod selection means.

Claims (1)

[Claims] 1. Data and a reference of a core flow rate and a reactor output distribution, which are separately introduced and include a detection means for detecting an event causing a stop of a recirculation pump when the core flow rate of a boiling water type light water reactor suddenly decreases, and a calculation section and a reference data storage section. A logical means for comparing the data and calculating the negative reactivity and its reactivity distribution necessary to secure the thermal margin of the reactor when the core flow rate suddenly decreases,
The optimum number of control rods and the insertion pattern for giving a negative reactivity are selected based on the negative reactivity distribution by the signal of the logic means, and the thermal margin of the in-core fuel rod is lost when the core flow rate is rapidly reduced. A reactor output control device comprising: a control rod selecting means for reducing the reactor output so as not to exist.