JP2000056068A - Control rod extraction monitoring device and control rod control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extract a control rod without reducing the output of a reactor when the neutron fluctuation quantity is increased and to prevent the extraction of the control rod against the local output increase in a core. SOLUTION: This control rod extraction monitoring device 2A is provided with an LPM output average processing device 3, an adjusting means 4, an amplifier 5, a low-pass filter 6, a comparator 7 and a decision criterion setter 8. The adjusting means 4 adjusts the gain of the amplifier 5 according to the size relation between the core average neutron bundle from an APRM 10 and the local average neutron bundle from the LPRM output average processing device. The amplifier 5 amplifies the local average neutron bundle at the adjusted gain and outputs it to the low-pass filter 6. The low-pass filter 6 suppresses the fluctuation of the neutron bundle included in an input signal and outputs a low-frequency signal. The comparator 7 outputs a control rod extraction blocking signal 9a when the level of the low-frequency signal exceeds a set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod pull-out monitoring device and a control rod control device, and more particularly to a control device for a boiling water reactor (referred to as BWR). The present invention relates to a control rod withdrawal monitoring device and a control rod control device that determine whether or not a control rod can be withdrawn based on a neutron flux.

[0002]

2. Description of the Related Art A BWR has a large number of fuel assemblies loaded in a core. These fuel assemblies have a plurality of fuel rods filled with nuclear fuel. In a conventional BWR, an average value of local outputs of neutron detectors located around a control rod selected as a target to be extracted during power operation of a nuclear reactor is obtained,
The set value of the reactor power determined by the core flow rate is compared with the average value of the local power, and when the average value of the local power exceeds the set value of the reactor power, the control rod withdrawal prevention signal is controlled by the control rod. A control rod withdrawal monitoring device that outputs to the means is applied. For example, if the control rod is accidentally withdrawn, the fuel rod may be damaged due to local power increase in the core.However, the control rod withdrawal monitoring device outputs the control rod withdrawal inhibition signal to output the control rod withdrawal signal. Prevent damage.

[0003] In the operation of the BWR, the reactivity of the core decreases with the burning of the nuclear fuel material. Therefore, the flow rate of the coolant (core flow rate) supplied to the core is increased to compensate for the decrease in the reactivity, and the atomic reactivity is reduced. Maintain constant furnace power. When the core flow rate reaches the maximum value, it is not possible to compensate for the decrease in the reactivity, and the reactor power cannot be maintained at the rated power (100% power). Therefore, it is necessary to increase the reactor power by pulling out the control rod. In the nuclear reactor, in order to maintain the integrity of the fuel rods and to prevent the reactor power from exceeding the rated power, the control rod is not pulled out near the rated power. When performing the control rod pull-out operation, the reactor power is sufficiently reduced by reducing the core flow rate, the control rod pull-out operation is performed, and then the reactor flow is increased to the rated power by increasing the core flow rate. Raising. However, once the reactor power is reduced, the capacity factor of the BWR plant is reduced, which is not desirable. In addition, it is not preferable from any point of view, such as causing a decrease in the core flow rate, an increase in the burden on the operator for the operation of the increasing rod, and an increase in disturbance to the power system due to a decrease in the reactor power. Therefore, it is preferable that the number of times the control rod is withdrawn while reducing the reactor power is as small as possible.

[0004] In recent years, from the viewpoint of effective utilization of uranium resources and securing of energy security, plans for using plutonium in a light water reactor (pluthermal) have been progressively advanced.
In the future, about one-third of the fuel assemblies loaded in the core of the operating boiling water nuclear power plant will be replaced with fuel assemblies containing mixed oxides (MOX) of uranium and plutonium (MOX) during production. (Referred to as a fuel assembly). The remaining 2/3 is a fuel assembly that does not contain a mixed oxide of uranium and plutonium at the time of manufacturing but contains uranium oxide (referred to as a uranium fuel assembly). Further, it is planned that all the fuel assemblies in the core be MOX fuel assemblies.

[0005]

The difference between a MOX fuel assembly and a uranium fuel assembly is that the MOX fuel assembly contains plutonium at the time of manufacture of the fuel assembly. Plutonium has a smaller delayed neutron ratio and a larger neutron absorption cross section than uranium. That is, the output change amount when the coolant void amount changes becomes large. This means that the void coefficient is large, and the amount of neutron fluctuation at high reactor power becomes large. It has been newly found by simulation that the fluctuation amount becomes ± 5% or more at the rated output.

A core loaded with a MOX fuel assembly (MO
Also in X fuel core), it is desirable to perform the operation of pulling out the control rod without lowering the reactor power as described above. As described above, in this core, the amount of neutron fluctuation changes by ± 5% or more of the rated power at the time of high reactor power. The control rod withdrawal monitoring device has a logic for inhibiting the control rod withdrawal operation when the output of the neutral detector exceeds 105%. For this reason, even if an attempt is made to perform the control rod withdrawal operation without lowering the reactor power, the control rod withdrawal is prohibited due to neutron fluctuation. In other words, in the MOX fuel core,
Control rod withdrawal is prohibited due to neutron fluctuations, and a problem arises in that it is impossible to perform control rod withdrawal operation without reducing the reactor power.

In a conventional BWR in which a uranium fuel assembly is loaded in a core, as the number of operation cycles increases, the produced plutonium is accumulated in the fuel assembly, and the void coefficient increases. Also, in the first half of one operation cycle, the core flow is decreased to increase the void volume, and thereby the plutonium is positively accumulated in the fuel assembly, and in the second half of the operation cycle, the void volume is reduced by the core flow increase and accumulated. Spectral shift operation is performed to burn the plutonium that has been burned to increase the utilization rate of nuclear fuel. In the first and second half of the operation cycle, the reactor power is maintained at the rated power by using the control rod operation and the core flow rate control together. When performing the spectrum shift operation, the neutron fluctuation amount in the first half of the operation cycle (when the core flow rate is low) is larger than that in the second half of the operation cycle, but at present, the control rod is pulled out due to the neutron fluctuation at the high reactor power. It will not be banned. However, when a fuel assembly with high enrichment is loaded, the operation cycle advances, or the operation cycle period itself becomes longer, the void coefficient increases and the amount of neutron fluctuation increases, so that the output of the neutral detector increases. There is a possibility that the reference value of the pullout monitoring device may exceed 105%. As a result, there arises a problem that control rod withdrawal at the time of high reactor power output is prohibited by the control rod withdrawal monitoring device.

[0008] An object of the present invention is to perform a control rod withdrawal operation without reducing the reactor power when the neutron fluctuation amount increases, and to prevent the control rod withdrawal against a local power increase in the core. It is an object of the present invention to provide a control rod pull-out monitoring device and a control rod control device.

[0009]

A first aspect of the present invention for achieving the above object is as follows.
The features of the invention are amplification means for amplifying the local average neutron flux, gain adjustment means for changing the gain of the amplification means when the local average neutron flux is lower than the core average neutron flux, and low frequency of the output signal of the amplification means. A low frequency pass filter for passing a signal and a comparing means for outputting a control rod withdrawal prevention signal when the low frequency signal from the low frequency pass filter exceeds a set level are provided.

A feature of the second invention that achieves the above object is that, when a control rod for performing a drawing operation is selected and a local average neutron flux is higher than a core average neutron flux, a local value is initially supplied to a low frequency pass filter. Set the value of the average neutron flux,
When the local average neutron flux is lower than the core average neutron flux, the low frequency pass filter is provided with filter initial value setting means for setting the value of the core average neutron flux as an initial value.

A third feature of the present invention to achieve the above object is that a plurality of low frequency pass filters for passing low frequency signals of neutron flux signals output from a plurality of neutron detectors arranged in the core, and a drawing operation Local average neutron flux output means for obtaining a local average neutron flux using output signals of a plurality of low-frequency pass filters that input neutron flux signals of a plurality of neutron detectors that measure neutron flux around the control rod that performs Amplifying means for amplifying the local average neutron flux, gain adjusting means for changing the gain of the amplifying means when the local average neutron flux is lower than the core average neutron flux, and passing a low frequency signal of the output signal of the amplifying means. A low frequency pass filter; and a comparing means for outputting a control rod withdrawal prevention signal when a low frequency signal from the low frequency pass filter exceeds a set level.

A fourth feature of the present invention to achieve the above object is that a plurality of low frequency pass filters for passing low frequency signals of neutron flux signals output from a plurality of neutron detectors arranged in the core, and a drawing operation A first local average neutron flux for obtaining a first local average neutron flux using output signals of a plurality of low frequency pass filters for inputting a neutron flux signal of a plurality of neutron detectors for measuring a neutron flux around the control rod for performing Flux output means, a second local average neutron flux output means for obtaining a second local average neutron flux using a neutron flux signal of a plurality of neutron detectors for measuring the neutron flux around the control rod performing the pull-out operation, Amplification means for amplifying the first local average neutron flux; gain adjustment means for changing the gain of the amplification means when the second local average neutron flux is lower than the core average neutron flux;
A low-pass filter that passes a low-frequency signal of the output signal of the amplifying unit; and a comparing unit that outputs a control rod withdrawal prevention signal when the low-frequency signal from the low-pass filter exceeds a set level. It is in.

A fifth feature of the present invention to achieve the above object is that a switching means for outputting one of an output signal of an amplifying means and an output signal of a low-frequency pass filter, and that the output signal of the switching means exceeds a set level And a comparison means for outputting a control rod withdrawal prevention signal.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A control rod pull-out monitoring device according to a preferred embodiment of the present invention will be described below with reference to FIG.

An outline of a BWR plant to which this embodiment is applied will be described first. The BWR plant has a reactor pressure vessel 1
3, a plurality of fuel assemblies (not shown) loaded in the core in the reactor pressure vessel 13 and a plurality of control rods 15 inserted between the fuel assemblies to control the reactor power. The control rod drive mechanism 16 is provided for each of these control rods 15. The control rod driving mechanism 16 has a motor 18, an electromagnetic brake 19 for stopping the rotation, and the like, and converts the amount of rotation of the motor 18 into linear motion to drive the control rod 15 in the vertical direction. . The position of the control rod 15 in the core height direction is detected by a position detector 17. The core is a MOX fuel core in which uranium fuel assemblies and MOX fuel assemblies are mixed.

By pulling out the control rod 15 from the core by driving the motor 18, the reactor power increases. Cooling water 14 supplied into the core by driving a recirculation pump (not shown) is heated by heat generated by the splitting of the nuclear fuel material while rising in the fuel assembly to become steam. The steam is passed through a main steam pipe 31 to a turbine (not shown).
Sent to Reactor power can also be controlled by controlling the core flow rate with a recirculation pump. As the core flow increases, the reactor power increases.

A plurality of neutron detector strings 12 are disposed in a water gap between the fuel assemblies where control rods 15 are not inserted. One cell is constituted by four fuel assemblies surrounding one control rod 15, and one neutron detector string 12 is arranged at one corner of the cell. Two neutron detectors A, B, C and D for measuring neutron flux at different heights are arranged in the neutron detector string 12, as shown in FIG. When these neutron detector strings 12 are combined into one quadrant of a core horizontal section divided into four quadrants, one neutron detector string 12 is arranged at each corner of each cell. In FIG. 2, the neutron flux at the other three corners of the cell is detected by neutron detectors 12A, 12B and 12C in neutron detector strings 12A, 12B and 12C located in a quadrant different from the neutron detector string 12 is located. Vessel A
~ D.

The control rod pull-out monitoring device of this embodiment comprises a control rod pull-out monitoring device 1 and N local output monitors (LPRMs).
-1 to LPRM-N) and average output monitor (APRM)
10 is provided. The control rod drive mechanism control device includes a control rod operation monitoring device 21 and a control rod controller 22.

When the core flow rate reaches the maximum value and the reactor power cannot be maintained at the rated power due to the increase in the core flow rate, selection is made by inputting an operation command 20 by an operator from a control panel (not shown). The withdrawn operation of the control rod 15 is performed. The control rod operation monitoring device 21 controls a command for pulling out one control rod (a plurality of control rods when operated in the gang mode) to the target position based on the input operation command 20. Output to the bar controller 22. Further, the control rod operation monitoring device 21 outputs the operation command 20
(Selection control rod information 25) indicating the position (position on the horizontal cross section of the core) of the one control rod (or a plurality of control rods) selected based on the above is output to the LPRM output averaging processing device 3 described later. .

The control rod controller 22 controls the corresponding control rod 14
Of the control rod from the position detector 17, and outputs a control command to the control rod driving device 23 to execute control to pull out the control rod to the target position. When the control command is input, the control rod driving device 23 applies the voltage of the power supply 24 to the motor 18 in order to rotate the motor 18 in a direction corresponding to the control rod driving direction. At this time, the electromagnetic brake 19 is released, and the motor 18 rotates. When the control rod 15 is pulled out to the target position, the control rod controller 22 stops outputting a control command, and the control rod driving device 23 cuts off the power supply to the motor 18 and the electromagnetic brake 19. The motor 18 stops and the control rod 15 is held at the target position by the braking force of the electromagnetic brake 19. The reactor power rises and is maintained at the rated power.

Next, the control rod withdrawal monitoring device 1 will be described. The output signals of the neutron detectors A to D in each neutron detector string 12 are LPRM-1, LPRM-2,..., LPRM-N which are local output monitors (LPRM).
Is input to the corresponding monitor. One LPRM
Is connected to one neutron detector. LPRM-1,
LPRM-2,..., LPRM-N receives the neutron flux signal measured by the corresponding neutron detector and compensates for the deterioration in sensitivity of the neutron detector (due to the consumption of uranium applied to the detector). , A filter process for suppressing the effect of high-frequency electrical noise of about 10 Hz or more, a gain process for responding to the reactor output, and the like, and output a neutron flux signal corresponding to the reactor output.

APRM 10 is LPRM-1, LPRM
−2,..., A core average neutron flux (average value of reactor power) is calculated and output using a neutron flux signal output from the LPRM-N. The control rod pullout monitoring device 1 includes control rod pullout monitoring devices 2A and 2B, control rod pullout monitoring device 2A.
And OR gate means 9 for inputting the output signals of FIG.

The control rod withdrawal monitoring device 2A has an LPRM output averaging device 3, an adjusting means 4, an amplifier 5, a low-pass filter 6, a comparator 7, and a criterion setting device 8. The low-pass filter 6 is a digital filter called a finite-length impulse response (FIR) shown in FIG. The delay element Z- ¹ delays the input signal by a filter operation cycle and outputs the delayed signal. a _{0 to} a _k are weighting coefficients, and 30
Is an adder. The FIR filter of FIG. 3 is also called a non-recursive filter, and a weight coefficient a is added to the current input signal x (t).
₀ and the past input signals x (t-1), x (t-
2),..., X (tk) multiplied by a weighting coefficient and added by the adder 30 to output y (t) as an output signal. The characteristics of the low-pass filter 6 are determined by the values of the weighting coefficients a _{0 to} a _k .

As the low-pass filter 6, an infinite-length impulse response (II
A filter called R) may be used. IIR in FIG.
The filter is also called a recursive filter, and forms a loop in which a part of the operation result is fed back to the input and circulates. The characteristics of the low-pass filter 6 are determined by the values of the weighting coefficients α ₁ and β ₁ .

The control rod pullout monitoring device 2B has the same configuration as the control rod pullout monitoring device 2A. The control rod withdrawal monitoring device 2A performs control rod withdrawal monitoring for neutron flux signals of the neutron detectors A and C. Control rod pull-out monitoring device 2B
Performs control rod withdrawal monitoring for neutron flux signals of the neutron detectors B and D. Since the control rod pullout monitoring device 2A and the control rod pullout monitoring device 2B perform the same operation, the operation of the control rod pullout monitoring device 2A will be described in detail below.

The output signals of the LPRMs to which the neutron flux signals of the neutron detectors A and C are input are input to the LPRM output averaging device 3. The LPRM output average processing device 3
The selection control rod information 25 is also input. The LPRM output averaging device 3 performs the neutron detectors A and C in the four neutron detector strings around the control rod 15 at the position specified by the selected control rod information 25 (for example, the neutron detector strings in FIG. 2). Neutron detectors A and C in 12, 12A, 12B and 12C are selected, and a neutron flux signal output from these neutron detectors is used to calculate a local average neutron flux which is an average of these signals. I do. The local average neutron flux is
Input to the amplifier 5. Since the output rise due to control rod withdrawal is small at the outermost periphery in the furnace, the output signals of neutron detectors A and C in three or two neutron detector strings are used for control rod withdrawal in this portion.

The adjusting means 4 has functions of a gain adjusting means and a filter initial value setting means. The function of the gain adjusting means in the adjusting means 4 will be described. The gain adjusting means in the adjusting means 4 adjusts the gain of the amplifier 5 to 1 when the core average neutron flux output from the APRM 10 is lower than the local average neutron flux output from the LPRM output average processing device 3. That is, in this case, the local average neutron flux is directly output from the amplifier 5 and input to the low-pass filter 6.

When the core average neutron flux is equal to or larger than the local average neutron flux, the gain adjusting means in the adjusting means 4 calculates a gain at which the core average neutron flux is equal to the local average neutron flux, and calculates the calculated gain. The gain of the amplifier 5 is adjusted so that The amplifier 5 amplifies the local average neutron flux by using the adjusted gain and outputs the amplified neutron flux to the low-pass filter 6. The gain of the amplifier 5 is fixed and not changed until the next control rod to be pulled out is selected.

Since the gain adjusting means in this embodiment performs the above-described gain adjustment, when the control rod 15 at the position where the neutron flux level is low is selected, if the value of the selected control rod 15 is high, The amount of control rod 15 that can be pulled out can be reduced, and the possibility of fuel rod damage due to the withdrawal of high-value control rod 15 can be prevented. Therefore, the safety of the fuel rod is improved. The gain adjusting means is also necessary for setting the initial value of the low-pass filter 6.

The low-pass filter 6 passes a neutron flux signal which is an output of the LPRM-1 or the like indicating an increase in the local power of the reactor core, and suppresses the fluctuation of the neutron flux included in the output of the neutron detector. It is. The low-pass filter 6 controls the fluctuation frequency included in the output signal of the amplifier 5.
(For example, 0.2 Hz) or more, and a low-frequency signal including a frequency component smaller than the fluctuation frequency is output.

The comparator 7 compares the level of the low-frequency signal from the low-pass filter 6 with the criterion value set in the criterion setter 8. The criterion values set in the criterion setter 8 have three criterion values linked to the core flow rate, as shown in FIG. The three determination reference values are a normal position serving as a threshold value for an upper level warning, an intermediate position serving as a threshold value for an intermediate level warning, and a low position serving as a threshold value for a low level warning. The comparator 7 compares the level of the low frequency signal with the above three determination reference values, and outputs an alarm when the level of the low frequency signal exceeds each of the determination reference values. When the level of the low-frequency signal exceeds the reference value for the correct position, the comparator 7
Outputs a control rod withdrawal prevention signal 9a. Since the core flow rate is 90% or more at the rated output, the control rod withdrawal prevention signal 9a is output when the level of the low frequency signal exceeds 105%.

When at least one of the control rod pull-out inhibiting signal 9a from the control rod pull-out monitoring device 2A and the control rod pull-out inhibiting signal 9b from the control rod pull-out monitoring device 2B is input to the OR gate means 9, the OR gate means The control rod withdrawal prevention signal 9c output from 9 is input to the control rod operation monitoring device 21. The control rod controller 22 controls the control rod withdrawal prevention signal 9c from the control rod operation monitoring device 21 to prevent further withdrawal of the control rod being withdrawn.
A control command for stopping the drawing is output to the control rod driving device 23. The control rod driving device 23 controls the motor 1 of the control rod driving mechanism 16 that operates the corresponding control rod based on the control command.
The supply of power to 8 is stopped. Thus, when the control rod withdrawal prevention signal 9c is output, the withdrawal of the corresponding control rod is stopped.

The LPR of the control rod withdrawal monitoring device 2B
The M-output averaging device 3 performs neutron detectors B and D in the four neutron detector strings around the control rod 15 at the position specified by the selected control rod information 25 (for example, the neutron detector strings in FIG. 2). The neutron flux signals output from the neutron detectors B and D) in 12, 12A, 12B and 12C are used to calculate a local average neutron flux which is an average of these signals. Since the output rise due to the control rod withdrawal is small at the outermost periphery in the furnace, the output signals of the neutron detectors B and D in the three or two neutron detector strings are used for the control rod withdrawal in this portion.

Since the low-pass filter 6 has the delay element Z- ¹ , it handles current and past local average neutron fluxes (output signals of the amplifier 5). When the output signal of the amplifier 5 is applied to the low-pass filter 6, the output of the delay element Z- ¹ is generally zero. For this reason, the level of the low-frequency signal output from the low-pass filter 6 gradually rises from zero and follows the local average neutron flux output from the amplifier 5. Until the level of the low-frequency signal follows the local average neutron flux which is the output of the amplifier 5, it is lower than the criterion value of the correct position, so that the output of the control rod withdrawal prevention signal 9a is actually required. This also causes a problem that the comparator 7 does not output the control rod withdrawal prevention signal 9a. This is because the initial value of the delay element Z ^-1 is zero. Therefore, when the selection control rod information 25 is input, the adjusting means 4 sets the value of the core average neutron flux output from the APRM 10 to the initial value of the delay element Z- ¹ . The initial value of each delay element Z ^-1 may be, for example, the value of the core average neutron flux output from the APRM 10 (or the value of the local average neutron flux output from the amplifier 5).

In the IIR filter of FIG. 4, the delay element Z ^-1
Is zero, the input signal is output as it is as an output signal, so that the initial value of the delay element Z- ¹ may remain zero. In addition, the FI of FIG.
The output of the R filter to which the IIR filter of FIG. 4 is connected may be used. In this case, too, the initial value of each delay element Z ^-1 of the FIR filter is, for example, the value of the core average neutron flux or the value of the amplifier 5. What is necessary is just to set to the value of the local average neutron flux which is an output.

If the initial value setting processing of each delay element Z ^-1 is not performed, the low-pass filter 6 applies the input signal (characteristic I in FIG. 6) to the input signal when the initial value of the delay element Z ^-1 is zero. On the other hand, it outputs a low-frequency signal whose level gradually rises from zero (characteristic II in FIG. 6), or the value of the local average neutron flux which is the output of the amplifier 5 for the fuel assembly adjacent to the control rod withdrawn last time. When this remains in the delay element Z- ¹ , this value is used as an initial value to output a low-frequency signal from an initial value different from that for the fuel assembly adjacent to the control rod to be withdrawn this time. On the other hand, the filter initial value setting means in the adjusting means 4 sets the initial value of each delay element Z ^{-1 to} the value of the core average neutron flux when the selection control rod information 25 for the control rod to be extracted this time is input. When the initial value setting process is performed, the low-pass filter 6 performs a low-frequency signal (characteristic III in FIG. 6) following the input signal from the time when the extraction control rod is selected for the input signal (characteristic I in FIG. 6). Is output.

In this embodiment, the filter initial value setting means in the adjusting means 4 further has the following functions. When the filter initial value setting means determines that the local average neutron flux output from the LPRM output average processing device 3 is higher than the core average neutron flux output from the APRM 10 when the selection control rod information 25 is input, The value of the local average neutron flux is set as the initial value of the delay element Z- ¹ . Also,
When it is determined that the local average neutron flux is equal to or less than the core average neutron flux, the filter initial value setting means sets the value of the core average neutron flux as an initial value of each delay element Z- ¹ .

If the filter initial value setting means does not have such a function, the following problem occurs.
That is, the output of the LPRM output averaging device 3 is APRM10
, The gain of the amplifier 5 is not changed, so that the signal input to the low-pass filter 6, ie, L
The output of the PRM output averaging device 3 is at a higher level than the initial value of the delay element Z- ¹ . In such a state, when the control rod is pulled out and the output level of the LPRM output averaging device 3 rises and the level of the low frequency signal becomes equal to or more than the reference value (105%) of the correct position, the comparison is performed. Vessel 7
Means that the control rod withdrawal prevention signal 9a is output much later because the initial value of the delay element Z ^-1 is lower than the output level of the LPRM output averaging device 3. The prevention of the output delay of the control rod withdrawal prevention signal 9a is achieved by the L
The filter initial value setting means may have an initial value setting function of the delay element Z- ¹ based on the magnitude relation between the local average neutron flux and the core average neutron flux output from the PRM output average processing device 3.

In this embodiment, the initial value of the delay element Z- ¹ is set by the filter initial value setting means in accordance with the magnitude relationship between the output of the LPRM output averaging device 3 and the output of the APRM 10, so that the low-pass filter 6 When the control rod is pulled out, the filter operation of the input signal can always be performed with the same value as the level of the output signal of the amplifier 5 first input to the low-pass filter 6 as the initial value. For this reason,
The output of the low-pass filter 6 can be prevented from being unnecessarily delayed, and the output delay of the control rod withdrawal prevention signal 9a from the comparator 7 can be prevented. There is no delay in the output of the control rod pullout prevention signal 9c from the control rod pullout monitoring device 1, and the pulling out of the corresponding pullout control rod can be prevented in a short time.

In this embodiment, the gain of the amplifier 5 is adjusted based on the output of the gain adjusting means, and the output of the amplifier 5 is input to the low-pass filter 6 to suppress the frequency components higher than the fluctuation frequency. In the reactor core, even if the local average neutron flux which is the output of the amplifier 5 due to an increase in the neutron fluctuation at the high reactor power, for example, the rated power, becomes a value exceeding the reference value of the normal position, the neutron It is possible to prevent the control rod from being pulled out due to the increase in the fluctuation amount. For this reason, when the amount of neutron fluctuation increases, the reactor power is not reduced by reducing the core flow rate, that is, the control rod 15 is pulled out at the rated power to compensate for the decrease in the reactor power accompanying the burning of nuclear fuel. it can. Further, in the present embodiment, when the local average neutron flux increases due to a local power increase in the core and the level of the low-frequency signal of the low-pass filter 6 exceeds the reference value of the correct position, the corresponding control is performed. Bar pull-out can be prevented.

Since the control rod can be pulled out to maintain the rated output without lowering the reactor power, B
The facility utilization rate of the WR plant can be improved, and the burden on the operator can be reduced. The present embodiment also has the effect of suppressing an increase in disturbance to the power system.

This embodiment is directed to a BW having a MOX fuel core.
Not only the R plant, but also a BWR plant equipped with a core loaded with a uranium fuel assembly and not loaded with a MOX fuel assembly, and a highly enriched uranium fuel assembly loaded with a spectrum shift operation. Thus, the present invention can be applied to a BWR plant having a core in which the amount of neutron fluctuation at the rated output increases, and in these cases, the above-described effect is also obtained.

A control rod pull-out monitoring device according to another embodiment of the present invention will be described below with reference to FIG. This embodiment is different from the embodiment of FIG. 1 in the installation location of the low-pass filter, and the adjusting means 4 has the function of the gain adjusting means and does not have the function of the filter initial value setting means. Other configurations of the present embodiment are the same as those of the embodiment of FIG.
The low pass filter ₆₁ to the LPRM-1, the low-pass filter 6 ₂ The LPRM-2, ......, a low-pass filter 6
_N is connected to LPRM-N, respectively. The configuration of the low-pass filters 6 ₁ , 6 ₂ ,..., 6 _N is the same as the configuration of the low-pass filter 6 in FIG. For this reason, in the present embodiment, regardless of whether or not the pullout control rod is selected,
A filtering process similar to that of the low-pass filter 6 in FIG. 1 is always individually performed on each output signal of LPRM-1 to LPRM-N. Therefore, the filter initial value setting means in the embodiment of FIG. 1 becomes unnecessary. This embodiment eliminates the need for the above-described initial value setting process for each low-pass filter, and, similarly to the embodiment of FIG. 1, does not perform a reactor power reduction operation due to a decrease in core flow when the neutron fluctuation amount increases. Thus, it is possible to compensate for a decrease in the reactor power caused by the burning of the nuclear fuel due to the withdrawal of the control rod 15. Further, in the present embodiment, when the local average neutron flux increases due to a local power increase in the core, it is possible to prevent the corresponding control rod from being pulled out.

In the embodiment shown in FIG. 7, the adjusting means 4 includes the APRM 10
The core average neutron flux and the local average neutron flux become equal when the core average neutron flux which is the output of the above is equal to or larger than the local average neutron flux which is the output of the LPRM output averaging device 3.
The gain of the amplifier 5 is calculated. However, in the embodiment of FIG. 7, since the low-pass filters 6 _{1 to} 6 _N are provided at the input stage of the LPRM output averaging device 3, the output of the LPRM output averaging device 3 is a signal having a time delay from the output of the APRM 10. And the calculation of the gain becomes complicated. Therefore, it takes a lot of time to calculate the gain, the output of the control rod withdrawal prevention signal 9c is delayed, and there is a possibility that the safety margin is reduced. This problem can be solved by the embodiment shown in FIG.

FIG. 8 shows a control rod pull-out monitoring device according to another embodiment of the present invention. In the present embodiment, an LPRM output averaging processor 3 'is newly provided in the control rod withdrawal monitoring device 2A, and the output of the LPRM output averaging processor 3' is used in the adjusting means 4 instead of the output of the LPRM output averaging processor 3 '. That is to input. Also in this embodiment, the control rod pullout monitoring device 2B has the same configuration as the control rod pullout monitoring device 2A. The LPRM output averaging device 3 '
M-1 to LPRM-N outputs are input as they are,
Similarly to the M output averaging device 3, the neutron detectors A and C in the four neutron detector strings around the control rod 15 at the position specified by the selection control rod information 25 are selected, and these neutron detectors are selected. Is used to calculate the local average neutron flux. When the core average neutron flux which is the output of the APRM 10 is lower than the local average neutron flux which is the output of the LPRM output averaging device 3 ', the adjusting means 4 adjusts the gain so that the core average neutron flux and the local average neutron flux become equal. Is calculated, and the gain of the amplifier 5 is adjusted so as to become the calculated gain. Otherwise, the adjusting means 4 sets the gain of the amplifier 5 to 1
To The output signal of the LPRM output averaging device 3 is amplified by the amplifier 5 whose gain has been adjusted as described above.
Other operations in this embodiment are the same as those in the embodiment of FIG.

In this embodiment, by newly providing the LPRM output averaging device 3 ', the gain of the amplifier 5 can be calculated more easily and faster than in the embodiment of FIG.
The response time to control rod withdrawal prevention can be further reduced.
This embodiment can also obtain the effect produced in the embodiment of FIG.

It should be noted that the embodiment of FIGS.
The output of -1~LPRM-N are input to APRM10 and the low-pass filter 6 ₁ to 6 _N. However, in the embodiment of FIGS. 7 and 8, receives the output of the LPRM-1~LPRM-N to the low pass filter 6 ₁ to 6 _N, the output of the low pass filter 6 ₁ ~6 _N APRM10 and LPRM output averaging processor If the number is input to 3, the following problem occurs.

Although not shown, the APRM 10 outputs a trip signal for scram to a reactor protection system (not shown) when the core average neutron flux exceeds a scram set value. The reactor protection system opens a solenoid valve provided on a line leading to an accumulator filled with high-pressure water based on the trip signal. The high-pressure water in the accumulator is supplied into the control rod drive mechanism 16 to rapidly insert the control rod 15 into the core. This scrams the BWR.

The response time required for the core average neutron flux to reach the set value and to cause scram is 90 ms or less,
Required for safety. For this reason, LPRM-1 to LPRM-1
The provision of large low-pass filter 6 ₁ to 6 _N of the time constant between the RM-N and APRM10, the scram response time 90ms
The following will not be satisfied. The embodiment of FIGS.
RM-1~LPRM-N and not by placing a low-pass filter 6 ₁ to 6 _N between APRM10, the response time to cause scram core average neutron flux reaches the set value 90ms
Below, the safety is extremely high.

Another embodiment of the present invention will be described below with reference to FIG. This embodiment is different from the embodiment of FIG. 1 in that the control rod withdrawal monitoring device 1 is replaced with a control rod withdrawal monitoring device 1A and a switch command switch 32 is further provided. The control rod pull-out monitoring device 1A includes control rod pull-out monitoring devices 2A and 2B like the control rod pull-out monitoring device 1. The control rod pull-out monitoring devices 2A and 2B of the control rod pull-out monitoring device 1A have a changeover switch 33. Changeover switch 3
3 is connected to the output terminals of the amplifier 5 and the low-pass filter 6 and to the input terminal of the comparator 7.

The changeover switch 33 is a changeover command switch 3
2 controls the connection state. Switch 33
Has a first contact that is normally open and a second contact that is normally closed. FIG. 9 shows a state in which the output signal of the amplifier 5 is input to the comparator 7 via the second contact in a normal state. Therefore, the output of the low-pass filter 6 is not input to the comparator 7. When the switch command switch 32 is closed,
Conversely, since the first contact is closed and the second contact is opened, the output of the low-pass filter 6 is output to the comparator 7, but the output signal of the amplifier 5 is not input to the comparator 7. That is, either the output of the amplifier 5 or the output of the low-pass filter 6 is selected by the switch command switch 32 and input to the comparator 7. The power supply 34A and the resistor 35
It is provided to output a switch command signal to the switch 33 when the switch command switch 32 is closed.

Further, the state of the switch command switch 32, that is, whether the output of the amplifier 5 or the output of the low-pass filter 6 is output to the comparator 7, can be monitored by the display 34. As will be described later,
The plant operator can easily recognize whether or not the low-pass filter 6 operates to suppress the neutron fluctuation according to the plant state. The display 34 is attached to a central control panel capable of being operated and monitored by a plurality of operators to easily and commonly determine whether or not the low-pass filter 6 is operated by the plurality of operators to suppress neutron fluctuation. Recognition can further improve the reliability of operation monitoring. The display 34 is supplied with necessary power from a power supply 34B.

The neutron fluctuation is caused by the rated output (10
0% output), up to about 1/3 of the replacement fuel is MO
About 3% in case of X fuel, MOX in the core
In the case of fuel, it becomes ± 5% or more. The control rod withdrawal monitoring device 1A has a logic that prohibits the control rod withdrawal operation when the neutron output exceeds 105%. Therefore, when about 1/3 of the replacement fuel is the MOX fuel core, The neutron fluctuation does not prevent the control rod withdrawal, but when the inside of the core is made of MOX fuel, the neutron fluctuation acts to prevent the control rod withdrawal. The low-pass filter 6 has an LPR that indicates a local power increase of the core.
The M output signal is passed to suppress the fluctuation of the neutron output signal, but has a time delay. If there is a time delay, the determination of control rod pull-out prevention is delayed by that much.

It is necessary to secure a safety margin in anticipation of the delay time.
The margin is slightly reduced. Up to about 1/3 of the replacement fuel
In the case of MOX fuel core, neutron output signal is 105%
In this case, the changeover switch 3
3 is controlled so that the output signal of the amplifier 5 is output to the comparator 7 and compared with the determination reference value, so that the safety margin can be further improved.

When everything in the core is MOX fuel,
Neutron fluctuation is more than ± 5%. In this case, the changeover switch 33 is controlled to output the output signal of the low-pass filter 6 to the comparator 7 so as to compare the output signal with the determination reference value, thereby preventing the control rod from being pulled out due to neutron fluctuation. In addition, it is possible to improve the capacity factor of plant operation. The same applies to the case where the neutron fluctuation amount increases due to the spectrum shift operation.
When it becomes 5% or more, the changeover switch 33 is controlled to output the output signal of the low-pass filter 6 to the comparator 7 so as to be compared with the judgment reference value, thereby preventing the control rod from being pulled out due to neutron fluctuation. Can be prevented.

The neutron output, which is the output signal of the APRM 10, is output to the operation monitoring panel (not shown).
The changeover switch 33 can be easily controlled by operating the changeover command switch 32 based on this instruction value. Also, the neutron output, which is the output signal of the APRM10, is not shown, but is taken into the core performance calculator.
It is determined whether or not the neutron output exceeds 105%, the result of the determination is output to a display device, a printer, or the like, and the changeover command switch 32 is operated based on the result to easily control the changeover switch 33. Is possible.

The comparator 7 compares the output of the changeover switch 33 with the judgment reference value from the judgment reference setting device 8, and if the output of the changeover switch 33 is larger than the judgment reference value 8, prevents the control rod from being pulled out. Rod removal prevention signal 9a
Is output. The control rod withdrawal prevention signal 9a is output to the control rod operation monitoring device 21 via the OR gate means 9, and the control rod withdrawal operation is prohibited.

If the low-pass filter 6 is not supposed to work, but the switch command switch 32 is touched by mistake and switched, and if the low-pass filter 6 is made to work, the timing for preventing the control rod from being pulled out is not reached. Even so, it is conceivable that the control rod withdrawal prevention determination is delayed and the safety is reduced. In order to solve such a problem, it is assumed that the changeover command switch 32 operates in a double action (for example, a switch that can be switched only after the switch lever is pulled up).

The control rod withdrawal monitoring device 1A can also be achieved by a computer, that is, by software processing, and the processing flow is as shown in FIG.

First, in step 1, various signals,
For example, an output signal of LPRM is taken. Then step 2
In, the average of the LPRM signal around the selection control rod is calculated. In step 3, the LPRM average value around the selected control rod selected in step 2 is compared with the APRM output value input in step 1, and a gain for amplifying the LPRM average value is calculated. The furnace average value (APRM output value) output from the average power monitor 10 is compared with the above-described LPRM average value. If the furnace average value is higher than the LPRM average value, the above-described LPRM average value is compared with the furnace average value. The gain is determined to be equal, and when the gain is low, the gain is set to 1. However, the determined gain is fixed until a new control rod is selected.

In step 4, the L selected in step 2
Multiply the PRM average value by the gain selected in step 3. In Step 5, it is determined whether or not the switch command switch 32 is ON (closed state). If not ON, go to step 7. If it is ON, go to step 6. In step 6, for example, the FIR filter (low-pass filter) shown in FIG.

If the switch command switch 32 has not been turned on, it is determined in step 7 whether the value obtained in step 4 is higher than the determination reference value (reference value in FIG. 5). If the changeover command switch 32 is ON, step 7
Then, it is determined whether the value after the filter processing in step 6 is higher than the comparison reference value (the reference value in FIG. 5). In either case,
If it is higher than the comparison reference value, the process proceeds to step S8. If it is lower than the comparison reference value, the process ends. In step 8, a control rod pull-out prevention signal is output. These processes are periodically executed at a predetermined cycle.

As described above, it is possible to prevent the control rod withdrawal from being prevented due to an increase in the amount of neutron fluctuation at high power, and the neutron output is increased due to a local increase in power in the core. In such a case, it is possible to prevent the control rod from being withdrawn.

Although not shown, the APRM 10 outputs a trip signal for scram to the reactor protection system when the average neutron output value (reactor average value) exceeds a predetermined value. It is required for safety that the time required for the average neutron output value to exceed a predetermined value to cause a scram is 90 ms or less.
Therefore, if a low-pass filter having a large time constant is provided between LPRM-1 to LPRM-N and the APRM 10, there is a problem that the scrum response of 90 ms is not satisfied. Therefore, such a configuration is not allowed for safety.

[0065]

According to the first aspect of the invention, when the local average neutron flux is lower than the core average neutron flux, the gain of the amplification means is changed based on the output of the gain adjustment means, and the gain is changed. To the low frequency pass filter to suppress the frequency components above the fluctuation frequency and pass the low frequency signal, avoiding prevention of control rod withdrawal due to the increase in neutron fluctuation at high reactor power. it can. For this reason, when the neutron fluctuation amount increases, it is possible to compensate for a decrease in the reactor power accompanying the burning of the nuclear fuel material by pulling out the control rod without performing a reactor power reduction operation due to a decrease in the core flow rate. Further, when the level of the low frequency signal exceeds the set level, it is possible to prevent the corresponding control rod from being pulled out.

According to the second invention, when the control rod for performing the drawing operation is selected and the local average neutron flux is higher than the core average neutron flux, the value of the local average neutron flux is set as an initial value in the low frequency pass filter. When the local average neutron flux is lower than the core average neutron flux, the core average neutron flux value can be set as the initial value in the low frequency pass filter, so the initial value of the low frequency pass filter becomes a value larger than zero. Thus, the output delay of the control rod pull-out prevention signal can be improved.
For this reason, it is possible to prevent the corresponding pull-out control rod from being pulled out in a short time.

According to the third invention, since each low-frequency signal passed through the plurality of low-pass filters is input to the local average neutron flux output means, the filter initial value setting processing of the second invention is not required. .

According to the fourth aspect, in addition to the first local average neutron flux output means for obtaining the first local average neutron flux around the relevant extraction control rod using the output signals of the plurality of low frequency pass filters, A second local average neutron flux output means for obtaining a second local average neutron flux around the extraction control rod by using the neutron flux signals of the plurality of neutron detectors measuring the neutron flux around the relevant extraction control rod; When the second local average neutron flux is lower than the core average neutron flux, the gain of the amplifying means is changed by the gain adjusting means, so that the gain of the amplifying means can be easily calculated, and the control rod withdrawal is prevented. Response time can be further reduced.

According to the fifth aspect, the switching means for outputting either the output signal of the amplifying means or the output signal of the low-pass filter, and the control rod withdrawal when the output signal of the switching means exceeds the set level Since the comparison means for outputting the inhibition signal is provided, the input signal to the comparison means can be selected as either the output signal of the amplification means or the output signal of the low-pass filter by operating the switching means. For this reason, even if the loading ratio of MOX fuel into the reactor core differs in the operation cycle, and as a result, the neutron fluctuation differs in the operation cycle, the control rod pullout monitoring device provided in common by one unit
Even if the neutron output exceeds the reference value for control rod withdrawal monitoring due to an increase in the amount of neutron fluctuation, it is possible to prevent control rod withdrawal at high power from being prevented from occurring and to prevent local When the neutron output rises due to the power rise, control rod withdrawal can be prevented.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a control rod pull-out monitoring device according to a preferred embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an arrangement relationship between a control rod and a neutron detector in a reactor core in the reactor pressure vessel of FIG.

FIG. 3 is a detailed configuration diagram of a low-pass filter (FIR filter) of FIG. 1;

FIG. 4 is a configuration diagram of an IIR filter which is another embodiment of the low-pass filter.

FIG. 5 is a characteristic diagram showing a relationship between a core flow rate and a control rod pullout prevention determination reference value.

FIG. 6 is an explanatory diagram showing an input signal and an output signal in the low-pass filter of FIG.

FIG. 7 is a configuration diagram of a control rod pull-out monitoring device according to another embodiment of the present invention.

FIG. 8 is a configuration diagram of a control rod pull-out monitoring device according to another embodiment of the present invention.

FIG. 9 is a configuration diagram of a control rod pullout monitoring device according to another embodiment of the present invention.

FIG. 10 is a flowchart showing a software processing flow of the control rod pull-out monitoring device of FIG. 9;

[Explanation of symbols]

1, 2A, 2B ... control rod pull-out monitoring device, 3 ... LPRM
Output averaging device, 4 ... adjustment means (gain adjustment means, filter initial value setting means), 5 ... amplifier, 6 ... low-pass filter, 7 ... comparator, 8 ... judgment criterion setter, 9 ... OR gate means, 10 ... Average output monitor (APRM), 12, 1
2A, 12B, 12C neutron detector strings, 13
... Reactor pressure vessel, 15 ... Control rod, 16 ... Control rod drive mechanism, 18 ... Motor, 21 ... Control rod operation monitoring device, 22 ...
Control rod controller, 25: selected control rod information, 32: switch command switch, 33: switch, A, B, C, D: neutron detector, LPRM-1, LPRM-2, LPRM-N
... local output monitor, Z ^-1 ... delay element.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Makoto Hasegawa 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. Hitachi, Ltd. Power and Electricity Development Division (72) Inventor Masaru Sasakawa, Sachimachi, Hitachi, Ibaraki Hitachi 1-1, Hitachi, Ltd. 3-chome, Hitachi, Ltd. (72) Inventor Takeshi Nozaki 3-1-1, Sakaimachi, Hitachi, Ibaraki, Japan Hitachi-Hitachi Ltd., Hitachi Plant (72) Inventor, Kazuhiko Ishii, Ibaraki 5-2-1, Omika-cho, Hitachi City Hitachi, Ltd. Omika Plant (72) Inventor Hiromi Maruyama 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref.Hitachi, Ltd. 72) Inventor Yoshihiko Ishii 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. Hitachi, Ltd.

Claims (10)

[Claims] 1. A core average neutron flux output means for obtaining a core average neutron flux using neutron flux signals output from a plurality of neutron detectors arranged in a core, and a neutron flux around a control rod for performing a drawing operation Local average neutron flux output means for obtaining a local average neutron flux using a neutron flux signal of a plurality of neutron detectors for measuring the neutron flux, amplification means for amplifying the local average neutron flux, and the local average neutron flux is the core average Gain adjusting means for changing the gain of the amplifying means when the neutron flux is lower than the neutron flux; a low-frequency pass filter for passing a low-frequency signal of the output signal of the amplifying means; and the low-frequency signal from the low-frequency pass filter And a comparison means for outputting a control rod withdrawal prevention signal when the control signal exceeds a set level. 2. The gain adjusting means changes a gain of the amplifying means so that the local average neutron flux becomes equal to the core average neutron flux when the local average neutron flux is lower than the core average neutron flux. The control rod withdrawal monitoring device according to claim 1. 3. When the control rod for performing the drawing operation is selected, and when the local average neutron flux is higher than the core average neutron flux, the value of the local average neutron flux is provided as an initial value to the low frequency pass filter. Filter initial value setting means for setting the value of the core average neutron flux as an initial value to the low frequency pass filter when the local average neutron flux is lower than the core average neutron flux, The control rod pullout monitoring device according to claim 1 or 2. 4. A method according to claim 1, wherein the plurality of neutron detectors disposed in the reactor core pass a low-frequency signal of a neutron flux signal output from the plurality of neutron detectors, and the plurality of neutron detectors disposed in the reactor core. A core average neutron flux output means for obtaining a core average neutron flux using the output neutron flux signal, and a plurality of neutron flux signals for inputting a neutron flux signal of a plurality of neutron detectors for measuring a neutron flux around a control for performing a drawing operation. Using an output signal of the frequency pass filter, a local average neutron flux output unit that obtains a local average neutron flux, an amplification unit that amplifies the local average neutron flux, and the local average neutron flux is lower than the core average neutron flux. A gain adjusting means for changing a gain of the amplifying means, a low-frequency pass filter for passing a low-frequency signal of an output signal of the amplifying means, and a low-frequency pass filter. Wherein when the low-frequency signal exceeds a predetermined level, the control rod withdrawal monitoring device characterized by comprising a comparison means for outputting a control rod withdrawal inhibit signal from. 5. The gain adjusting means changes the gain of the amplifying means so that when the local average neutron flux is lower than the core average neutron flux, the local average neutron flux becomes equal to the core average neutron flux. The control rod withdrawal monitoring device according to claim 4. 6. A low-pass filter for passing low-frequency signals of neutron flux signals output from a plurality of neutron detectors disposed in the core, and a plurality of neutron detectors disposed in the core. A core average neutron flux output means for obtaining a core average neutron flux using the output neutron flux signal, and a plurality of neutron flux signals for inputting a neutron flux signal for a plurality of neutron detectors for measuring a neutron flux around a control rod for performing a drawing operation. First local average neutron flux output means for obtaining a first local average neutron flux using an output signal of a low frequency pass filter, and the plurality of neutron detectors for measuring neutron flux around a control rod performing the drawing operation A second local average neutron flux output means for obtaining a second local average neutron flux using the neutron flux signal;
Amplifying means for amplifying the first local average neutron flux; gain adjusting means for changing a gain of the amplifying means when the second local average neutron flux is lower than the core average neutron flux; and an output of the amplifying means. A low-frequency pass filter for passing a low-frequency signal of the signal; andcomparing means for outputting a control rod withdrawal prevention signal when the low-frequency signal from the low-frequency pass filter exceeds a set level. Control rod pull-out monitoring device. 7. The amplifying means so that the second local average neutron flux becomes equal to the core average neutron flux when the second local average neutron flux is lower than the core average neutron flux. The control rod pullout monitoring device according to claim 6, wherein the gain of the control rod is changed. 8. A control rod pull-out monitoring device according to claim 1, a control rod drive mechanism for pulling out a control rod inserted into a reactor core, and inserting said control rod pull-out. A control means for stopping the withdrawal operation of the control rod drive mechanism connected to the control rod to be withdrawn on the basis of the control rod withdrawal prevention signal from a monitoring device. 9. A core average neutron flux output means for obtaining a core average neutron flux using neutron flux signals output from a plurality of neutron detectors disposed in the core, and a neutron flux around a control rod for performing a drawing operation. Local average neutron flux output means for obtaining a local average neutron flux using a neutron flux signal of a plurality of neutron detectors for measuring the neutron flux, amplification means for amplifying the local average neutron flux, and the local average neutron flux is the core average Gain adjusting means for changing the gain of the amplifying means when the neutron flux is lower than the neutron flux; a low-frequency pass filter for passing a low-frequency signal of an output signal of the amplifying means; an output signal of the amplifying means and the low-frequency pass Switching means for outputting one of the output signals of the filter, and comparing means for outputting a control rod withdrawal prevention signal when the output signal of the switching means exceeds a set level Control rod withdrawal monitoring device characterized by comprising a. 10. The control rod pull-out monitoring device according to claim 9, wherein the switching means can switch the output signal only after performing the operation two or more times.