Abnormality detection device for control rod drive mechanism and abnormality detection method for control rod drive mechanism

Abstract

The present invention comprises: a current acquisition unit 14 that acquires at least one phase current of an electric motor 1; a zero-cross section extraction unit 15, 15A that determines a zero-cross section including a zero-cross point from time-series data of the phase current acquired by the current acquisition unit 14 and extracts a current waveform of the zero-cross section; a feature amount calculation unit 16 that calculates a feature amount for detecting an abnormality of a control rod drive mechanism 11 from the current waveform of the zero-cross section; an abnormality diagnosis unit 17 that diagnoses the abnormality by using the feature amount; and a transmission unit 18 that transmits the diagnosis result of the abnormality diagnosis unit 17.

Description

Control rod drive mechanism abnormality detection device and control rod drive mechanism abnormality detection method

[0001] The present invention relates to a technique for detecting an abnormality in a control rod drive mechanism driven by an electric motor in a device related to nuclear power generation.

[0002] Patent Document 1 describes a reactor output control rod drive mechanism that is provided within a housing at the bottom of the reactor and transmits the rotation of an electric motor to a hollow piston lifting mechanism to lift and lower the hollow piston, thereby inserting and extracting the control rod into the reactor core. The control rod drive mechanism includes a spool piece provided at the bottom end of the housing, an outer yoke connected to the rotating shaft of the electric motor, an outer magnet provided in the outer yoke, an inner yoke provided inside the spool piece, an inner magnet provided in the inner yoke, and a rotation detection means for the inner magnet.

[0003] Japanese Patent Application Publication No. 10-10264

[0004] In recent years, the spread of the Internet of Things (IoT), the advancement of artificial intelligence (AI), and the decline in the working population have all coincided, and technology that actively uses information obtained by sensors and other devices in maintenance work has been attracting attention, with the aim of improving the efficiency of maintenance work and reducing the number of people required.

[0005] Among these, diagnostic methods using sensor information have been proposed for the control rod drive mechanisms of nuclear power plants, with the aim of reducing the amount of inspection work.

[0006] Typical types of sensors include vibration sensors and position sensors, but the method using current sensors is particularly noteworthy due to its ease of installation.

[0007] For example, Patent Document 1 proposes a method of detecting slippage of a magnetic coupling caused by an increase in load exceeding a permissible value based on the magnitude of the current of the motor.

[0008] However, the method of Patent Document 1 may not be applicable when the drive source of the control rod drive mechanism is other than an induction motor. When a load change occurs in an induction motor, regardless of the type of control or whether control is in use, a phenomenon called "slip" occurs, in which the motor shaft rotates out of synchronization with the frequency of the motor current, resulting in a large current change. Therefore, Patent Document 1 detects an abnormality based on the magnitude of the current.

[0009] However, the aforementioned "slip" does not occur in motors other than induction motors, such as magnet motors and stepping motors including hybrid types, and when these motors are controlled in an open loop, the effective value and maximum value, which are representative values ​​of the current magnitude, do not change much (less than 1%) even when there is a large change in load.

[0010] In other words, in magnet motors and stepping motors, it may not be possible to detect abnormalities based on the "magnitude of current" such as the effective value or maximum value. Therefore, in order to take advantage of the easy-to-use current sensor, a technology is needed that can detect abnormalities in the control rod drive mechanism based on something other than the magnitude of the current.

[0011] The present invention has been made in consideration of the above-mentioned problems of the prior art and in order to solve these problems.

[0012] Therefore, an object of the present invention is to provide an abnormality diagnosis device and an abnormality diagnosis method that detect an abnormality in a control rod drive mechanism using a feature other than the "magnitude of current" obtained from the motor current.

[0013] The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0014] The present invention includes a plurality of means for solving the above-described problems. One example is an apparatus for diagnosing an abnormality in a control rod drive mechanism that performs control rod insertion and extraction operations by driving a ball screw with an electric motor, the apparatus comprising: a current acquisition unit that acquires at least one phase current of the electric motor; a zero-crossing interval extraction unit that determines a zero-crossing interval including a zero-crossing point from time-series data of the phase current acquired by the current acquisition unit and extracts a current waveform in the zero-crossing interval; a feature calculation unit that calculates a feature for detecting an abnormality in the control rod drive mechanism from the current waveform in the zero-crossing interval; an abnormality diagnosis unit that diagnoses an abnormality using the feature; and a transmission unit that transmits the diagnosis result of the abnormality diagnosis unit.

[0015] According to the present invention, an abnormality in a control rod drive mechanism driven by a magnet motor or a stepping motor can be detected by a characteristic quantity other than the "magnitude of the current" obtained from the motor current. Problems, configurations, and effects other than those described above will become clear from the following description of the embodiment.

[0016] FIG. 1 is a schematic diagram of a control rod drive mechanism. FIG. 2 is a configuration diagram of an abnormality diagnosis device for a control rod drive mechanism according to a first embodiment of the present invention. FIG. 3 is a diagram showing the processing flow of a zero crossing interval extraction unit. FIG. 4 is time series data of motor current. FIG. 5 is a diagram supplementing the explanation of a specific interval. FIG. 6 is a diagram supplementing the explanation of a feature amount. FIG. 7 is a diagram supplementing the explanation of a feature amount. FIG. 8 is a diagram showing the processing flow of a zero crossing interval extraction unit in an abnormality diagnosis device for a control rod drive mechanism according to a second embodiment.

[0017] Hereinafter, embodiments of the control rod drive mechanism anomaly detection device and control rod drive mechanism anomaly detection method of the present invention will be described with reference to the drawings. In the drawings used in this specification, identical or similar reference numerals are used to designate identical or corresponding components, and repeated description of these components may be omitted.

[0018] Representative embodiments of the present invention disclosed in the present application will be described in detail. Reference numerals in the drawings are merely illustrative of components included in the concept of the components to which they are attached.

[0019] First Embodiment A first embodiment of the control rod drive mechanism abnormality detection device and control rod drive mechanism abnormality detection method of the present invention will be described with reference to FIGS. 1 to 7. FIG.

[0020] First, the overall configuration of the control rod drive mechanism that is the target of anomaly diagnosis by the anomaly detection device will be described with reference to Figure 1. In the first embodiment of the present invention, a device will be described that diagnoses anomalies in the control rod drive mechanism using a one-phase motor current as an input. Figure 1 shows a schematic diagram of the control rod drive mechanism.

[0021] 1 is a device for vertically moving (inserting / withdrawing) the control rods 6 for controlling the power output of the nuclear reactor. The device configuration is shown below.

[0022] The power supply 7 drives the electric motor 1 by supplying a periodic three-phase rectangular wave voltage (each phase being offset by 120 degrees) to the electric motor 1 under open loop (feedforward) control.

[0023] In the description of this embodiment, the electric motor 1 is either a stepping motor or a magnet motor, and the description will be limited to a rectangular wave voltage, but the description will be similar even if the electric motor is driven by a sine wave voltage.

[0024] The shaft of the electric motor 1 driven by the power source 7 is connected to the ball screw 3 by the coupler 2 and rotates in synchronization. This rotation causes the ball nut 4 to move up and down, and the connecting rod 5 and control rod 6 riding on the ball nut 4 also move up and down.

[0025] In the control rod drive mechanism 11 having such a configuration and operation, abnormal changes in sliding resistance may occur due to, for example, the inclusion of foreign matter, insufficient lubrication, contact with the housing, or the detachment of parts. From here, the configuration of the abnormality diagnosis device 13 for detecting abnormalities in the control rod drive mechanism 11 will be described with reference to Figure 2 and other figures.

[0026] The timing for operating the abnormality diagnosis device 13 or the timing for executing the method for detecting abnormalities in the control rod drive mechanism is preferably basically during periodic inspection, but it may also be configured to perform detection at all times during normal power generation.

[0027] 2 shows an outline of the configuration of the control rod drive mechanism 11 and the abnormality diagnosis device 13. The control rod drive mechanism 11 is the same as that shown in FIG. 1, so a description thereof will be omitted.

[0028] The abnormality diagnosis device 13 shown in FIG. 2 is a device that diagnoses abnormalities in the control rod drive mechanism 11, which drives the ball screw 3 using the electric motor 1 to insert and extract the control rod 6, and is composed of a current acquisition unit 14, a zero-cross interval extraction unit 15, a feature calculation unit 16, an abnormality diagnosis unit 17, and a transmission unit 18.

[0029] Of these, the zero crossing interval extraction unit 15, the feature calculation unit 16, the abnormality diagnosis unit 17, and the transmission unit 18 each have a central processing unit (CPU) and a memory connected to the CPU.

[0030] The control processes for the operations to be executed may be integrated into one program, or may be divided into multiple programs, or may be a combination of these.

[0031] Some or all of the programs stored in each device may be implemented using dedicated hardware or may be modularized. Furthermore, various programs may be installed in each device from a program distribution server or external storage media, or existing devices may be updated.

[0032] Furthermore, each device may be an independent device connected to a wired or wireless network, or two or more devices may be integrated together.

[0033] The current acquiring unit 14 is configured with a known sensor that acquires at least one phase current of the electric motor 1. The current acquiring unit 14 preferably executes a current acquiring step of acquiring at least one phase current of the electric motor 1.

[0034] The zero-crossing interval extraction unit 15 determines zero-crossing intervals including zero-crossing points from the time-series data of the phase current acquired by the current acquisition unit 14, and extracts the current waveform of the zero-crossing interval. The feature calculation unit 16 is the main entity that executes the zero-crossing interval extraction step, which determines zero-crossing intervals including zero-crossing points from the time-series data of the phase current acquired in the current acquisition step, and extracts the current waveform of the zero-crossing interval.

[0035] The feature quantity calculation unit 16 calculates, from the current waveform in the zero crossing section, a feature quantity for detecting an abnormality in the control rod drive mechanism 11. This feature quantity calculation unit 16 is the main body that executes the feature quantity calculation step of calculating a feature quantity for detecting an abnormality in the control rod drive mechanism 11 from the current waveform in the zero crossing section.

[0036] The abnormality diagnosis unit 17 diagnoses an abnormality using the feature amount. The abnormality diagnosis unit 17 is the main body that executes the abnormality diagnosis step of diagnosing an abnormality using the feature amount.

[0037] The transmission unit 18 transmits the diagnosis result of the abnormality diagnosis unit 17. The transmission unit 18 is the entity that executes the transmission step of transmitting the diagnosis result in the abnormality diagnosis step.

[0038] The abnormality diagnosis device 13 first receives as input at least one phase of motor current (time-series data) acquired by a current acquisition unit 14, and extracts only current data for a specific interval including the timing at which the current crosses zero in a zero-crossing interval extraction unit 15. Next, the extracted interval of current data is used in a feature calculation unit 16 to calculate feature amounts used for abnormality diagnosis. The calculated feature amounts are then input to an abnormality diagnosis unit 17, which performs an abnormality diagnosis using machine learning, statistical methods, or the like, and transmits the abnormality diagnosis results to an operator or other systems in a transmission unit 18.

[0039] From here on, each processing block will be described in detail.

[0040] First, the zero-crossing interval extraction unit 15 will be described with reference to Fig. 3. As shown in Fig. 3, the zero-crossing interval extraction unit 15 is configured with a steady state standby unit 20 and an interval current holding unit 21. In this embodiment, the input motor phase current (diagnosis data) is input to the steady state standby unit 20. In this manner, in this embodiment, the zero-crossing interval extraction unit 15 extracts the current waveform from the data acquired from the current acquisition unit 14.

[0041] As shown in Figure 4, the input motor current undergoes transient changes in frequency and amplitude as the motor accelerates and decelerates immediately after operation starts (when the ball screw starts to drive) and immediately before stopping. In this transient state, the phase and amplitude of the current may change depending on the initial conditions of the motor, ball screw, etc., so there is a high possibility that more suitable data exists to use for abnormality diagnosis.

[0042] Therefore, it is desirable that the steady state standby unit 20 waits until the steady state is reached, using as criteria whether the amplitude and frequency fall within a certain range (for example, within ±1%) or whether a specific time or period has passed since the start of operation, as shown in Fig. 4. The steady state standby unit 20 outputs data from the section in which the steady state is reached to the section current holding unit 21.

[0043] Thereafter, the section current holding unit 21 of the zero cross section extraction unit 15 extracts and holds the current value of a specific section including the zero cross point required for abnormality diagnosis.

[0044] Here, the section current holding unit 21 can extract the zero-cross section from a steady state in which the steady state waiting unit 20 has determined that the maximum value or effective value of the current of the motor 1 is within a predetermined error range with respect to a predetermined value, or from a steady state that has been determined to be after a specific time has elapsed since the motor 1 started to operate or after the periodic fluctuation of the phase current has reached a specific number of times.

[0045] Then, the zero-crossing section can be extracted from within a predetermined range centered on the zero-crossing point, or from a predetermined section of the current cycle starting from the time when the phase current reaches its maximum value, or a point can be arbitrarily selected within a predetermined section of the current cycle starting from the time when the phase current reaches its maximum value and determined from within a predetermined range centered on that point.

[0046] As an extraction method, for example, the zero crossing point (in this example, this refers to the zero crossing point in the direction of increasing current; the same process applies to the opposite direction) calculated from current data in a normal state can be set as the center of the coordinate (time), and a range of ±M points from there (M is an integer value greater than or equal to zero; when M=0, only the zero crossing points are included) can be used as the specific section data. An image is shown in Figure 5. The horizontal axis of the figure is time, and the vertical axis is current value, with waveforms in normal and abnormal states overlaid.

[0047] As can be seen from the figure, when an abnormality occurs, the maximum current value and cycle do not change, and the largest difference occurs in the current value near the zero crossing point. Therefore, when diagnosing an abnormality, it is effective to use the current value in this vicinity (the current value near the zero crossing point is not generally referred to as the "magnitude of the current"), so only the current in the section including the zero crossing point is extracted.

[0048] In addition to the above, the zero-crossing interval extraction unit 15 may extract an interval including a zero-crossing point in an interval 4 / 6 to 5 / 6 of the current period from the coordinate (time) where the current is maximum, as shown in Figure 5, or it may select any point within the interval and use a range of ±M points from that point as data for the specific interval. Furthermore, when the U-phase current is used, it is possible to use the command value of the voltage applied to the motor 1 to extract data from a period where the UV voltage is positive, the VW voltage is zero, and the WU voltage is negative.

[0049] Next, the feature amount calculation unit 16 will be described.

[0050] In the feature calculation unit 16, examples of the feature can be the current value at the coordinate of the zero cross point when the control rod drive mechanism 11 is in a normal state, or the current value of any one point in the zero cross section of the phase current when the control rod drive mechanism 11 is in a normal state, or the integrated value of the current in a range equally spaced to the left and right of the coordinate of the zero cross point when the control rod drive mechanism 11 is in a normal state.

[0051] Specifically, if the aforementioned variable M is zero, the measured phase current value (diagnosis data) itself at the zero-crossing point coordinate (time) in a normal state can be used as a feature quantity, as shown in FIG. 6 . Also, the current value itself at any point in the section from 4 / 6 to 5 / 6 of the current period from the coordinate (time) where the current is maximum can be used as a feature quantity. On the other hand, if M≠0, the integrated value of the current value within a specific section can be used as a feature quantity. Furthermore, as shown in FIG. 7 , the zero-crossing point coordinate (time) can also be calculated for the diagnostic data, and the coordinate value (time) from the start of the specific section can be used as a feature quantity.

[0052] Next, the abnormality diagnosis unit 17 will be described. The abnormality diagnosis unit 17 can, for example, determine whether the feature amount is equal to or greater than a threshold value and output binary data indicating normality / abnormality. Furthermore, the abnormality diagnosis unit 17 can acquire multiple samples of the feature amount in normal states, calculate the mean value and variance in advance, calculate the Mahalanobis distance (MTD) using the following formula (1), and output the calculated continuous value as the degree of abnormality.

[0053] MTD=(Feature_meas−Feature_normal_ave) 2 / Feature_normal_disp (1) Here, in equation (1), Feature_meas is the feature amount at the measured current, Feature_normal_ave is the average value of the feature amount in the normal state, and Feature_normal_disp is the variance of the feature amount in the normal state.

[0054] Furthermore, the method for calculating the degree of abnormality is not limited to the above-mentioned method, and machine learning or deep learning techniques such as One Class SVM may also be used. Furthermore, if both the mean value and variance of the feature amounts of the normal data and the diagnostic data are calculated, the mean value can be tested, and the test result can be output.

[0055] Finally, the transmission unit 18 will be described.

[0056] The transmission unit 18 is a device that transmits the abnormality diagnosis results to at least one of the control device of the control rod drive mechanism 11, the control device of the electric motor 1, the power plant control system, a display, a speaker, and a lamp, and has the role of informing workers of the status of the control rod drive mechanism 11.

[0057] In addition to the above, the results of the abnormality diagnosis unit 17 can be sent to the motor control device, the control device of the control rod drive mechanism 11, or the power plant control device / system, and used as feedback data for equipment control such as emergency shutdown and degenerate operation.

[0058] Next, the effects of this embodiment will be described.

[0059] The abnormality diagnosis device 13 for the control rod drive mechanism 11 according to the first embodiment of the present invention described above is a device for diagnosing abnormalities in the control rod drive mechanism 11, which drives the ball screw 3 using the electric motor 1 to insert and extract the control rod 6, and includes: a current acquisition unit 14 for acquiring at least one phase current of the electric motor 1; a zero-crossing interval extraction unit 15 for determining a zero-crossing interval including a zero-crossing point from time-series data of the phase current acquired by the current acquisition unit 14 and extracting a current waveform in the zero-crossing interval; a feature calculation unit 16 for calculating a feature for detecting an abnormality in the control rod drive mechanism 11 from the current waveform in the zero-crossing interval; an abnormality diagnosis unit 17 for diagnosing an abnormality using the feature; and a transmission unit 18 for transmitting the diagnosis result of the abnormality diagnosis unit 17.

[0060] This makes it possible to provide a new device and method for diagnosing abnormalities in the control rod drive mechanism 11 using a feature other than the "magnitude of current" obtained from the current of the electric motor 1.

[0061] Furthermore, the zero-cross interval extraction unit 15 extracts the zero-cross interval from a steady state in which the maximum value or effective value of the current of the motor 1 is within a predetermined error range with respect to a predetermined value, or from a steady state after a specific time has elapsed since the motor 1 started to drive or after the periodic fluctuation of the phase current has reached a specific number of times. This makes it possible to extract the zero-cross interval from data in a steady state excluding intervals in which the frequency or amplitude of the current changes transiently, thereby enabling more accurate abnormality diagnosis.

[0062] Furthermore, the zero-crossing interval extracting unit 15 extracts the current waveform from the data acquired from the current acquiring unit 14, thereby improving the processing speed.

[0063] Furthermore, by setting the characteristic quantity to the current value at the coordinate of the zero cross point when the control rod drive mechanism 11 is in a normal state, or the current value of any one point in the zero cross section of the phase current when the control rod drive mechanism 11 is in a normal state, or the integrated value of the current in a range equally spaced to the left and right of the coordinate of the zero cross point when the control rod drive mechanism 11 is in a normal state, it is possible to more accurately distinguish between normal and abnormal states.

[0064] Furthermore, by extracting the zero-crossing section from within a predetermined range centered on the zero-crossing point, or from a predetermined section of the current cycle starting from the time when the phase current is at its maximum value, or by arbitrarily selecting a point within the predetermined section of the current cycle starting from the time when the phase current is at its maximum value and determining it from within a predetermined range centered on that point, it is possible to extract as the zero-crossing section a section that reliably includes the current value near the zero-crossing point where the difference between when an abnormality occurs and when normal is large, thereby making it possible to detect abnormalities more reliably.

[0065] Furthermore, the transmission unit 18 transmits the abnormality diagnosis results to at least one of the control device of the control rod drive mechanism 11, the control device of the electric motor 1, the power plant control system, a display, a speaker, and a lamp, thereby enabling workers to reliably grasp the occurrence of an abnormality, or enabling the system to automatically take action after detecting an abnormality.

[0066] An apparatus for detecting an abnormality in a control rod drive mechanism and a method for detecting an abnormality in a control rod drive mechanism according to a second embodiment of the present invention will be described with reference to Fig. 8. Note that a description of the same processes as those in the first embodiment will be omitted, and the description will focus on the differences from the first embodiment.

[0067] The difference between the control rod drive mechanism anomaly detection device of this embodiment shown in Fig. 8 and the abnormality diagnosis device 13 for the control rod drive mechanism 11 of the first embodiment is the processing content of the zero crossing interval extraction unit 15A. Fig. 8 shows the processing content.

[0068] The zero-crossing interval extraction unit 15A of this embodiment first waits until the measured current value (diagnostic data) reaches a steady state, as in the first embodiment, and then identifies a data interval including a zero-crossing point (identification interval setting unit 31). This prevents the actual zero-crossing points from being obscured by frequency decomposition in the FFT unit 32 (described later), making it impossible to identify the zero-crossing points.

[0069] On the other hand, the difference with the zero-crossing interval extraction unit 15 is that in the zero-crossing interval extraction unit 15A, the measured current value after the steady state is subjected to frequency decomposition (FFT: Fast Fourier Transform) by an FFT unit 32 so that the extracted current waveform is data composed of a portion of the frequency components contained in the data acquired from the current acquisition unit 14. A specific frequency component selection unit 33 selects at least one frequency component that changes significantly when an abnormality occurs (e.g., the 5 and 7 o'clock components of the current waveform). The IFFT unit 34 then reconverts the data into time-series data (IFFT: Inverse Fast Fourier Transform). Finally, a section data holding unit 35 extracts data for the section determined by the specific section setting unit 31 from the output result of the IFFT unit 34.

[0070] The subsequent processing is the same as in the first embodiment.

[0071] The other configurations and operations are substantially the same as those of the control rod drive mechanism anomaly detection device and control rod drive mechanism anomaly detection method of the first embodiment described above, and details thereof will be omitted.

[0072] The control rod drive mechanism anomaly detection device and control rod drive mechanism anomaly detection method according to the second embodiment of the present invention also provide substantially the same effects as those of the control rod drive mechanism anomaly detection device and control rod drive mechanism anomaly detection method according to the first embodiment described above.

[0073] Furthermore, the zero-crossing interval extraction unit 15A extracts a current waveform that is composed of data consisting of some of the frequency components contained in the data acquired from the current acquisition unit 14. This makes it possible to remove frequency components that change little when an abnormality occurs, making it easier to see current changes and enabling diagnosis of even minor abnormalities.

[0074] <Others> The present invention is not limited to the above-described examples, and includes various modifications. The above-described examples have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the described configurations.

[0075] It is also possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of one embodiment to the configuration of another embodiment.It is also possible to add, delete, or replace part of the configuration of each embodiment with the configuration of another embodiment.

[0076] DESCRIPTION OF SYMBOLS 1... Electric motor 2... Coupler 3... Ball screw 4... Ball nut 5... Connecting rod 6... Control rod 7... Power supply 11... Control rod drive mechanism 13... Abnormality diagnosis device 14... Current acquisition unit 15... Zero cross section extraction unit 15A... Zero cross section extraction unit 16... Feature calculation unit 17... Abnormality diagnosis unit 18... Transmission unit 20... Steady state standby unit 21... Section current holding unit 31... Specific section setting unit 32... FFT unit 33... Specific frequency component selection unit 34... IFFT unit 35... Section data holding unit

Claims

1. An apparatus for diagnosing abnormalities in a control rod drive mechanism that uses an electric motor to drive a ball screw to insert and extract a control rod, comprising: a current acquisition unit that acquires at least one phase current of the electric motor; a zero-crossing interval extraction unit that determines a zero-crossing interval including a zero-crossing point from time series data of the phase current acquired by the current acquisition unit and extracts a current waveform in the zero-crossing interval; a feature calculation unit that calculates a feature for detecting abnormalities in the control rod drive mechanism from the current waveform in the zero-crossing interval; an abnormality diagnosis unit that diagnoses abnormalities using the feature amount; and a transmission unit that transmits the diagnosis result of the abnormality diagnosis unit.

2. The control rod drive mechanism abnormality diagnostic device according to claim 1, wherein the electric motor is either a stepping motor or a magnet motor.

3. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 2, wherein the zero cross section extraction unit extracts the zero cross section from within a steady state in which the maximum value or effective value of the electric motor current falls within a prescribed error range with respect to a prescribed value.

4. An abnormality diagnosis device for a control rod drive mechanism as described in claim 2, wherein the zero-crossing interval extraction unit extracts the zero-crossing interval from within a steady state after a specific time has elapsed since the motor started to drive or after the periodic fluctuation of the phase current has reached a specific number of times.

5. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the zero-crossing interval extraction unit extracts the current waveform from either the data itself acquired from the current acquisition unit, or data composed of a portion of the frequency components contained in the data acquired from the current acquisition unit.

6. An abnormality diagnosis device for a control rod drive mechanism according to claim 3 or 4, wherein the characteristic amount is a current value at the coordinate of the zero cross point when the control rod drive mechanism is in a normal state.

7. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the characteristic quantity is a current value at any one point in the zero cross section of the phase current when the control rod drive mechanism is in a normal state.

8. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the characteristic quantity is an integral value of the current in an equally spaced range on the left and right sides, centered on the coordinate of the zero cross point when the control rod drive mechanism is in a normal state.

9. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the zero cross section is determined from within a predetermined range with the zero cross point as the centre.

10. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the zero crossing section is determined by extracting a predetermined section of the current cycle from the time when the phase current is at its maximum value.

11. An abnormality diagnosis device for a control rod drive mechanism as claimed in claim 3 or 4, wherein the zero-crossing section is determined by arbitrarily selecting a point within a predetermined section of the current cycle from the time when the phase current is at its maximum value, and determining the zero-crossing section from within a predetermined range centered on the point.

12. An abnormality diagnosis device for a control rod drive mechanism as described in claim 1, wherein the transmission unit transmits the abnormality diagnosis result to at least one of the control device for the control rod drive mechanism, the control device for the electric motor, the power plant control system, a display, a speaker, and a lamp.

13. A method for diagnosing an abnormality in a control rod drive mechanism which performs control rod insertion and extraction operations by driving a ball screw with an electric motor, comprising: a current acquisition step for acquiring at least one phase current of the electric motor; a zero crossing interval extraction step for determining a zero crossing interval including a zero crossing point from time series data of the phase current acquired in the current acquisition step and extracting a current waveform in the zero crossing interval; a feature calculation step for calculating a feature for detecting an abnormality in the control rod drive mechanism from the current waveform in the zero crossing interval; an abnormality diagnosis step for diagnosing an abnormality using the feature; and a transmission step for transmitting a diagnosis result from the abnormality diagnosis step.

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