WO2005017919A1

WO2005017919A1 - Apparatus for preventing unwanted control rod dropping in nuclear power plant and method thereof

Info

Publication number: WO2005017919A1
Application number: PCT/KR2003/001841
Authority: WO
Inventors: Hyeong-Pyo Hong; Yong-Hun Kim
Original assignee: Korea Power Engineering Company, Inc.
Priority date: 2003-08-14
Filing date: 2003-09-06
Publication date: 2005-02-24
Also published as: KR100619663B1; KR20050018507A; AU2003258885A1

Abstract

Provided is a method and apparatus for preventing unexpected dropping of a control rod in a nuclear power plant. The apparatus includes a first current detector which detects current supplied to a control rod drive mechanism, a coil current monitoring module which compares a value of the current detected by the first current detector with a predetermined value and outputs a subsitute power request signal requesting a substitute power to be supplied to the control rod drive mechanism if it is determined that the value of the detected current is not suitable for preventing the dropping of the control rod, and a substitute power supplying unit which supplies the substitute Power to the control rod drive mechanism if the substitute power request Signal is output from the coil current monitoring module. Since the unexpected dropping of the control rod due to a wide variety of single failures and errors can be prevented, the safe operation of the nuclear power plant can be ensured. Also, since costs arising due to an abrupt shutdown can be reduced, the economical effeciency of the nuclear power plant can be improved.

Description

APPARATUS FOR PREVENTING UNWANTED CONTROL ROD DROPPING IN NUCLEAR POWER PLANT AND METHOD THEREOF

Technical Field The present invention relates to a method and system for preventing unexpected dropping of a control rod in a nuclear power plant, and more particularly, to a method and apparatus for preventing unexpected dropping of a control rod in a nuclear power plant when a power failure to a latch coil that holds the control rod in a control rod drive mechanism is caused by a single failure of a control rod control system which controls a position of the control rod to adjust an output of a nuclear reactor.

Background Art A control rod control system in a nuclear power plant controls an output of a nuclear reactor by inserting or withdrawing neutron absorbing control rods according to an operator's manual control command or an automatic control command of a reactor regulating system during plant operation, drastically reduces the output of the nuclear reactor by dropping preselected control rods by gravity into a core to eliminate an output unbalance between the nuclear reactor and a turbine generator caused due to a load cutback or feed pump failure, and safely shutdowns the nuclear reactor by dropping all the control rods by gravity into the core of the nuclear reactor in response to a nuclear reactor trip signal output from a reactor protection system. A control rod control system in a pressurized water reactor (PWR) nuclear power plant is used to supply a direct current of a predetermined magnitude to latch coils and lift coils of a pertinent control rod drive mechanism according to an electric signal, and maintain, insert, or withdraw control rods by operating an internal mechanical drive mechanism using a mangetic force induced by the coils. The control rod drive mechanism is classified into a three-coil type and a four-coil type according to its structure and operating method. The four-coil type is structured to have one more lift coil than the three-coil type. During an output operation of the nuclear power plant, most control rods are withdrawn maximally high to secure a shutdown margin and maintained in a stopped position. In this state, a holding current of a predetermined magnitude is supplied to the latch coils. That is, when the control rods are not in motion, a direct current is constantly supplied to the latch coils and the pertinent control rods are maintained in the stopped state. Most control rods are maintained in the stopped state during a first operation period of approximately 18 months. If a single failure in a control circuit that supplies power to the control rod drive mechanism causes a power failure, the pertinent control rods cannot be maintained in their positions any longer and are forced to be dropped by gravity into the core of the nuclear reactor. For example, a single component, such as a circuit breaker of a power control system which supplies current to the latch coils, a protecting fuse, a silicon controlled rectifier (SCR) switching elemement which supplies a direct current to each coil comprised in the control rod drive mechanism in response to a sequence control signal, a diode, or a connector, could be abruptly failed or interrupted during plant operation, and electronic circuits for controlling the SCR may undergo malfunction and failure. Once this problem occurs, the holding current supplied to the latch coils that hold the control rods is suddenly cut off and the pertinent control rods (one control rod or one subgroup comprising four control rods) can be inserted into the core of the nuclear reactor. If one control rod or one subgroup of control rods are abruptly dropped into the core during a normal output operation of the nuclear power plant, output distribution within the core is changed, and accordingly, a nuclear reactor output at other regions within the core increases to compensate for an output loss at the region to which the control rods are dropped, leading to a damage to nuclear fuels at the regions where the output increases. The output distribution of the nuclear reactor is always monitored using a nuclear measuring instrumentation to avoid such abnormal results. If it is detected that the output distribution variations of the nuclear reactor due to the unexpected dropping of the control rods may deteriorate the safety of the nuclear reactor, power supplied to the rest control rods is also cut off by the nuclear reactor protection system and all the control rods are dropped by gravity into the core. Then, the nuclear reactor is safely stopped and the turbine generator cannot produce any more power. Therefore, if the control rods are unnecessarily dropped due to a failure of part of the system during the normal plant operation, power generation is stopped, resulting in a considerable economic loss. Accordingly, attempts to develop technology and apparatuses for preventing unexpected dropping of a control rod to prevent the economic loss have been continuously made. FIG. 1 is a block diagram of a power control system of a control rod control system which supplies power to a conventional four-coil type control rod drive mechanism. The power control system employs a failure detecting electronic circuit to minimize an ununcessary dropping of the control rod and protect coils of control rod drive mechanism. When abnormal current of a latch coil is detected, the power control system supplies a holding current to the other latch coil and provides an alarm signal. The power control system will be explained in detail hereinbelow. A three-phase alternating voltage of 240V supplied from a system power source 180 is input to a votlage control circuit 131 through a zero cross detector 121 of a subgroup power control circuit 120, which detects a point whose voltage is 0V in a waveform of the alternating voltage, to generate an SCR trigger pulse. The voltage control circuit 131 generates the SCR triggger pulse capable of producing a high voltage used in driving the control rod drive mechanism 110 by using an input of the zero cross detector and producing a low voltage used in maintaining the driving state. A control rod controller 133 sequentially outputs, according to an operation sequence under the control of a system controller 150 to a voltage selecting circuit 132, a control signal which determines whether to provide the high voltage or the low voltage to four coils of the control rod drive mechanism 110, or cut off the power supply. The voltage selecting circuit 132 determines and provides a trigger pulse signal suitable for a supply votlage of a SCR trigger circuit 122 according to the sequence control signal of the control rod controller 133. The SCR trigger circuit 122 triggers an SCR 123 using a signal output from the votlage selecting circuit 132, and the SCR 123 respectively supplies a predetermined direct current to an upper lift coil 111 , an upper latch coil 112, a lower lift coil 113, and an lower latch coil 114 of the control rod drive mechanism 110. If there occurs a problem in a circuit which transmits a signal to the subgroup power control circuit 120, and thus, no signal is transmitted from the voltage selecting circuit 132 to the SCR trigger circuit 122, the SCR trigger circuit 122 unconditionally triggers the SCR 123 so that the SCR 123 can output a high voltage to the upper latch coil 1 12 to prevent the control rod from being dropped, and then generates an alarm signal. Further, the control rod controller 133 monitors current supplied to one of the latch coils 1 12 and 1 14, which holds the control rod, through a first current detector 160. If an abnormal condition is detected, the control rod controller 133 supplies current to the other latch coil so as for the other latch coil to hold the control rod and generates an alarm signal. Once the alarm signal occurs, the operator checks the abnormal subgroup, and operates a manual power control board 141 of a substitute power supply module 140 installed in a system cabinet to turn on a switch 170, which connects a power supplier 142 to the pertinent latch coil, and permit a substitute power to be supplied to the pertinent latch coil. Next, the operator stops the operation of the abnormal subgroup power control circuit 120 to investigate the cause and performs a repair work. While the substitute power is supplied to the specific subgroup, the control rod is impossible to be withdrawn or inserted and is maintained in the current position for the repair period. However, according to the conventional system of FIG. 1 , to reduce the unexpected dropping of the control rod due to a failure of a certain single component, an urgent alarm signal is provided after the pertinent latch coil is replaced with a new one by a current monitoring circuit and all the three-phase SCR of the upper latch coil is triggered to output a high voltage. Thus, the application of the conventional system is limited to just some failures which can be corrected by replacing the latch coil with a new one or triggering all the three-phase SCR of the upper latch coil to output the high voltage. As a consequence, the conventional system cannot prevent the dropping of the control rod caused for other various failure reasons. In particular, in the event that power and coil power line connecting pins are damaged, diodes and fuses are damaged, SCR control card connector pins are poorly connected, an SCR phase control card has a defect, or a current detector is out of order, the conventional system cannot prevent the control rod from dropping. Furthermore, to trigger all the three-phase SCR of the upper latch coil to output the high voltage may cause a coil damage when the three-phase SCR is exposed to such a high voltage condition for a long time due to a delayed action by the operator, resulting in holding current failure and control rod dropping. Moreover, if the coil of the control rod drive mechanism is damaged, the control rod cannot be driven. Accordingly, the damaged coil should be replaced with a normal coil. Due to the replacement, the maintenance persons are unavoidably exposed to radiation, and lots of time and cost are spent.

Disclosure of the Invention The present invention provides a system which can ensure a stable operation of a nuclear power plant by preventing unexpected dropping of a control rod due to a wide variety of single failures through the improvement of a control rod control system, which controls a position of the control rod to adjust an output of a nuclear reactor in the nuclear power plant, and also can improve the economical efficiency of the nuclear power plant by reducing costs arising due to an abrupt shutdown. The present invention provides a method of driving the above-mentioned system. According to an aspect of the present invention, there is provided an apparatus for preventing unexpected dropping of a control rod in a control rod control system which supplies current to a control rod drive mechanism using a predetermined power control circuit in a nuclear power plant, the apparatus comprising: a first current detector which detects the current supplied to the control rod drive mechanism; a coil current monitoring module which compares a value of the current detected by the first current detector with a predetermined value, and outputs a substitute power request signal requesting a substitute power to be supplied to the control rod drive mechanism if it is determined that the value of the detected current is not suitable for preventing the dropping of the control rod; and a substitute power supplying unit which supplies the substitute power to the control rod drive mechanism if the substitute power request signal is output from the coil current monitoring module. The substitute power supplying unit may include: a substitute power supply module which supplies the substitute power; a switch which switches a connection between the substitute power supply module and the control rod drive mechanism; and an automatic substitute power controller which operates the switch if the substitute power request signal is output from the coil current monitoring module. The apparatus may further comprise a second current detector which detects current flowing between the switch and the control rod drive mechanism and sends a detected result to the automatic substitute power controller. If the substitute power supplying unit has already supplied the substitute power, the substitute power supplying unit may not respond to a received new substitute power request signal, and if a reactor power cutback signal for drastically reducing an output of a nuclear reactor by dropping preselected control rods is received, the substitute power supplying unit may not respond to the received substitute power request signal. The substitute power supplying unit may be converted into a manual mode by an operator, and may continuously supply power to the control rod drive mechanism even when the substitute power supplying unit is converted into the manual mode while the substitute power is being supplied to the control rod drive mechanism. According to another aspect of the present invention, there is provided a method of preventing unexpected dropping of a control rod in a control rod control system which supplies current to a control rod drive mechanism using a predetermined power control circuit in a nuclear power plant, the method comprising: detecting the current supplied to the control rod drive mechanism; comparing a value of the detected current with a predetermined value and determining whether the value of the detected current is not suitable for preventing the dropping of the control rod; and supplying a substitute power to the control rod drive mechanism if it is determined that the value of the detected current is not suitable for preventing the dropping of the control rod. The substitute power supplying step may not be performed if the substitute power has already been supplied to other control rod drive mechanism and if a reactor power cutback signal for drastically reducing an output of a nuclear reactor by dropping preselected control rods is received.

Brief Description of the Drawings FIG. 1 is a block diagram of a conventional power control system of a control rod control system which supplies current to a four-coil type control rod drive mechanism. FIG. 2 is a block diagram of a power control system of a control rod control system which supplies current to a control rod drive mechanism according to the present invention. FIG. 3 is a block diagram illustrating a configuration of a coil current monitoring module according to the present invention. FIG. 4 is a flow chart illustrating a method according to the present invention.

*Explanation on Reference Numerals for the Major Elements of the Drawings 1 10 Control rod drive mechanism 1 1 1 Upper lift coil 1 12 Upper latch clil 1 13 Lower lift coil 1 14 Lower latch coil 120 Subgroup power control circuit 121 Zero cross detector 122 Silicon controlled rectifier (SCR) trigger circuit 123 SCR 130 Subgroup logic circuit 131 Voltage control circuit 132 Voltage selecting circuit 133 Control rod controller 140 Substitute power supply module 141 Manual power control board 142 Power supplier 150 System controller 160 First current detector 170 Switch 180 System power source 181 , 182 Circuit breaker 210 Coil current monitoring module 220 Subgroup signal integrating module 230 Second current detector 240 Automatic substitute power controller

Best mode for carrying out the Invention Preferred embodiments of the present invention will be explained below in detail with reference to the appended drawings. FIG. 2 is a block diagram of a system according to the present invention. The system includes a subgroup power control circuit 120, a subgroup logic circuit 130, a control rod drive mechanism 110, a system controller 150, and a system power source 180. The subgroup power control circuit 120 includes a zero cross detector 121 , a silicon controlled rectifier (SCR) trigger circuit 122, and an SCR 123.

The subgroup logic circuit 130 includes a voltage control circuit 131 , a voltage selecting circuit 132, and a control rod controller 133. The control rod drive mechanism 110 includes four coils, namely, an upper lift coil 111 , an upper latch coil 112, a lower lift coil 113, and a lower latch coil 114, or three coils (not shown), namely, a lift coil, a movable latch coil, and a stationary latch coil. Those elements have been explained previously in the related art, and thus, features of the present invention will be importantly explained hereinbelow. A coil current monitoring module 210 is installed on each control rod drive mechanism 110, and determines whether a substitute power request signal is output to an automatic substitute power controller 240 based on current detected by a first current detector 160. While just one first current detector 160 is shown in FIG. 2 for simplicity, the first current detector 160 is provided to each coil to detect current flowing through the coil. That is, when four coils are used to constitute one control rod drive mechanism 110 as shown in FIG. 2, four first current detectors 160 are provided, and when three coils are used, three first current detectors 160 are provided. Specifically, the first current detector 160 detects current flowing through each coil, converts the current into a voltage signal proportional to the detected current, and provides the voltage signal to the coil current monitoring module 210. A detailed configuration of the coil current monitoring module 210 is illustrated in FIG. 3. The coil current monitoring module 210 includes a signal conditoning circuit 211 , an analog to digital (A D) converting circuit 212, a substitute power request signal output circuit 213, and a coil current waveform transmitting circuit 214. A signal output from the first current detector 160 is input to the signal conditioning circuit 211 of the coil current monitoring module 210, such that an offset voltage and noise contained in the signal are removed and the signal is amplified enough to be input to the A/D converting circuit 212. After the signal is converted into a digital signal by the A D converting circuit 212, the converted signal is input to the substitute power request signal output circuit 213 comprising a microprocessor. The substitute power request signal output circuit 213 compares a value of the input current with a predetermined value to determine whether the supply current is in an abnormal condition, and outputs a substitute power request signal if it is determined that the measured value of the current is beyond the predetermined value and the currently supplied current is difficult to maintain a control rod. Here, the substitute power request signal output circuit 213 outputs the substitute power request signal all the time when the measured value of the current is beyond the predetermined value. This is because when the current is abnormally low, it is not suitable for controlling the control rod, causing the control rod to drop, and when the current is abnormally high, a damage to a coil occurs due to a high voltage, causing the control rod to drop. Since the substitute power request signal output circuit 213 always is provided with a normally energized relay (not shown) and a relay control circuit (not shown), it is preferable that a signal with which the relay is not energized is used as the substitute power request signal. Through this, it is possible to prevent a possible dropping of the control rod by supplying an output signal from the de-energized relay automatically when the relay cannot be controlled due to cut-off of a power supplied to the coil current monitoring module 210 or the relay control circuit has a defect. Meantime, the coil current monitoring module 210 periodically checks the signal conditioning circuit 211 , the A/D converting circuit 212, and the substitute power request signal output circuit 213, and provides the checking results, namely, information on their state through a light emitting diode (LED) or an alarm output. One subgroup generally consists of four control rod drive mechanisms 110. One coil current monitoring module 210 is provided to each control rod drive mechanism 110. The substitute power request signal output from each of the coil current monitoring modules 210 provided to one subgroup is integrated in a subgroup signal integrating module 220 to be input to the automatic substitute power controller 240. If the substitute power request signal is output from any one of the coil current monitoring modules 210 provided to the four control rod drive mechanisms 110 per subgroup, the subgroup signal integrating module 220 sends the substitute power request signal with respect to the pertinent subgroup to the automatic substitute power controller 240. The reason why the substitute power request signal is processed by subgroup is that if all the substitute power request signals of the coil current monitoring modules 210 are individually input to the automatic substitute power controller 240, the capacity of the automatic substitute power controller 240 needs to increase, coil distribution is complicated, and management becomes difficult. However, it is possible that the substitute power request signal generated by control rod drive mechanism is individually input to the automatic substitute power controller 240. Regardless of whether the substitute power request signal generated by control rod drive mechanism 110 is individually input to the automatic substitute power controller 240 or signals are integrated by subgroup and then a subgroup substitute power supply signal is input to the automatic substitute power controller 240, it is preferable to set power supplied from a substitute power supply module 140 by subgroup. In the event that the substitute power request signal generated by an individual control rod drive mechanism is individually input to the automatic substitute power controller 240, however, it is also possible to set power supplied from the substitute power supply module 140 by individual control rod drive mechanism 110. If the subgroup substitute power request signal is input, the automatic substitute power controller 240 operates a switch 170, which connects the pertinent subgroup to the power supplier 142 of the substitute power supply module 140, to supply the substitute power to the control rod drive mechanisms 110 of the pertinent subgroup. It is easier in design and installation that the substitute power supplied to the subgroup is supplied to the upper latch coil 112 among the latch coils of each control rod drive mechanism 1 10 of the subgroup. However, the substitute power can be supplied to the lower latch coil 1 14 instead of the upper latch coil 1 12, and can be supplied to both the upper latch coil 1 12 and the lower latch coil 1 14 as well. The previously described construction can be equally applied to a control rod drive mechanism including three coils (not shown), namely, a lift coil, a movable latch coil, and a stationary latch coil. The switch 170 is a switching element for introducing the substitute power generated by the substitute power supply module 140 to the upper latch coil 112 of the control rod drive mechanism 1 10 of the subgroup determined by the automatic substitute power controller 240. The switch 170 operates according to a switch command of the automatic substitute power controller 240, and can be constructed using a mechanical relay or a semiconductor device. To confirm that power is properly supplied to the latch coil through the operation of the switch 170, a second current detector 230 is installed on an output power line of the substitute power supply module 140. The result obtained by the second current detector 230 is input to the automatic substitute power controller 240. It is preferable that the operator can see the result with naked eyes by watching a separate display window. Whether or not the substitute power is properly supplied from the power supplier 142 to the pertinent subgroup can be determined by virtue of the second current detector 230. While one switch 170 is shown in FIG. 2 for convenience in illustration, the switch 170 is provided to the upper latch coil 1 12 of each of the control rod drive mechanisms 1 10 constituting the subgroup. If it is determined by information stored in the automatic substitute power controller 240 that power has already been supplied to a specific subgroup, it is preferable that even though another substitute power request signal is received, the automatic substitute power controller 240 is .set to inform the operator of the receipt of the substitute power request signal but not to respond to the substitute power request signal. This is because the capacity of the power supplier 142 is generally as large as to supply the substitute power to one subgroup, and accordingly, if the substitute power request signal is set to supply the subsititute power to all the received subgroups, overload and malfunction are caused. Furthermore, it is a typical design condition that the substitute power is supplied only to one subgroup. Moreover, in a state where the substitute power has already been supplied to the specific subgroup in an automatic mode, it is desirable that the substitute power is continuously supplied even though the substitute power supply to the pertinent subgroup is mannualy set by the operator through a manual power control board 141. However, if the substitute power request signal is input when the substitute power supply to the pertinent subgroup has already been manually set by the operator through the manual power control board 141 , it is preferable that the automatic substitute power controller 240 does not respond to the substitute power request signal. It is also preferable that the automatic substitute power controller 240 does not respond to the substitute power request signal when a reactor power cutback signal for drastically reducing an output of a nuclear reactor by dropping preselected control rods through the system controller 150 is input to the automatic substitute power controller

240. FIG. 4 is a flow chart illustrating a method according to the present invention. Functions of the elements have already been explained with reference to FIGS. 2 and 3, and the method according to the present invention will be explained in short. First, in step 401 , the first current detector 160 detects current supplied to the control rod drive mechanism 1 10 using the system power source 180, converts the current into a voltage proportional to the current, and outputs the voltage to the coil current monitoring module 210. In step 402, the coil current monitoring module 210 compares a value of the current output from the first current detector 160 with a predetermined value and determines whether the supply current is difficult to maintain the control rod. If it is determined that the supply current is difficult to maintain the control rod, the process continues with step 403. In step 403, the substitute power request signal is output. In step 404, if the automatic substitute power controller 240 receives the substitute power request signal from the coil current monitoring module 210 or the subgroup signal integrating module 220, the automatic substitute power controller 240 operates the pertinent switch 170 to supply the substitute power from the power supplier 142 to the upper latch coil 112 of the control rod drive mechanism 110 corresponding to the coil current monitoring module 210 or the the subgroup signal integrating module 220, which outputs the substitute power request signal. Here, if the substitute power has already been supplied to a specific subgroup by the automatic substitute power controller 240, the mode has already been converted into the manual mode by the operator, or the reactor power cutback signal is received from the system controller 150, the automatic substitute power controller 240 does not respond to the substitute power request signal even though the substitute power request signal is received, as previously described. While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Industrial Applicability As described above, in contrast to the conventional art which triggers all the three-phase SCR of the upper latch coil to generate a high voltage when the SCR control signal is lost, the present invention does not trigger the SCR to output the high voltage. Accordingly, problems caused when the operator's actions are delayed and the three-phase SCR is exposed to the high voltage state for a long time can be solved. Moreover, since the present invention can prevent unexpected dropping of the control rod due to a wide variety of single failures and human errors, the stable operation of the nuclear power plant can be ensured, costs arising due to an abrupt shutdown can be reduced, and the economical efficiency of the nuclear power plant can be improved. Additionally, the present invention can minimize costs caused by preventive inspections and repair works for the control rod control system, which are excessively performed every regular checkup with concern over the unexpected dropping of the control rod due to a single faillure of the control rod control system, and reduce a mental burden to people in charge.

Claims

What is claimed is: 1. An apparatus for preventing unexpected dropping of a control rod in a control rod control system which supplies current to a control rod drive mechanism using a predetermined power control circuit in a nuclear power plant, the apparatus comprising: a first current detector which detects the current supplied to the control rod drive mechanism; a coil current monitoring module which compares a value of the current detected by the first current detector with a predetermined value, and outputs a substitute power request signal requesting a substitute power to be supplied to the control rod drive mechanism if it is determined that the value of the detected current is not suitable for preventing the dropping of the control rod; and a substitute power supplying unit which supplies the substitute power to the control rod drive mechanism if the substitute power request signal is output from the coil current monitoring module.

2. The apparatus of claim 1 , wherein the substitute power supplying unit includes: a substitute power supply module which supplies the substitute power; a switch which switches a connection between the substitute power supply module and the control rod drive mechanism; and an automatic substitute power controller which operates the switch if the substitute power request signal is output from the coil current monitoring module.

3. The apparauts of claim 2, further comprising a second current detector which detects current flowing between the switch and the control rod drive mechanism and sends a detected result to the automatic substitute power controller.

4. The apparatus of claim 1 , wherein if the substitute power supplying unit has already supplied the substitute power, the substitute power supplying unit does not respond to a received new substitute power request signal.

5. The apparatus of claim 1 , wherein if a reactor power cutback signal for drastically reducing an output of a nuclear reactor by dropping preselected control rods is received, the substitute power supplying unit does not respond to the received substitute power request signal to supply a substitute power.

6. The apparatus of claim 1 , wherein the substitute power supplying unit can be converted into a manual mode by an operator, and continuously supplies power to the control rod drive mechanism even when the substitute power supplying unit is converted into the manual mode while the substitute power is being supplied to the control rod drive mechanism.

7. A method of preventing unexpected dropping of a control rod in a control rod control system which supplies current to a control rod drive mechanism using a predetermined power control circuit in a nuclear power plant, the method comprising: detecting the current supplied to the control rod drive mechanism; comparing a value of the detected current with a predetermined value and determining whether the value of the detected current is suitable for preventing the dropping of the control rod; and supplying a substitute power to the control rod drive mechanism if it is determined that the value of the detected current is not suitable for preventing the dropping of the control rod.

8. The method of claim 7, wherein the substitute power supplying step is not performed if the substitute power has already been supplied to other control rod drive mechanism.

9. The method of claim 7, wherein the substitute power supplying step is not performed if a reactor power cutback signal for drastically reducing an output of a nuclear reactor by dropping preselected control rods is received.