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Development of Moving Coil Type Control ROD Drive Mechanism
Copyright © IFAC Control Science and T echnology (8th Triennial W orld Congr=) Kyo to. Japan. 1981
DEVELOPMENT OF MOVING COIL TYPE CONTROL ROD DRIVE MECHANISM T. Shibata*, S. Ishihara*, K. Hosono**, T. Nakajima** and T. Kanazawa*** -Research Reactor Inst£tute of Kyoto Un£vers£ty, Noda, Kumator£-cho , Sennan-gun, Osaka 590-04, japan "Control Eng£neer£ng Department, Techn£cal Research Cent er, N£ppon Kokan, No . 1-1, Minam£-Watarida -cho, Kawasak£- ku, Kawasaki 210, japan "-Nuclear System Development Sect£on, Ocean Engineering Department, Nzppon Kokan, No . 1-1-2, Marunouchz', Chz'yoda -ku, Tokyo 100, japan
Abstract. Nuclear reactors are controlled by movable control rods made of neutron absorbing materials. These rods are driven by control rod drive mechanism which is directed by servo-contro1 circuit. New drive mechanism was developed. Magnetic plunger connected with control rods in guide tube is exerted by the magnetic coil outside of the guide tube. Special circuit for precise positioning of the plunger relative to the coil have been developed. The drive mechanism has no vessel penetration such as rotary seal or sliding seal, so that no leakage of primary cooling water of high radioactivity was reached. Keywords. Actuators; electromagnet.
nuclear
reactors;
position
control;
power
control;
INTRODUCTION In general, nuclear reactors are controlled by movable control rods made of neutron absorbing materials . These rods are driven by control rod drive mechanism (hereinafter referred to CRDM) which is directed by servo-contro1 circuit. Conventional CRDM has penetrations with water sealing through the reactor pressure vessels. In these water sea1ings, some water leakages are induced. The water which is called primary reactor coolant has very high radioactivity. It is very important to prevent the leakage of the primary coolant for safety. New CRDM has been developed after some investigations for various types of drive mechanism. In the drive mechanism magnetized plunger engaged with control rods in guide tube which is the part of pressure boundary is exerted by the magnetic coil outside of the guide tube as shown in Fig. 1. The drive mechanism has no penetration and induces no leakage of the primary coolant in principle. However there are technical difficulties especially for precise positioning of the plunger because of the isolated mechanical structure with magnetic coupling. The difficulties of the precise positioning are due to mainly the mechanical friction between the plunger and the guide tube and hydraulic effec t. To obtain the precise positioning of the
plunger, the special control devices for automatic adjustment of the magnetic field has been developed. The control rod can be placed at any desired positions. The structure of the developed CRDM is rather simple and the rod drop at emergency reactor shut down is reliable. While the mechanism was deve10poed primarily for KUHFR (Kyoto University High Flux Reactor), it is applicable to other reactors or other plants with similar requirements. COMPARISON OF THREE TYPES OF DRIVE MECHANISMS Requirements for Control Rod Drive Mechanism of Nuclear Reactors The requirements follows;
for CRDM are described as
The control rod is required to have a smooth motion and constant velocity in driving. It is also required that the rod can be stopped at any desired position. A backlash on its travel has to be sufficiently small. For emergency reac tor shutdown, the control rod has to be inserted into the reactor core immediately by disengagement from the driving mechanism (which is called scram). CRDM has to be sufficiently durable and reliable.
2 14 7
T . Shib ata et al .
2 148
In the case of KUHFR, performances are as follows;
the
required
10 cm/min. Control rod driving velocity 65 cm Control rod driving stroke Minimum durable traveling 10,000 m length of rod 400,000 times Minimum inching cycles 200 times Minimum number of scrams (mechanical reliability for shock) In addition to these requirements for KUHFR, next two items are also required.
(l)
Holding force is greater than that of the other two types and the device can be made smaller.
(2)
This type of mechanism enables the accurate positioning of the rod relatively better than that of the linear motor type. Continuous positioning is possible whi le the other two types are dr iven stepwise. The plunger is compelled to contac t with the guide tube due to the radial magnetic force and the plunger's stickslip movement is caused by the ununiform fric t ion force between the plunger and the guide tube. Similarly backlash is resulted when the moving direction of the coil is reversed.
(3)
(4)
1) The mechanism must be no leaking structure
for safety. mechanism must be composed of usual parts and material for easy maintenance.
2) The
Under these conditions, three types of drive mechansims were examined as follows;
(5 )
Summary of Comparison Linear Motor Type A experimental device shown in Fig. 2-(1) was examined. Many small coils are located outside the guide tube coaxially over the stroke length. Each coi 1 is energized one after another for producing magnetic field movement. The magnetic field movement drives the magnetic plunger located coaxially in the coils. As a result of the examinations, the following characteristics were obtained. A large number of small coils has to be provided to move the rod smooth and stop precisely it at any desired position. However, it is practically very difficult to realize it. This type may be appropriate for a drive mechanism of long stroke compared with each coil size. Friction Grip Magnetic Jack Type A experimental device shown in Fig. 2-(2) was examined. This type of mechanism drives the rods in the guide tube stepwise by exciting the lift coil, grip coil, pull down coil and hold coil sequentially. This type was examined and the results are listed below. (1) (2)
Holding force is generated with friction so that slips are apt to take place and more load results in more slips. As it takes time to switch the current for the coils, it is difficult to obtain continuous and smooth movement of the rod.
Moving Coil Type A experimental device shown in Fig. 2-(3) was examined. The coil outside the guide tube is motor-dr iven vert ically, and the plunger magnetically coupled with coil is moved in the guide tube. The results of the examination were as follows;
As a result of investigation of these mechanisms, the following conclusions were made. The moving coil type still has problems in accurate pos~t~oning because of such a stickslip and backlash. Since this type of mechanism was considered the most suitable for KUHER, it was decided to adopt this mechanism. But it required further improvements and detailed design. IMPROVEMENTS AND DESIGN STUDY OF MOVING COIL TYPE CRDM The Characteristics of Primitive Moving Coil Type CRDM In this type of electromagnet, the position of plungers depends on such factors as the weight of the plunger, magnetic field and friction. In other words, the position of the plunger is not potentially stable. As the magentic field is not perfectly symmetric due to structural nonsymmetry, plungers are compelled to contact with the guide tube. Friction force was taken place between plungers and the guide tube. Consequent ly, the plungers' movement looses fidelity to the coils movement. Backlash and stickslip appear in the plunger movement. The detailed mechanisms of the backlash and sticks lip are described schematically as follows; The magnetic axial force acting on the plunger vs. the relative position of the plunger to the coil is shown curve (a) in Fig. 3-(1). In the ideal case of no friction presence, the plungers must stop at point A, where the weight W balances with the magentic axial force. Ac tually the fric tion force between plungers and the guide tube due to magnetic field nonsymmetry is induced and when the coils are moved upward, the relative position
Deve l opme nt of Mov ing Coil Type Control Rod Drive Me chanism
of plungers to the coils is changed to point B, where magnetic axial force increases by amount of friction force, Fr, relative to the magnetic axial force of point A. When they are moved downward, the relative position of plungers to the coils is changed to point C similarly. The distance BC is the backlash. The point Band C fluctuate due to the local change of friction force. This fluctuation is sticks lip. Factors Affecting the Fidelity The stoppage period (backlash) depends on (1) the drive velocity of coil, (2) the yoke's thickness, and (3) the relation between the plunger's length and the length of the coil and yoke. As it is not practicable to adjust the drive velocity for improving the fidelity, the other two effects were investigated. Thickness of the yoke. A thicker yoke increases the amount of backlash and worsens fidelity of plunger motion. Because of this, and to reduce the weight of the magnet, the yoke's thickness should be reduced as much as possible. On the other hand, an excessively thinned yoke results in greater magnetic resistance and thus the holding force of the magnet decreases. In addition, the yoke's thickness should be decided in consideration of its structural strength. After investigating every condition mentioned above, the optimum thickness of the yoke was decided. The plunger's length against the coil. Backlash increases when the length of the plunger is too great. For this reason, the plunger should be as short as possible. However, the holding force generated by the magnet is decreased if the plunger length is too short. In order to determine optimum plunger length against the coil, the magnetic force against the plunger length was examined experimentally. Further Improvements of Fidelity Structural optimization of the electromagnets was achieved by various improvments as mentioned above. However, the backlash problems could still not be satisfactorily solved. For these circumstances, we tried to solve the stickslip and backlash problems by utilizing special control system of coil current. The details of the method are explained below. The main reason for stickslip and backlash phenomena is due to the nonsymmetry of magnetic field, as mentioned above. The magnetic field nonsymmetry causes the radial force acting on the plungers and the friction between the plungers and the guide tube. As this friction force is not uniform, the curve (b) in Fig. 3-(2), which is the the relative position of the plungers to the coils for coils' position curve (a), fluctuates.
2 149
Principles of the speical coil explained with Fig. 4 as follows;
system
is
The coils as shown in Fig. 4-(1) are divided into two parts. The plunger is also divided into two parts correspondingly. The upper part coil is a holding coil which mainly support the plunger weight. The upper coil also gives the upward driving force to overcome friction force by increasing the current when it drives upward. The lower part coil is a auxiliary coil which gives the downward driving force to overcome the friction force to move it smooth when it dr ives downward. The coi ls are mechanically connected to each other and so are the plungers. The group of these coils is called "coil-train", and the group of these plungers is ca lled "p lunger-train". The current of both coils is adjusted automatically, depending on the moving direction, to obtain the highest fidelity of the plunger-train. The principle shall matically below;
be
explained
sche-
Curve (a) in Fig. 4-(2) corresponds to the curve (a) in Fig. 3-(1). Curve (b) shows the downward force of auxiliary coil. When the plunger-train is driven upward, curve (a) is changed to curve (a') by increasing the holding coil current to produce the upward force corresponding to friction force Fr. On the other hand, when the plunger-train driven downward, curve (b) is changed to curve (b') by increasing the auxiliary coil current to produce the downward force corresponding to friction force Fr. In the practical device, various procedures including feedback control as mentioned later are utilized. A large backlash appears in the case of the primitive CRDM as shown in Fig. 3-(2) curve (b). The backlash was reduced considerably by above mentioned improvements. The typical curve of relative pos~t~on of the plunger-train to the coil-train is shown as curve (a) in Fig. 5. But it is not sufficient yet. Although the optimizations of the currrent control were examied, the improvements was not sufficient yet. Therefore, further advanced improvements was required. There is also a time dependent factor for backlash, i.e., the plunger-train velocity can not follow the coil-train velocity immediately and the delay of acceleration is taken place when it starts to move, as shown in Fig. 6. The difference of relative position, which corresponds to the area between the velocity curves of plunger-train and coil-train, is produced in a certain time interval. A feedback control method corresponding to the lack of uniformity of friction and the time
2150
T. Shiba ta et al.
dependent position difference was employed. The current is controlled corresponding to the signal of relative position of the plunger-train to the coil-train. The signal of the position was generated from the differential transformer mounted at the bottom part of the coil-train. Applying this method, extremely satisfactory performance was obtained as shown the curve (b) in Fig . 5. Stickslip was also reduced sufficiently with this method. PROTOTYPE CRDM FOR KUHFR Mechanical Structure A prototype CRDM for KUHFR was fabricated according to the previously mentioned developments. The device is the type which should be installed beneath the reactor vessel as shown in Fig. 7. It drives the control rod into and out of the reactor core (nuclear fuel region). The control rod is fully inserted into the core when the control rod comes to the bottom end of the stroke. Usually the control rod is inserted upward when the CRDM is installed beneath the reactor vessel. But in this case, the control rod is withdrawn upward from the core with a supporting rod and the control rod inserted into the core downward. The control rod is connected with plungertrain. The length of the device is about 3.5 m and its weight is about 100 Kg. The material of the guide tube which constitutes the pressure boundary and is in contact with primary cooling water, is nonmagnetic stainless steel and the material of plungers is magentic stainless steel. The guide tube is 50 mm in outside diameter with 1 mm thickness and the plunger is about 60 mm long and is about 47 mm in diameter. Each coil, which is cuprum winding of about 700 turns and 50 mm long, is installed within a yoke which is about 66 mm in outside length. The coil current is about from 3.5A to 4 . 0A for holding coils and about from 1.0A to 4.0A for auxiliary coils. Each coil is air-cooled to remove the joule heat . The weight of the driven part is about 30 Kg. The total load on the driving mechanims is the sum of the weight and the load due to hydraulic force of the primary cooling water. The primary coolant flows from the upper side of the core to the bottom. The estimated hydraulic force is about several kilograms. Six holding coils and two auxiliary coils were equipped to the CRDM. The coils outside of the guide tube is driven upward and downward by a screw and moving nut mechanism.
device is equipped with a snubber at the bottom to absorb the mechanical shock in scram ac tion. Control and Electric Equipments The control and electric equipments for the CRDM are described as follows; The block diagram of the control and electric equipments of the CRDM is shown in Fig. 7. The up or down drive signal of the control rod given from the reactor console desk goes to the AC motor at the top of the device and two direct current sources for holding coils and auxiliary coils. The AC motor drives the coil-train upward or downward according to the signals. The signal to the current sources controls the currents of both coils. In addition to the current control , the signal of the difference between the plunger-train and the coil-train initiates some further current increment to keep the relative position constant. The differential transformer is installed at the b:>ttom part of the coil-train and the plunger-train to give the difference signal. The absolute position of the control rod is obtained by the signal from the differential transformer and the coil-train's position signal . The finish of the control rod insertion into the core in scram action is confirmed with a signal from magnetic sensor outside the snubber, which is generated when the snubber piston is inserted into the snubber cylinder. The temperature of the coils is observed with bimetals and is monitored. DURABILITY EXAMINATION Full scall test instrumenL In order to examine the function of equipment and reliability over long term, the full scale drive mechanism was fabricated. The photograph of the equipment is shown in Fig .
S. The control rod mock-up was employed to simulate the load of practical control rod. Long term examination. The running test was carried out from January 1979 to August 1979 . It was found that life travel was over 10,000 m, inching cycles was over 400,000 times and the durable number of scram shock was over 200 times. The precision of the plunger position of 0.2 mm was obtained. It is recognized through the examination that the control rod drive mechanism is satisfactory. CONCLUSION
In an emergency of nuclear reactor, the scram is carried out by inserting the control rod into the core immediately . In the present case, the control rod falls and is inserted into the core by disenergizing the coils. The
The new type of nuclear reactor control rod The drive mechanism has been developed. mechanism has no penetration through the pre ssure vesse 1. The difficulties in presice
Development of Moving Coil Type Control Rod Drive Mechanism
isolated from positioning resulting the mechanical structure and mechanical problems have breen overcome by using the speical devices. The durability of prototype machine was tested by traveling the plunger a distance of 10,000m. It was successful in controlling the precise positioning of the plunger to 0.2mm. The mechanism is applicable to various nuclear reac tors and other plants wi th s imi lar requirements.
2151
REFERENCES Freund G.A. and co-workers (1962). Materials for Control Rod Drive Mechanism. Rowman and Littlefield, New York. McLain S., and J.H. Hartens (1964). Reactor Handbook. John Wiley & Sons, New York. Thompson T.J., and J.G. Beckerley (1970). The Technology of Nuclear Reactor Safety vol-.-1, vol. 2, The M.I.T. Press.
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2152
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Development of Moving Coil Type Control Rod Drive Mechanism Discussion to Paper 93.1 Pan Fong (China): Since you use the dynamic process for the tracking welding I want to know how do you measure the dynamic accuracy and which system is better, the image system or the tracking system? M. Kawahara (Japan): In the arc welding field, the static accuracy of tracking is much more important than the dynamic accuracy from the practical point of view. However, the dynamic accuracy, including the detection accuracy and the positioning accuracy, has been determined as described in our paper. The tracking accuracy of our method can be changed by an optical magnification and obtained a higher accuracy than the ordinary methods.
2153
D. Leckie (USA): What is the distance between the sensor image on the weldment and the torch? Is there some means to control the torch to compensate for weld gap changes of direction within the distance between the sensor and the torch? M. Kawahara (Japan): The distance between the point of detection and the welding torch is about 10 centimetres. The point of detection is ahead of the torch by the distance of about 10 centimetres, but the detected information is delayed for a corresponding period of time, and then fed into the servomechanism as the control target. Therefore, the welding torch position can be controlled accurately even if the welding line curves.
DEVELOPMENT OF MOVING COIL TYPE CONTROL ROD DRIVE MECHANISM T. Shibata*, S. Ishihara*, K. Hosono**, T. Nakajima** and T. Kanazawa*** -Research Reactor Inst£tute of Kyoto Un£vers£ty, Noda, Kumator£-cho , Sennan-gun, Osaka 590-04, japan "Control Eng£neer£ng Department, Techn£cal Research Cent er, N£ppon Kokan, No . 1-1, Minam£-Watarida -cho, Kawasak£- ku, Kawasaki 210, japan "-Nuclear System Development Sect£on, Ocean Engineering Department, Nzppon Kokan, No . 1-1-2, Marunouchz', Chz'yoda -ku, Tokyo 100, japan
Abstract. Nuclear reactors are controlled by movable control rods made of neutron absorbing materials. These rods are driven by control rod drive mechanism which is directed by servo-contro1 circuit. New drive mechanism was developed. Magnetic plunger connected with control rods in guide tube is exerted by the magnetic coil outside of the guide tube. Special circuit for precise positioning of the plunger relative to the coil have been developed. The drive mechanism has no vessel penetration such as rotary seal or sliding seal, so that no leakage of primary cooling water of high radioactivity was reached. Keywords. Actuators; electromagnet.
nuclear
reactors;
position
control;
power
control;
INTRODUCTION In general, nuclear reactors are controlled by movable control rods made of neutron absorbing materials . These rods are driven by control rod drive mechanism (hereinafter referred to CRDM) which is directed by servo-contro1 circuit. Conventional CRDM has penetrations with water sealing through the reactor pressure vessels. In these water sea1ings, some water leakages are induced. The water which is called primary reactor coolant has very high radioactivity. It is very important to prevent the leakage of the primary coolant for safety. New CRDM has been developed after some investigations for various types of drive mechanism. In the drive mechanism magnetized plunger engaged with control rods in guide tube which is the part of pressure boundary is exerted by the magnetic coil outside of the guide tube as shown in Fig. 1. The drive mechanism has no penetration and induces no leakage of the primary coolant in principle. However there are technical difficulties especially for precise positioning of the plunger because of the isolated mechanical structure with magnetic coupling. The difficulties of the precise positioning are due to mainly the mechanical friction between the plunger and the guide tube and hydraulic effec t. To obtain the precise positioning of the
plunger, the special control devices for automatic adjustment of the magnetic field has been developed. The control rod can be placed at any desired positions. The structure of the developed CRDM is rather simple and the rod drop at emergency reactor shut down is reliable. While the mechanism was deve10poed primarily for KUHFR (Kyoto University High Flux Reactor), it is applicable to other reactors or other plants with similar requirements. COMPARISON OF THREE TYPES OF DRIVE MECHANISMS Requirements for Control Rod Drive Mechanism of Nuclear Reactors The requirements follows;
for CRDM are described as
The control rod is required to have a smooth motion and constant velocity in driving. It is also required that the rod can be stopped at any desired position. A backlash on its travel has to be sufficiently small. For emergency reac tor shutdown, the control rod has to be inserted into the reactor core immediately by disengagement from the driving mechanism (which is called scram). CRDM has to be sufficiently durable and reliable.
2 14 7
T . Shib ata et al .
2 148
In the case of KUHFR, performances are as follows;
the
required
10 cm/min. Control rod driving velocity 65 cm Control rod driving stroke Minimum durable traveling 10,000 m length of rod 400,000 times Minimum inching cycles 200 times Minimum number of scrams (mechanical reliability for shock) In addition to these requirements for KUHFR, next two items are also required.
(l)
Holding force is greater than that of the other two types and the device can be made smaller.
(2)
This type of mechanism enables the accurate positioning of the rod relatively better than that of the linear motor type. Continuous positioning is possible whi le the other two types are dr iven stepwise. The plunger is compelled to contac t with the guide tube due to the radial magnetic force and the plunger's stickslip movement is caused by the ununiform fric t ion force between the plunger and the guide tube. Similarly backlash is resulted when the moving direction of the coil is reversed.
(3)
(4)
1) The mechanism must be no leaking structure
for safety. mechanism must be composed of usual parts and material for easy maintenance.
2) The
Under these conditions, three types of drive mechansims were examined as follows;
(5 )
Summary of Comparison Linear Motor Type A experimental device shown in Fig. 2-(1) was examined. Many small coils are located outside the guide tube coaxially over the stroke length. Each coi 1 is energized one after another for producing magnetic field movement. The magnetic field movement drives the magnetic plunger located coaxially in the coils. As a result of the examinations, the following characteristics were obtained. A large number of small coils has to be provided to move the rod smooth and stop precisely it at any desired position. However, it is practically very difficult to realize it. This type may be appropriate for a drive mechanism of long stroke compared with each coil size. Friction Grip Magnetic Jack Type A experimental device shown in Fig. 2-(2) was examined. This type of mechanism drives the rods in the guide tube stepwise by exciting the lift coil, grip coil, pull down coil and hold coil sequentially. This type was examined and the results are listed below. (1) (2)
Holding force is generated with friction so that slips are apt to take place and more load results in more slips. As it takes time to switch the current for the coils, it is difficult to obtain continuous and smooth movement of the rod.
Moving Coil Type A experimental device shown in Fig. 2-(3) was examined. The coil outside the guide tube is motor-dr iven vert ically, and the plunger magnetically coupled with coil is moved in the guide tube. The results of the examination were as follows;
As a result of investigation of these mechanisms, the following conclusions were made. The moving coil type still has problems in accurate pos~t~oning because of such a stickslip and backlash. Since this type of mechanism was considered the most suitable for KUHER, it was decided to adopt this mechanism. But it required further improvements and detailed design. IMPROVEMENTS AND DESIGN STUDY OF MOVING COIL TYPE CRDM The Characteristics of Primitive Moving Coil Type CRDM In this type of electromagnet, the position of plungers depends on such factors as the weight of the plunger, magnetic field and friction. In other words, the position of the plunger is not potentially stable. As the magentic field is not perfectly symmetric due to structural nonsymmetry, plungers are compelled to contact with the guide tube. Friction force was taken place between plungers and the guide tube. Consequent ly, the plungers' movement looses fidelity to the coils movement. Backlash and stickslip appear in the plunger movement. The detailed mechanisms of the backlash and sticks lip are described schematically as follows; The magnetic axial force acting on the plunger vs. the relative position of the plunger to the coil is shown curve (a) in Fig. 3-(1). In the ideal case of no friction presence, the plungers must stop at point A, where the weight W balances with the magentic axial force. Ac tually the fric tion force between plungers and the guide tube due to magnetic field nonsymmetry is induced and when the coils are moved upward, the relative position
Deve l opme nt of Mov ing Coil Type Control Rod Drive Me chanism
of plungers to the coils is changed to point B, where magnetic axial force increases by amount of friction force, Fr, relative to the magnetic axial force of point A. When they are moved downward, the relative position of plungers to the coils is changed to point C similarly. The distance BC is the backlash. The point Band C fluctuate due to the local change of friction force. This fluctuation is sticks lip. Factors Affecting the Fidelity The stoppage period (backlash) depends on (1) the drive velocity of coil, (2) the yoke's thickness, and (3) the relation between the plunger's length and the length of the coil and yoke. As it is not practicable to adjust the drive velocity for improving the fidelity, the other two effects were investigated. Thickness of the yoke. A thicker yoke increases the amount of backlash and worsens fidelity of plunger motion. Because of this, and to reduce the weight of the magnet, the yoke's thickness should be reduced as much as possible. On the other hand, an excessively thinned yoke results in greater magnetic resistance and thus the holding force of the magnet decreases. In addition, the yoke's thickness should be decided in consideration of its structural strength. After investigating every condition mentioned above, the optimum thickness of the yoke was decided. The plunger's length against the coil. Backlash increases when the length of the plunger is too great. For this reason, the plunger should be as short as possible. However, the holding force generated by the magnet is decreased if the plunger length is too short. In order to determine optimum plunger length against the coil, the magnetic force against the plunger length was examined experimentally. Further Improvements of Fidelity Structural optimization of the electromagnets was achieved by various improvments as mentioned above. However, the backlash problems could still not be satisfactorily solved. For these circumstances, we tried to solve the stickslip and backlash problems by utilizing special control system of coil current. The details of the method are explained below. The main reason for stickslip and backlash phenomena is due to the nonsymmetry of magnetic field, as mentioned above. The magnetic field nonsymmetry causes the radial force acting on the plungers and the friction between the plungers and the guide tube. As this friction force is not uniform, the curve (b) in Fig. 3-(2), which is the the relative position of the plungers to the coils for coils' position curve (a), fluctuates.
2 149
Principles of the speical coil explained with Fig. 4 as follows;
system
is
The coils as shown in Fig. 4-(1) are divided into two parts. The plunger is also divided into two parts correspondingly. The upper part coil is a holding coil which mainly support the plunger weight. The upper coil also gives the upward driving force to overcome friction force by increasing the current when it drives upward. The lower part coil is a auxiliary coil which gives the downward driving force to overcome the friction force to move it smooth when it dr ives downward. The coi ls are mechanically connected to each other and so are the plungers. The group of these coils is called "coil-train", and the group of these plungers is ca lled "p lunger-train". The current of both coils is adjusted automatically, depending on the moving direction, to obtain the highest fidelity of the plunger-train. The principle shall matically below;
be
explained
sche-
Curve (a) in Fig. 4-(2) corresponds to the curve (a) in Fig. 3-(1). Curve (b) shows the downward force of auxiliary coil. When the plunger-train is driven upward, curve (a) is changed to curve (a') by increasing the holding coil current to produce the upward force corresponding to friction force Fr. On the other hand, when the plunger-train driven downward, curve (b) is changed to curve (b') by increasing the auxiliary coil current to produce the downward force corresponding to friction force Fr. In the practical device, various procedures including feedback control as mentioned later are utilized. A large backlash appears in the case of the primitive CRDM as shown in Fig. 3-(2) curve (b). The backlash was reduced considerably by above mentioned improvements. The typical curve of relative pos~t~on of the plunger-train to the coil-train is shown as curve (a) in Fig. 5. But it is not sufficient yet. Although the optimizations of the currrent control were examied, the improvements was not sufficient yet. Therefore, further advanced improvements was required. There is also a time dependent factor for backlash, i.e., the plunger-train velocity can not follow the coil-train velocity immediately and the delay of acceleration is taken place when it starts to move, as shown in Fig. 6. The difference of relative position, which corresponds to the area between the velocity curves of plunger-train and coil-train, is produced in a certain time interval. A feedback control method corresponding to the lack of uniformity of friction and the time
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dependent position difference was employed. The current is controlled corresponding to the signal of relative position of the plunger-train to the coil-train. The signal of the position was generated from the differential transformer mounted at the bottom part of the coil-train. Applying this method, extremely satisfactory performance was obtained as shown the curve (b) in Fig . 5. Stickslip was also reduced sufficiently with this method. PROTOTYPE CRDM FOR KUHFR Mechanical Structure A prototype CRDM for KUHFR was fabricated according to the previously mentioned developments. The device is the type which should be installed beneath the reactor vessel as shown in Fig. 7. It drives the control rod into and out of the reactor core (nuclear fuel region). The control rod is fully inserted into the core when the control rod comes to the bottom end of the stroke. Usually the control rod is inserted upward when the CRDM is installed beneath the reactor vessel. But in this case, the control rod is withdrawn upward from the core with a supporting rod and the control rod inserted into the core downward. The control rod is connected with plungertrain. The length of the device is about 3.5 m and its weight is about 100 Kg. The material of the guide tube which constitutes the pressure boundary and is in contact with primary cooling water, is nonmagnetic stainless steel and the material of plungers is magentic stainless steel. The guide tube is 50 mm in outside diameter with 1 mm thickness and the plunger is about 60 mm long and is about 47 mm in diameter. Each coil, which is cuprum winding of about 700 turns and 50 mm long, is installed within a yoke which is about 66 mm in outside length. The coil current is about from 3.5A to 4 . 0A for holding coils and about from 1.0A to 4.0A for auxiliary coils. Each coil is air-cooled to remove the joule heat . The weight of the driven part is about 30 Kg. The total load on the driving mechanims is the sum of the weight and the load due to hydraulic force of the primary cooling water. The primary coolant flows from the upper side of the core to the bottom. The estimated hydraulic force is about several kilograms. Six holding coils and two auxiliary coils were equipped to the CRDM. The coils outside of the guide tube is driven upward and downward by a screw and moving nut mechanism.
device is equipped with a snubber at the bottom to absorb the mechanical shock in scram ac tion. Control and Electric Equipments The control and electric equipments for the CRDM are described as follows; The block diagram of the control and electric equipments of the CRDM is shown in Fig. 7. The up or down drive signal of the control rod given from the reactor console desk goes to the AC motor at the top of the device and two direct current sources for holding coils and auxiliary coils. The AC motor drives the coil-train upward or downward according to the signals. The signal to the current sources controls the currents of both coils. In addition to the current control , the signal of the difference between the plunger-train and the coil-train initiates some further current increment to keep the relative position constant. The differential transformer is installed at the b:>ttom part of the coil-train and the plunger-train to give the difference signal. The absolute position of the control rod is obtained by the signal from the differential transformer and the coil-train's position signal . The finish of the control rod insertion into the core in scram action is confirmed with a signal from magnetic sensor outside the snubber, which is generated when the snubber piston is inserted into the snubber cylinder. The temperature of the coils is observed with bimetals and is monitored. DURABILITY EXAMINATION Full scall test instrumenL In order to examine the function of equipment and reliability over long term, the full scale drive mechanism was fabricated. The photograph of the equipment is shown in Fig .
S. The control rod mock-up was employed to simulate the load of practical control rod. Long term examination. The running test was carried out from January 1979 to August 1979 . It was found that life travel was over 10,000 m, inching cycles was over 400,000 times and the durable number of scram shock was over 200 times. The precision of the plunger position of 0.2 mm was obtained. It is recognized through the examination that the control rod drive mechanism is satisfactory. CONCLUSION
In an emergency of nuclear reactor, the scram is carried out by inserting the control rod into the core immediately . In the present case, the control rod falls and is inserted into the core by disenergizing the coils. The
The new type of nuclear reactor control rod The drive mechanism has been developed. mechanism has no penetration through the pre ssure vesse 1. The difficulties in presice
Development of Moving Coil Type Control Rod Drive Mechanism
isolated from positioning resulting the mechanical structure and mechanical problems have breen overcome by using the speical devices. The durability of prototype machine was tested by traveling the plunger a distance of 10,000m. It was successful in controlling the precise positioning of the plunger to 0.2mm. The mechanism is applicable to various nuclear reac tors and other plants wi th s imi lar requirements.
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REFERENCES Freund G.A. and co-workers (1962). Materials for Control Rod Drive Mechanism. Rowman and Littlefield, New York. McLain S., and J.H. Hartens (1964). Reactor Handbook. John Wiley & Sons, New York. Thompson T.J., and J.G. Beckerley (1970). The Technology of Nuclear Reactor Safety vol-.-1, vol. 2, The M.I.T. Press.
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Development of Moving Coil Type Control Rod Drive Mechanism Discussion to Paper 93.1 Pan Fong (China): Since you use the dynamic process for the tracking welding I want to know how do you measure the dynamic accuracy and which system is better, the image system or the tracking system? M. Kawahara (Japan): In the arc welding field, the static accuracy of tracking is much more important than the dynamic accuracy from the practical point of view. However, the dynamic accuracy, including the detection accuracy and the positioning accuracy, has been determined as described in our paper. The tracking accuracy of our method can be changed by an optical magnification and obtained a higher accuracy than the ordinary methods.
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D. Leckie (USA): What is the distance between the sensor image on the weldment and the torch? Is there some means to control the torch to compensate for weld gap changes of direction within the distance between the sensor and the torch? M. Kawahara (Japan): The distance between the point of detection and the welding torch is about 10 centimetres. The point of detection is ahead of the torch by the distance of about 10 centimetres, but the detected information is delayed for a corresponding period of time, and then fed into the servomechanism as the control target. Therefore, the welding torch position can be controlled accurately even if the welding line curves.