GB967513A - Improvements in and relating to actuating mechanism for control rods - Google Patents

Abstract

967, 513. Controlling nuclear reactors. NUCLEAR POWER PLANT CO. Ltd., and A.E.I. - JOHN THOMPSON NUCLEAR ENERGY CO. Ltd., [trading as NUCLEAR POWER GROUP]. Dec. 17, 1962 [Jan. 26, 1962], No. 2968/62. Heading G6C. An actuating mechanism for. a nuclear reactor control rod 10 comprises a winding drum 7 around which passes a rope or chain 9 supporting the rod and which is driven by a motor 1 through a gear reduction unit 4 housed in a casing mounted on bearings so as to be free to rotate about the axis of its coaxial input and output shafts 1a, 4a, an electromagnetic clutch 2 acting, when energized, on the casing to'lock it in position to enable the drive from the motor 1 to be transmitted to the drum 7. As shown, the clutch 2 comprises a coil and a magnetic core fixed to the frame 3, the magnetic flux from the core traversing a small air gap to a disc 2b pf magnetic material rigidly fixed to the gear casing. The disc 2b and the magnetic core are provided with a number of slots which form in effect magnetic teeth on each member and this provides a strong magnetic lock between the core and the disc when the coil is energized. Under emergency conditions, the clutch 2 is released and the rod 10 falls under its own weight into the reactor core. The tendency for the torque produced by the weight of the rod 10 acting on the drum radius to drive the motor 1 when the gear casing is held fixed by the clutch 2 can be overcome by incorporating a single worm-wheel in the gear unit 4, the torque exerted by the rod 10 then being solely opposed by the action of the clutch 2. Supplementary means for raising or lowering the rod and also for locking it in position and which are manually operated, but which could be operated from a separate power supply, comprise a spring loaded shaft 14, the upper end of which is covered by a plate 15 and at the lower end of which is a pinion 16. On removal of the plate 15, a winding tool 17 with a reversible ratchet 17a may be inserted and downward pressure applied to the shaft 14 to cause the pinion 16 to engage a gear ring 4c on the gear casing. A drive can then be transmitted to the drum 7 and the control rod raised or lowered, depending upon which way the ratchet 17a is connected. The provision of the ratchet 17a serves to lock the rod 10 in any given position except when the tool 17 is being turned. With the actuating mechanism described, a large gear reduction can be employed between the driving motor and the winding drum and hence the .notor used can operate at mains frequency and still drive the control rod at the desired low speeds. The high inertia of the reduction gear does not affect the rate at which the control rod falls under emergency conditions because the larger part of the reduction gear is by-passed by virtue of the gear casing being free to rotate.