Patent GB-1515782-A (GB)
Title: Anti-ejection system for a reactor control rod drive assembly
IPC: G21C7/14
Filed: 1976-01-20
Abstract: 1515782 Controlling reactors DIAMOND POWER SPECIALTY CORP 20 Jan 1976 [18 Feb 1975] 02187/76 Heading G6C An anti-ejection system for a control rod drive assembly of a nuclear reactor comprises a linearly movable positioning member driven by the control rod drive assembly for positioning a control rod in the reactor core, latch means engageable with the positioning member to move with the positioning member, whenever this moves in a first direction but remaining disengaged from the positioning member to permit movement thereof in a second direction opposite the first direction, and deformable means for binding the positioning member to prevent further movement thereof in response to the latch means moving a predetermined distance in the first direction. In a preferred embodiment (Fig. 1, not shown), the positioning member comprises a threaded leadscrew 20 linearly movable within a tubular pressure housing (12) pressurized to the pressure of the reactor by a sealed connection thereto through its open end (11). Since the reactor pressure may be in the region of 2000 p.s.i., should the housing (12) rupture, the leadscrew 20 could be shot out of the housing (12) due to the pressure difference between the ambient and the housing (12). To prevent this from occurring, an anti-ejection assembly 52 (Fig. 2) is made engageable with the threads 34 of the leadscrew 20 whenever the stator windings (32) of the control rod drive assembly are de-energized. The leadscrew 20 may move in a "scram direction" by ratchetting with the assembly 52 but should it start to move in the opposite direction, indicating an attempted ejection, 52 will then move with the leadscrew 20 to press into and deform a collet 54 causing it to wedge the leadscrew 20 to the inside wall 78 of a carrier body 30 within the housing (12). The anti-ejection assembly 52 comprises a pair of latch pawls 56 which are linearly movable a predetermined distance as allowed by slots 58 formed in the carrier body 30, the pawls 56 having pivots 60 located in respective slots 58 and engaging the threads 34 of the leadscrew 20 by the action of springs 62. Each latch pawl 56 is also provided with a pin 66 engageable by respective lever arms (14, 16) of the control rod drive assembly to pivot the pawls 56 against the springs 62 and so decouple them from the threads 34 whenever the stator windings (32) are energized. Thus the assembly 50 does not interfere with the normal positioning and hold functions of the control rod drive assembly. When the stator windings (32) are de-energized, the lever arms (14, 16) release the pins 66 and the latch pawls 56 pivot under the force of the springs 60 to engage the threads 34. The teeth 68 of the pawls 56 have inclined surfaces 70 which allow the pawls 56 to ratchet and the leadscrew 20 to drop down into the reactor core in a "scram" condition, and flat surfaces 74 parallel with the threads 34 to prevent the ratchetting of the pawls 56 in the opposite or ejection direction of the leadscrew 20. Therefore, should a rupture occur in the housing (12), the pawls 56 will move with the leadscrew the predetermined distance as allowed by the clearance provided for the pivots 60 within the slots 58 and engage the collet 54. The collet 54 is then driven up an inclined surface 76 of the carrier body 30 and wedged between the leadscrew 20 and the inside wall 78 of the carrier body 30. The force of the pawls 56 driving up against a flange #0 of the collet 54 causes the flange 80 to pivot around a projection 82 and the collet 54 to move along the inclined surface 76. Vertical slots 84 in the collet 54 enable it to be easily deformed to the smaller diameter necessitated by its wedging action. When the stator windings (32) are energized, the leadscrew 20 is retained by roller nuts (18) carried by the lever arms (14, 16). As an additional precaution, however, the end of the leadscrew 20 could be provided with a pop-out section such as a spring loaded pin (not shown) which mould normally drag along the inside wall of the housing (12) but which would engage either the collet 54 or the pawls 56 should the end of the leadscrew 20 be ejected beyond the roller nuts (18).