Patent GB-1223277-A (GB)
Title: Nuclear reactor control element drive assembly
IPC: G21C7/20
Filed: 1968-10-08
Abstract: 1,223,277. Controlling reactors. COMBUSTION ENG. Inc. 8 Oct., 1968 [10 Oct., 1967], No. 47679/68. Heading G6C. A control element drive assembly for a nuclear reactor cooled by pressurized liquid coolant comprises a vertically movable member connected to the control element and having an enlarged member at its upper end, drive means for reciprocating the movable member, and a liquid-filled guide tube through an opening in the lower end of which the movable member extends with its enlarged portion within the guide tube, the enlarged portion and the guide tube forming a piston-cylinder arrangement acting on the liquid in the guide tube for the control of the downward movement of the control element. In the control element drive assembly 18 of a pressurized water nuclear reactor, the control element (not shown) is connected to a rack 26 driven by a pinion gear 30 mounted in a gear housing 28 within a pressure housing 22 filled with coolant water under reactor pressure. The upper end of the pressure housing 22 is closed by a plug 44 which forms part of an autoclave type closure. Mounted within the pressure housing 22 is a support tube 60 provided with a lower end plate 70 through which pass apertures 72, 74. The gear housing 28 is bolted to the end plate 70. Mounted on the upper end of the rack 26 is an enlarged cylindrical portion which functions as a piston 76 and which is housed in a guide tube 78 filled with coolant water. A bushing inserted into the bottom of the guide tube 78 forms a stop 80. Holes 82 suitably spaced along the length of the guide tube 78 permit water to flow in and out when the piston 76 moves to and fro. When the rack 26 is released for a scram, it will descend at a controlled rate and be slowed to a gradual stop when the piston 76 enters the lower end portion of the guide tube where there are no holes 72. During this latter portion of travel, the water will flow up around the piston 76 and down through the clearance between the rack 26 and the stop 80. The clearance between the guide tube 78 and end plate 70 permits water to flow back up to the upper portion of the guide tube 78. The guide tube 78 is supported within the support tube 60 by means of a collar 84 which rests on a plate 86 extending across the support tube 60 near its upper end. Should a scram occur when there is no water in the guide tube 78, the force of impact will be transferred through the guide tube 78 to the collar 84 and the shear pins 90 with which it is fastened to the guide tube. An additional collar 88 provides a backup shearing means. The upper end of the guide tube 78 is open to permit access to means for disconnecting the control element from the rack-and-pinion drive means. This disconnecting means comprises a nut 94 screwed on to a rod which extends down through the centre of the rack 26 and which supports clamping means for the control element, and a nut turning and locking means 96. A tool is inserted through an access tube 54 through the plug 44 into engagement with the means 96 which is operated to turn the nut 94 to either engage or disengage the control element.