Patent GB-1344013-A (GB)
Title: Control rod drive for a water cooled nuclear reactor
IPC: G21C7/16
Filed: 1971-06-17
Abstract: 1344013 Controlling reactors ASEA-ATOM AB 17 June 1971 [18 June 1970] 28373/71 Heading G6C A control rod drive for a water-cooled nuclear reactor comprises a drive means operated hydraulically to insert a control rod into the reactor core, and a housing which surrounds the drive means and is intended to be connected vertically to the lower side of the reactor, the housing being of double-walled construction and intended normally to contain at least a substantial part of the driving water within the annular gap between its walls, which gap is connected at its lower end through at least one flow passage to the inside of the housing and to the drive means for rapid insertion of the control rod, and is provided with an inlet which is intended to be connected to a pressure medium source through a valve which is normally shut but opened in an emergency, for pressurizing the driving water. The drive means 16 shown in Fig. 2, suitable for use with a boiling water reactor, has a mechanical drive means for normal movement of the control rod 15 and an hydraulic driving system for rapid insertion of the control rod into the reactor core. The drive housing 22 comprises outer and inner casings 32, 33 and the annular gap 35, containing substantially all the driving water, is provided at the top with an inlet 37 for gas under high pressure and is joined at the bottom to the space enclosed by the inner casing 33 by means of a plurality of throttled flow passages 38. Normally a hollow piston-rod 21 carrying the control rod 15 rests on the nut 20 of the mechanical drive means and upon emergency insertion of the control rod is pushed up by the hydraulic pressure and leaves its position on the nut 20. Spring-loaded blocks 30 (only one of which is shown) on the piston-rod 21 engage with toothed racks 44 arranged inside the inner casing 33 and prevent the control rod 15 from leaving the reactor core when the hydraulic pressure drops. Upon an emergency shutdown, the electric motor 18 of the mechanical drive means is started and the nut 20 screwed up to its top position, automatically releasing the blocks 30 from the racks 44, after which the downward operation can start. Except for the outer casing 32, the entire drive means forms a unit which can be inserted into the outer casing 32 and screwed thereto by means of screws 42. The outer casing 32 forms a control drive connection piece which is permanently secured to the reactor pressure vessel 6. The gas used for pressurizing the driving water is preferably a 95 : 5 N 2 /H 2  gas mixture, the H 2  serving to counteract the formation of HNO 3  when the N 2  reacts with the free O 2  in the reactor and is absorbed into the reactor coolant/moderator. Where the reactor is of the direct cycle type, the driving gas flows with the steam to a turbine condenser and is then drawn off by the normal ejector vacuum pumps of the condenser. In order to prevent a temperature shock on the occasion of an emergency shutdown and also to prevent impurities in the form of radio-active slime from falling down and collecting in the drive means, a small flow of water is taken from the reactor and pressurized, then led through the inlet 37 to the interior of the drive means where most of it flows down through the gap 35 and then through the passages 38 into the inner casing 33, most of it then passing up through the hollow piston-rod 21 and out through a narrow  flow passage 62 to the upper part of the outer casing 32 and to the control rod guide tube 17. A small quantity of the water flows from the upper part of the gap 35 through a narrow flow passage 63, and similarly a further small quantity of the water flows past a seal 25 surrounding the piston-rod 21 through a narrow flow passage 64 in the top of the inner casing 33. This normal water flow is insufficient to lift the piston-rod 21 from the nut 20, but instead leaks out through the narrow flow passages 62, 63 and 64 to the reactor interior.