Patent GB-1170879-A (GB)
Title: Improvements in or relating to nuclear reactors.
IPC: G21C7/14
Filed: 1968-06-04
Abstract: 1,170,879. Controlling reactors. GULF GENERAL ATOMIC Inc. 4 June, 1968 [7 June, 1967], No. 26447/68. Heading G6C. A control rod drive system comprises means for supporting and adjusting a control rod with respect to the reactor core, and means for coupling the control rod to the supporting and adjusting means including a latch releasable in response to a scram signal, rod means interconnecting the control rod to the latch and having two aligned sections of different cross-sectional areas with the smaller section positioned toward the latch, a cylinder mounted in fixed position around at least parts of the two sections, and means for maintaining a fluid under constant pressure in the cylinder for rapidly displacing the rod means toward the reactor core upon release of the latch. One such system is illustrated in connection with a reactor having a core comprising a plurality of fuel modules or boxes 11 containing fuel rods 12 and supported on a grid 13. Each fuel module has a central passageway 17 in which a control rod 18 may be moved. The upper ends of the fuel modules fit in the square spaces formed by the grid 13 and each is suspended on a supporting sleeve 26. The lower end of the sleeve 26 has outwardly turned projections 27 which are positioned underneath projections 24 on a collar 22 supported by four webs 23 extending diagonally from the corners of the fuel module. When the sleeve 26 is turned sufficiently, the projections 27 pass between the projections 24 in order to release the fuel module and permit its removal from the reactor core. The upper end of the sleeve 26 is threaded into the interior of a nut 28. A load transfer sleeve 29 surrounds the sleeve 26 from which the nut 28 projects a sufficient distance to extend over the upper end of the sleeve 29. The lower end of the sleeve 29 rests upon a collar 31 supported by four webs 32 extending diagonally from the corners of a square space defined by the grid 13. By sufficiently tightening the nut 28, the weight of the fuel module is transferred to a bearing load on the top of the grid 13. A cylinder 33 is supported on a shield plug 30 attached to the sleeve 26 and a plug 35 is threadably secured to the sleeve 26 at the top of the cylinder 33. An upper pistonrod 34 extends from the upper end of the cylinder 33 and a lower piston-rod 36 from the lower end, the rods being connected inside the cylinder by a short cylindrical member 69 which has a diameter greater than that of either of the rods but less than the internal diameter of the cylinder. The lower piston-rod 36 is connected to the control-rod 18 by a breech type coupling. A magnetic latch which connects the upper piston-rod 34 to a drive-plate 38 includes a pair of plates 39, 41 which are securely held together upon energization of an electromagnetic coil 42. A screw-drive mechanism for the vertical positioning of the plate 38 includes a pair of drive-screws 43, 44 which pass in threaded engagement through the plate 38. The screws are rotated by gears 58, 59 which are both driven by a drive-gear 61 rotated by a synchronous reversible drive motor 62. The ends of the cylinder 33 are sealed by annular seals 64, 66 which enable the rods 34, 36 to slide axially through the closed ends of the cylinder. A passage 67 connects the interior of the cylinder 33 with a source of hydraulic pressure 68 and since the rod 36 is of larger cross-sectional area than the rod 34, a constant downward bias is exerted on the rods 34, 36. Consequently, downward movement of the control-rod 18 occurs with minimal delay upon the initiation of a scram signal and de-energization of the magnetic latch. During the scram, the cylindrical member 69 moves into a tapered section 71 at the lower end of the cylinder 33, the pressure on the underside of the member increasing due to the diminishing area available for passage of fluid around the member so as to exert a braking force. A crushable shock-absorber 75 may be positioned to stop the control-rod in the event of a failure and consequent free fall.