Patent GB-1255927-A (GB)
Title: Emergency nuclear reactor shutdown systems
Inventor: BENNETT JOHN CHARLES
IPC: G21C9/027
Filed: 1967-11-30
Abstract: 1,255,927. Controlling reactors. ATOMIC POWER CONSTRUCTIONS Ltd. 2 Dec., 1968 [30 Nov., 1967], No. 54503/67. Heading G6G. An emergency shut-down system for a nuclear reactor comprises means for inserting a shutdown material into the reactor core through its bottom face, the material when inserted being located in the core in at least one upright column and being retained in the core, once inserted, by material in at least one second column, the material in the at least one second column exerting a retaining force on the shutdown material in the at least one upright column. In a gas-cooled reactor, a number of U-shaped tubes 2 are provided in the core 1 for the insertion of neutron absorbing bodies 3. These bodies 3 are hollow spheres formed of, for example, stainless steel internally coated with Cd or B 4 C, or a boron steel, and each has a small hole drilled in it to prevent crushing or bursting due to changes in external pressure. The upper end 5 of each tube 2 is partially closed by a restrictor 7 which serves to prevent ejection of the bodies 3 into the core surroundings. Each tube 2 also contains heavy bodies 4 of stainless steel. The other end 6 of each tube 2 is connected via a valve 9 to a cylinder 10 of high pressure gas. A gas line 14 is connected to each tube 2 via a valve 15 for pressure release. The bodies 3, 4 are normally retained within each tube 2 by a removable barrier 16, but may be  withdrawn through a tube 17 for replacement or inspection. During normal operation, the bodies 3 are outside the core 1, the position of the first body 3 being shown by the arrow 27 and that of the first body 4 by the arrow 28. Because the height of the bodies 3 is the same in each limb of the tube 2 and the bodies 4 are in a horizontal part of the tube 2, this standby position of the bodies 3, 4 is stable. When the valve 9 is opened, high pressure gas from the cylinder 10 blows the bodies 3, 4 along the tube 2 and the bodies 3 fill the tube within the core 1, their weight being balanced by that of the bodies 4. Controlled retardation of the train of bodies 3, 4 as they approach the fully inserted position is achieved either by means of a spring device in the upper end of the tube 2 or by modifying the insertion gas flow during the latter part of the insertion. This flow modification can be obtained by increasing the diameter of the tube 2 towards its upper end, or by introducing side gas ports into the upper end of the tube. In a further embodiment a pressure relief tube (not shown) is run from the upper end 5 of the tube 2 to a point between the position 28 and the cylinder 10, incorporating a one-way valve which only allows gas to flow from the end 5 to the cylinder 10. The purpose of this tube is to prevent the bodies 3, 4 being blown back down the tube 2 should the pressure at the end 5 rise due, for example, to a fire in the reactor heating and expanding the gas within the reactor. Such a pressure rise would cause gas to flow down the pressure relief tube through the one way valve to equalize the pressure at the ends of the train of bodies 3, 4.