Patent GB-951276-A (GB)
Title: A nuclear reactor and method of varying its reactivity
IPC: G21C7/16
Filed: 1961-11-20
Abstract: 951,276. Controlling nuclear reactors. GENERAL NUCLEAR ENGINEERING CORPORATION. Nov. 20, 1961 [Dec. 15, 1960], No. 41393/61. Heading G6C. The reactivity in the core of a nuclear reactor is varied by the use of movable poison members disposed in laterally spaced vertical control passageways up through which a fluid flow is maintained at all times, the rate of flow of fluid through separate passageways or small predetermined groups of passageways being independently controlled and such that at the highest rate of flow the poison members are moved upwardly through the passageways to a position above the core and retained in this position, but are returned to the core by gravity on restriction of the fluid supply. In the embodiment described, the reactor core con tains a plurality of separate fuel assemblies 19 (Fig. 4), hexagonal in transverse section, and each comprising two annular pairs of rows of fuel elements 20, 22 separated by a passageway 24 with a passageway 26 inwardly of the pair of rows 22. This particular configuration provides for good neutron flux distribution. Motor driven Y-shaped control rods 30 are positioned in the channels 28 between the various fuel assemblies 19. A tubular member 32 extends throughout the length of each passageway 26 (Fig. 2) and is joined at the top to another tubular member 34. The lower end of tubular member 32 is provided with a bellows 54 from which extends a cylindrical member 56 which engages a connector 58 secured to the end of a distribution tube 50 of small diameter which enters the reactor vessel through a control rod thimble 51. The passageway 26 contains a number of balls 38 which include a neutron poison such as boron or cobalt, the position of these balls 38 being controlled by the passage of a fluid, e.g. steam or water from the reactor cooling system, from the tube 50 up through the tubular members 32, 34 and out through openings 62 at the upper end of tubular member 34. The fluid system shown in Fig. 3 comprises a distribution header 40 supplied with water through supply conduit 42 and valve 44, the water being conveyed to the various passageways 26 by means of valves 48, subheaders 46, distribution tubes 50 and valves 52, the latter each being provided with a by-pass flow. When a particular valve 52 is open, the fluid flow drives the balls 38 up the respective passageway 26 until the lower ball 38 which is of greater diameter than the  others engages a seat 61 formed at the lower end of the tubular member 34. This allows only a small flow of fluid to pass by, sufficient for cooling purposes. When the valve 52 is closed, the balls 38 descend by gravity into the reactor core, the lower ball 38 being supported by stops 60. The bypass flow from valve 52 provides for cooling when the balls 38 are in this position. Individual control passageways or groups of passageways can be controlled in the above manner to obtain a uniform power distribution across the reactor core. In a modification (Fig. 6, not shown) the balls 38 are all of the same size and the seat 61 is dispensed with, the upper ball 38 making an imperfect fit with an opening in the top of the tubular member 34 to allow a small fluid flow through the passageway 26 when the corresponding valve 52 is open. In order to reduce the height of the tubular member 34 it may be in the form of a spiral (Fig. 7 not shown), or a hopper (Fig. 8, not shown) of larger diameter than the tubular member 32. In an alternative embodiment (Fig. 9), each poison member comprises telescopic tubular members 68 occupying the position as shown when the corresponding valve 52 is closed but being displaced upwardly with the lower member sliding into the upper when the valve is opened. Another embodiment is shown in Fig. 10, the poison member being in the form of a rod 70 having a guide ball 72 secured to each end. The location of a ruptured or leaky fuel element may be determined by successively moving the poison into and out of the inactive region in one passageway or groups of passageways at a time while the effluent from the reactor is being monitored, a sudden increase in radio-activity when the poison is moved out of a passageway or group of passageways indicating the presence there of a defective fuel element.