Patent GB-1205731-A (GB)
Title: Nuclear reactor control device
IPC: G21C7/08
Filed: 1969-01-03
Abstract: 1,205,731. Controlling reactors. WESTING- HOUSE ELECTRIC CORP. 3 Jan., 1969 [24 Jan., 1968 (2)], No. 469/69. Heading G6C. A nuclear reactor has disposed within a vessel a reactor core including a plurality of fuel assemblies each having a number of separately movable control rods, at least one drive mechanism for moving the control rods and means for holding selected control rods in predetermined positions, the or each drive mechanism having a drive train extending into the reactor vessel and adapted to engage at least some of the separately movable control rods in at least one of the fuel assemblies for movement of the control rods engaged by the drive train, but not held by the holding means. In one embodiment, the fuel assemblies each include a plurality of guide tubes which house either rod cluster control units for primary and shutdown control or fine flux trim rods. The guide tubes housing fine flux trim rods from a number of fuel assemblies are connected by curved guide channels with the upper guideway extensions 92 of a drive mechanism 64 (Fig. 5) mounted on a head penetration adaptor of the reactor vessel. Pistons 74 are attached at their lower ends to ball cables 78 which are in turn attached at their lower ends to the fine flux trim rods (not shown). The trim rods in their in-core position hang on the ball cables 78 attached to pistons 74, which are supported on a ledge, and are moved to their out-of-core position (r.h.s. in Fig. 5) through the extensions 92. The force required to lift the pistons 74 is obtained by connecting an upper cavity 94 with a low pressure water retention tank 96. Energizing the coil 98 of an electromagnetic valve 100 lifts a tapered valve tip 102 from its associated seat 104. Connection is then made through channels 106, 107 between the cavity 94 and the tank 96. The trim rods are selectively returned to their in-core position under the action of gravity when the coil 98 is de-energized. A plurality of electromagnets 114 is arranged symmetrically about the mechanism head, one electromagnet being positioned adjacent the top position of each piston 74. When the coils 118 of the electromagnets 114 are energized, the corresponding pistons 74 are pulled towards the polepieces 116. A tapered portion 120 of the piston 74 abuts a ledge 122 which aids in holding the piston in its top position. If one or more trim rods must be withdrawn from the in-core position, it is first necessary in this embodiment to withdraw all the rods by opening the valve 100 and then to selectively energize the magnetic coils 118 to hold in the out-of-core position those rods which are to remain withdrawn. An A.C. signal may be superimposed on the D.C. current of the electromagnets 114 to ascertain which pistons 74 are in their uppermost position, the coil impedance being higher in the latter case than when a piston has been accidentally dropped or has been unable to complete its upward movement due to some mechanical misalignment. In a modification, the necessity of lifting all the trim rods when withdrawal of less than all is desired is eliminated by providing electromagnets at the location of the pistons when the rods are in their in-core position. It is then only necessary to energize selectively those electromagnets associated with the trim rods which are to remain in the core, before opening the valve 100. In an alternative embodiment, the trim rods are suspended from a movable block of magnetic material disposed in a housing and a magnetic structure comprising at least one electromagnet is located exteriorly of and movable along the housing. This mechanism accomplishes the same functions as its hydraulic counterpart in the first embodiment. The individual fine flux trim rods are of the same neutron absorbing configuration as the individual rod cluster control units, and may comprise Ag-In-Cd absorbers clad with stainless steel.