Patent Number: 048329029
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION U.S. Pat. No. 4,511,531 granted Apr. 16, 1985 to Kenneth J. Swidwa, Leonard P. Hornak and Edward F. Kowalski for Transfer of Nuclear Reactor Component Assemblies and assigned to Westinghouse Electric Corporation (herein Swidwa). Swidwa is incorporated herein by reference. BACKGROUND OF THE INVENTION This invention relates to the art of nuclear reactor power plants. It has particular relationship to the refueling of the reactors of such plants. In carrying out the refueling, the reactor to be refueled is at the base of a pit in a containment filled with water to a depth of 20 or 30 feet. During refueling, component assemblies of the reactor or from refueling racks are engaged by grippers or grapples of a mast assembly, raised, transported from their position of origin and lowered into the position where they are to be deposited. The component assemblies are highly radioactive and are engaged, raised, transported and lowered under a substantial depth of water. To carry out this operation, there is provided apparatus including a bridge moveable along a track on the containment. A trolley is moveable on a track on the bridge. The trolley carries a mast assembly having a rotatable supporting mast from which the component-assembly engaging-raising-and-lowering means is suspended. This means is sometimes herein referred to as "component-assembly handling mechanism" or "assembly-handling mechanism" or simply "mechanism". With the bridge and trolley at selectably different positions along their tracks, the mast assembly is suspended with the assembly-handling mechanisms at selectably different positions of the area of the pit or of the reactor within the pit. It is indispensable to successful refueling that the mechanism on the mast assembly be positioned during each operation to engage and raise the exact assembly which is selected for transport. Since the reactor is under water, the positioning of the mechanisms must be carried out with the necessary precision with at best a heavily clouded view of the nuclear core. It is also necessary that the depth of the mechanisms in the water be known. The tracks on the bridge and containment define a coordinate system whose coordinates serve to determine the positions of the mechanisms on the mast assembly. The tracks in Swidwa, and predominantly in prior art apparatus, are linear and at right angles to each other defining a Cartesian coordinate system. With the tracks of other configurations, other coordinate systems may define the positions of these mechanisms. For example, if the bridge moves in a circular track on the containment and the trolley moves on a radial track, the coordinate system would be a polar coordinate system. The coordinates of the positions of the bridge and trolley along the tracks are magnitudes which indicate the positions of the engaging-raising-and-lowering means over the pit. In addition, there are provided indications of the vertical positions of these mechanisms. The mechanisms are raised or lowered by a hoist which is typically a winch. In accordance with the teachings of the prior art the positions of the mast assembly along the tracks are derived from selsyn-type indicators. The readings of the indicators for different positions of the mechanisms of the mast assembly are compared with marks along the containment walls. The marks are coordinated with the indications of the selsyns by a calibration process. Impairment of the calibration was experienced and when this occurred, recalibration was a difficult and time consuming experience. The height of the mechanisms on the mast assembly was determined from marks on a tape along the assembly. Recalibration in this case was also difficult and time consuming. In accordance with the teachings of the prior art, index marks were also provided on the trolley and bridge to locate the component assemblies to be moved in the reactor core. The index marks served to position the mast assembly over the theoretical location of a component assembly. If the assembly is out of position, difficult visual determinations were required to make the necessary adjustments. This operation was time consuming and required the utmost caution to preclude damage to the component assemblies. It is the object of this invention to overcome the difficulties and drawbacks of the prior art. An object of this invention is to provide refueling apparatus for a reactor having facilities, whose calibration shall be maintained throughout a refueling operation, for reliably and precisely identifying the position of the mast assembly with reference to the reactor core. A further object of this invention is to provide refueling apparatus including position-visual facilities for aligning the mast assembly with the component assembly to be transported for identifying and recording the location of component assemblies to be transferred and, if desirable, to provide a permanent record of the refueling and core-mapping operation. SUMMARY OF THE INVENTION In apparatus in accordance with this invention, the trolley is provided with a pulse generator which produces pulses as the trolley moves along the track on the bridge. The pulses are entered as counts in a computer. Each pulse enters in the computer a positive count for each increment of movement of the trolley in a forward direction away from a reference point on the track and a negative count for each increment of the movement of the trolley in a backward direction towards the reference point. There is thus a one-to-one relationship between the position of the trolley on the track and the count of each train of pulses measuring the increments from the reference point to a position. Likewise the bridge is provided with a pulse generator which produces pulses resulting in a positive count in the computer for each increment of the forward movement of the bridge on the track on the containment walls from a reference point and a negative count for each increment of backward movement. The drive for the hoist also has a pulse generator which produces pulses resulting in a positive count in the computer for each increment of forward, typically downward, movement of the component-assembly-handling mechanism from a reference point and a negative count for each increment of backward, typically upward, movement of this assembly-handling mechanism from the reference point. For each set of positions of the trolley, bridge and handling mechanism, the computer contains digital entries which identify these positions. The apparatus may be calibrated so that each set of counts defines the position of the trolley, bridge and component-assembly handling mechanism relative to a known predetermined component assembly in the core. To test the integrity of the calibration and reset the coordinate system if the calibration is impaired, a limit switch is actuated by each, the trolley, bridge and handling mechanism at a predetermined position along its path. The switch may be carried by the vehicle and may be actuated by a cam along its path or it may be provided at the position along the path and actuated by a cam on the vehicle. The actuation by the trolley and bridge takes place at a convenient position along their tracks. In the case of the handling assembly, the limit switch is typically actuated at the maximum height of this assembly. The actuation of each limit switch is entered in the computer together with the count at the position where each limit switch is actuated. The memory of the computer carries intelligence of the counts that should be entered for the actuation of the limit switches if the calibration is maintained. The memory is programmed to recalibrate the coordinate system if there is a disparity between a count entered in the memory and a count entered on actuation of a limit switch.