Patent Number: 046860789
Section: summary

BACKGROUND OF THE INVENTION Nuclear reactor ultrasonic inspection devices have long been available for scanning welds when a reactor is shut down for refuelling and its open reactor vessel is submerged in a water pool. Such devices typically include a spider assembly having legs spanning the reactor vessel flange at its open upper end, a telescopic vertical column mounted in the hub of the spider coaxial with the vessel, and a telescopic horizontal boom at the lower end of the column carrying the scanner. U.S. Pat. No. 3,780,571 is typical of the disclosures of such devices. It is characteristic of prior art inspection apparatus that they involve complicated telescopic and rotational drives often with pulley and cable systems, all of relatively high cost and substantial weight. It is the principal purpose of the present invention to provide a method and apparatus for displacing a reactor weld scanner into and out of position by a system which is inexpensive and which can be assembled at the site with lightweight individual parts transportable by one man. SUMMARY OF THE INVENTION The invention provides a method of displacing a weld scanner in and out of a horizontal reactor vessel nozzle, wherein vertical guide studs extend upwardly from an upper circular vessel flange with the studs and vessel submerged in a water pool. The scanner is mounted on a horizontal positively driven telescopic boom to which a vertical column is attached which is slideable between stop limits within a hub of a spider having legs adapted to span the flange with clamps on the outer ends of at least two legs adapted to slide on and grip two of the studs. Any or all of the boom and column and spider legs defines a buoyancy chamber into which air and water may be introduced. The method comprises the steps of first introducing air into the buoyancy chamber. The scanner-boom-colummn-spider assembly is then floated in the pool with the upper end of the column extending above and the scanner and boom extending below the water surface. Some water is then introduced into the chamber to cause the scanner-boom-column to sink relative to the spider until the column is stopped at its lower limit in the hub. Positioning rod means are then affixed coaxially to the upper end of the column. More water is then introduced into the buoyancy chamber to sink the scanner-boom-column-spider assembly at substantially neutral buoyancy with the rod means projecting at its upper end above the pool surface. The rod means are then manipulated and then more water is introduced into the buoyancy chamber so that the leg clamps are lowered about the studs and the spider legs descend onto the vessel flange. The clamps are then closed to fix the spider relative to the vessel. The rod means are next moved vertically and rotated to bring the scanner to a level coaxial with the nozzle. The telescopic boom is then positively extended to enable inspection by the scanner in the nozzle. After inspection the boom is positively retracted and the spider clamps are opened. Air is then introduced into the chamber until the entire assembly is buoyant. The rod means are manipulated as the assembly lifts upwardly off the studs and to the surface and then the rod means are detached. In a preferred form of the method the spider has three legs, two of which have clamps slideable on two studs, each of the three legs having a buoyancy chamber. Each of the boom and column may also define a buoyancy chamber. The column may be rotatable on the hub and adapted to be braked with respect to the hub. After retraction the boom may be rotated by the rod means to be under one of the spider legs. After the boom is retracted and before the clamps are opened air may be introduced into the spider legs chambers to render them positively buoyant while the other chambers are still negatively buoyant. The water introduced into the buoyancy chambers may be maintained separate from the pool water. The boom may be provided with two fore and aft buoyancy chambers. In combination with a weld scanner mounted on a positively driven telescopic boom to which a perpendicular column is attached, the invention also provides apparatus for buoyantly displacing the scanner in and out of a horizontal reactor vessel nozzle wherein vertical guide studs extend upwardly from an upper circular vessel flange with the studs and vessel submerged in a water pool. The apparatus of the invention comprises a spider having legs adapted to span the flange. A central hub is provided in the spider in which the column is slideable. Stop means are included for limiting the slideable movement of the column in the hub. Clamps operable by remote control are included on the outer ends of at least two legs adapted to slide and grip two of the studs. At least one of the boom and column and spider legs defines a buoyancy chamber. Means operable by remote control are included for introducing air into and releasing it from the chambers. Means operable by remote control are also included for introducing water into and draining it from the chambers. Positioning rod means are provided coaxially attachable to the column remote from the boom for manipulating the scanner-boom-column-spider assembly when submerged. The weight of this apparatus is such that it is buoyant in water when air is introduced into the chambers. In a preferred form of the apparatus two guide studs extend from the flange approximately 120 degrees apart and the spider has three substantially equally spaced legs with clamps included on two of the legs. Each of the boom and column and spider legs may define a buoyancy chamber and the boom may define two fore and aft chambers. The water supply for all of the chambers may be separate from the pool water. Braking means operable by remote control may be provided for controlling axial and rotational displacement of the column in the spider hub.