Patent Number: 061378534
Section: summary

FIELD OF THE INVENTION This invention relates generally to nondestructive examination of material, such as metal, for voids, flaws, cracks and other defects that can be detrimental to the integrity of the material. Specifically, the invention relates to the ultrasonic inspection of nozzles located high on the periphery of penetrations in the bottom head of the reactor pressure vessel of a boiling water reactor. BACKGROUND OF THE INVENTION The structure of a water-cooled and water-moderated nuclear reactor of the boiling water type is well known. (See, e.g., U.S. Pat. Nos. 4,548,785 and 5,118,464 to Richardson et al.) As depicted in FIG. 1, a boiling water reactor 2 includes a reactor pressure vessel 4 containing a nuclear reactor core (not shown) submerged in a coolant-moderator such as light water. The core, which is surrounded by an annular shroud 6, includes a plurality of replaceable fuel assemblies (not shown) arranged in spaced relation between an upper core support grid 8 and a lower core support plate 10. A plurality of control rod drive housings (not shown) penetrate the bottom head of the reactor pressure vessel 4 and house control rod drives by which a plurality of control rods (not shown) are selectively insertable among the fuel assemblies for controlling the core reactivity. Each control rod and the four fuel assemblies comprise a fuel cell of the core. The four fuel assemblies are laterally supported at their upper ends in an opening in the upper core support grid 8 formed by intersecting and interlocking beams. At their lower ends the four fuel assemblies are vertically supported on the fuel assembly support member fitted to the top end of the control rod guide tube, lateral support being provided by passage of the guide tube through an aperture or hole in the lower core support plate 10. In addition, a plurality of nozzles (only one nozzle 20 of which is shown in FIG. 1) penetrate the bottom head of the reactor pressure vessel. These nozzles are of two types: differential pressure nozzles which provide means for monitoring the differential pressure across the fuel core and liquid poison nozzles which provide means for supplying liquid neutron absorber to the fuel core in the event of a transient overpower event with inability to scram. Each nozzle 20 is supported by an outer tube 12 and extends to the elevation of the upper core support grid 8. The outer tubes 12 are affixed to bottom-head penetrations. Penetration of the differential pressure and liquid poison nozzles through the bottom head of the reactor pressure vessel 4 is accomplished using stub tubes 14 (see FIG. 7A). Each stub tube, suitably shaped at its bottom end to fit the curvature of the bottom head at its particular location, is secured in a corresponding aperture or hole in the bottom head by a circumferential weld 16. The outer tube 12 is welded to the top end of the stub tube 14 by a circumferential weld 18 after the outer tube 12 is properly positioned vertically. As is evident from the foregoing, the stub tubes 14 become a part of the pressure vessel boundary and any defect (e.g., cracks) therein can jeopardize the integrity of the pressure system. Under certain conditions, the stub tubes are found to undergo stress corrosion cracking in the heat-affected zone adjacent to the upper weld 18 joining the outer tube and the stub tube. This stress corrosion cracking may result in water leakage from the vessel through the narrow gap between the outer tube 12 and the stub tube 14, an undesirable event necessitating repair. For the foregoing reasons, the welds which attach the outer tube to the reactor pressure vessel are required to be examined periodically to determine their structural integrity. However, the differential pressure and liquid poison nozzles are inherently difficult to access. Therefore, means for remotely and automatically inspecting the welds by which the differential pressure and liquid poison nozzles are attached to the reactor pressure vessel are needed. SUMMARY OF THE INVENTION The present invention is a method and an apparatus for ultrasonically inspecting the welds which attach the differential pressure and liquid poison nozzles to the bottom head of the reactor pressure vessel. The scanning apparatus is lowered from the refueling bridge into position around the outer tube and seated on the upper taper of the outer tube. Since there is no access to the top of outer tube when the nozzles is installed, the scanner is installed from the side. The apparatus has a cutaway section which allows installation from the side. The apparatus incorporates a stationary frame and vertical and circumferential positioning mechanisms which are operated remotely to scan transducer means over the circumferential welds and heat-affected zones thereof. The circumferential positioning means includes a rotating frame which sits in a V-guide on the stationary frame, on which it can rotate 360.degree.. The rotating frame has a transducer carriage mounted thereof. The transducer carriage, which carries the transducer means, is vertically displaceable relative to the rotating frame. The vertical motion path of the transducer carriage is maintained by linear slide assemblies located on opposing sides of the stationary frame. The vertical and angular motion motors can be controlled together to provide the desired path for the transducer means around the stub tube. The invention further comprises means for following the contour of the inclined surfaces of the reactor pressure vessel bottom head. These contour following means ensure that the transducer means follow the contour of the weld, in position and angle. The vessel contour follower is a free-hinged frame with weighted rollers at the end which lay flat on the vessel surface regardless of the angle of inclination thereof. To achieve this function, the vessel contour follower is pivotably mounted on an up-and-down hinge, which is in turn mounted on a side-to-side hinge at the bottom of the transducer carriage.