Core Shroud Inspection Coverage and Inspection Time Improvements through Phased Array Technique and Manipulator Improvements

In December 2006, the Boiling Water Reactor Vessel and Internals Project (BWRVIP) issued a letter mandating two-sided coverage for the volumetric examination of BWR core shroud welds. In this letter, they noted that: “Ultrasonic flaw detection and sizing performance is degraded by stainless steel or nickel allow weld metal. The base metal examination volumes on both sides of the weld should be examined using sound beams that have not passed through the weld metal. Whenever possible, stainless steel or nickel-alloy weld joints should be examined by scanning on both of the two components that are jointed by the weld. When one side of the weld is examined using only beams that have passed through the weld metal, that side of the weld should be considered to have received a “best effort” examination.” This long awaited letter and the associated new requirements coupled with continued customer interest in improved coverage and reduced outages have supported AREVA’s comprehensive program to modify their core shroud inspection tools and techniques. The basic approach was to address all welds from the annulus between the vessel and the shroud so as to minimize interference with refueling, to examine horizontal, vertical, and ring segment welds with maximum coverage, and to complete the task in the shortest possible time. This paper will discuss: the UT techniques adopted to achieve twosided phased array coverage of the welds; the tool modifications particularly regarding the reduction in tool height to maximize coverage; software features to enhance the tool ergonomics and minimize the possibility of operator errors; and auxiliary tool features to facilitate completion of the inspection in the shortest possible time and with minimal dependence on site resources including the polar crane. The tool has been deployed to two sites following the improvements discussed herein and those field performances are also discussed.