INSPECTION AND REPAIR TECHNIQUES AND STRATEGIES FOR ALLOY 600 PWSCC IN REACTOR VESSEL HEAD CRD NOZZLES AND WELDS

摘要

As a result of the Alloy 600 PWSCC CRD nozzle leaks discovered in the fall of 2000 and spring of 2001 in several US plants, the NRC has recommended a more pro-active effort by U.S. utilities to inspect similarly susceptible nozzles in all US plants. The primary safety concern is circumferential cracks that can permit the nozzle to separate from the head at high velocity and produce a large-break leak in the reactor vessel. A secondary concern is head leakage from any through-wall cracks in the nozzle or Jgroove weld area. Although the fundamental weld and seal design are similar for all US PWR plants, the various surrounding geometry and repair probability considerations require multiple inspection and repair alternatives. Geometry issues include the head insulation design that influences the ability to perform visual examinations from above the head, and the presence or absence of thermal sleeves and funnels governing the type of NDE probes than can be used. Repair probability considerations primarily include the likelihood for repair of a small or large number of nozzles and the length of time the repair must last before a head replacement. This paper discusses the various inspection and repair alternatives offered by one service vendor and discusses a decision process for planning the inspection and repair effort.