An update of the alloy 600 mitigation/repair technologies

Many utilities are currently implementing their Alloy 600 Management plans. Mitigation techniques have been developed to assure the long term operability of the components containing Alloy 600/82/182, especially those with high susceptibility that will develop cracks from Primary Water Stress Corrosion Cracking (PWSCC) during the remaining life of the plants. Stress Corrosion Cracking (SCC) requires the confluence of a susceptible material, a chemical environment conducive to cracking, and tensile stresses of significant magnitude on the material in contact with the primary water. Mitigation is intended to extend the life of components by altering one or more of the conditions necessary for SCC to occur in PWR's. Based on the susceptibility ranking of the Alloy 600 locations in the Westinghouse designed PWR's, the priority has been given to the mitigation of the pressurizer nozzle locations which is about being complete in the US and has largely started in Europe. The next most susceptible locations in Westinghouse designed PWR's are the Reactor Vessel hot leg nozzle to safe-end dissimilar metal welds. A few plants have implemented repairs or mitigation techniques on the RV nozzle locations and many utilities are currently planning these activities. The development of mitigation, repair or replacement processes by the industry has been a major effort over the last years. This paper provides an update of the main techniques that are currently available or under evaluation, namely: the Structural Weld Overlay (SWOL) process, the Mechanical Stress Improvement Process (MSIP), the Underwater Laser Beam Welding (ULBW) process and the Safe-End Replacement (SER) process. For each mitigation/repair/replacement technique, the paper addresses the recent developments in terms of process and qualification, tooling and equipment, evolutions of the ASME Code and applications. (authors)