44 PWR Reactor Vessel Alloy 600 Issues
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Jeffrey Gorman, Steve Hunt, Pete Riccardella authored Chapter 44 for the previous edition that has been updated by Pete Riccardella and Glenn White for this edition. They have considerable expertise and experience in handling PWR Reactor Vessel Alloy 600 and related issues confronted by the industry. Considering the extreme importance of this topic the authors have covered concerns pertinent to several ramifications of the problem.
Primary water stress corrosion cracking (PWSCC) of Alloy 600 nickel-chromium-iron base metal and related Alloy 82/132/182 weld metal has become an increasing concern to commercial pressurized water nuclear power plants. Cracks and leaks have been discovered in Alloy 600/82/182 materials at a number of locations in PWR reactor vessels and other reactor coolant loop components worldwide. These locations include control rod drive mechanism (CRDM) nozzles, bottom head instrument nozzles, reactor vessel nozzle butt welds, and pressurizer nozzle welds. The consequences of PWSCC have been significant including numerous leaks, many cracked nozzles and welds, expensive inspections, more than 60 reactor vessel heads replaced, and extensive repair and mitigation activities on reactor coolant loop butt welds. A number of plants experienced months-long outage extensions to repair leaks, and one plant was down for over two years as a result of regulatory action following the detection of extensive corrosion to the vessel head resulting from a leaking CRDM nozzle. This chapter addresses Alloy 600/82/182 material locations in reactor vessels, operating experience, causes of PWSCC, inspection methods and findings, safety considerations, degradation predictions, repair methods, remedial measures, and strategic planning to address PWSCC at the lowest possible net present value cost. Recent industry and ASME Code activities to address these concerns are also discussed.