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Chapter 21
PWR Reactor Vessel Alloy 600 Issues

Excerpt

Primary water stress corrosion cracking (PWSCC) of alloy 600 nickel-chromium-iron base metal and related alloys 82 and 182 weld metal has become a principal materials degradation concern for commercial pressurized water reactor (PWR) plants. Cracks and leaks have been discovered in alloys 600/82/182 materials at numerous PWR plant primary coolant system locations, including at several locations in the reactor vessels. The reactor vessel locations include top head control rod drive mechanism (CRDM) nozzles, top head thermocouple nozzles, bottom head instrument nozzles, and reactor vessel outlet nozzle butt welds. The consequences of this PWSCC have been significant worldwide with about 73 leaks through July 2013 (∼57 CRDM nozzles, 13 reactor vessel closure head thermocouple nozzles, 2 reactor pressure vessel bottom-mounted instrument nozzles, and 1 piping butt weld), many cracked nozzles and welds, expensive inspections, more than 100 heads replaced, several plants with several-month outage extensions to repair leaks, and a plant shutdown for more than 2 years due to extensive corrosion of the vessel head resulting from leakage from a PWSCC crack in a CRDM nozzle. This chapter addresses alloys 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.

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