IMPLEMENTATION OF ASME CODE, SECTION XI, CODE CASE N-770, ON ALTERNATIVE EXAMINATION REQUIREMENTS FOR CLASS 1 BUTT WELDS FABRICATED WITH ALLOY 82/182

摘要

In May 2010, the U.S. Nuclear Regulatory Commission (NRC) issued a proposed notice of rulemaking (75 FR 24324) [1] that includes a new section to its rules to require licensees to implement ASME Code Case N-770, "Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated with UNS N06082 or UNS W86182 Weld Filler Material With or Without the Application of Listed Mitigation Activities, Section XI, Division 1," [2] with 15 conditions. Code Case N-770 contains baseline and inservice inspection (ISI) requirements for unmitigated Alloy 82/182 butt welds and preservice and ISI requirements for mitigated Alloy 82/182 butt welds. The NRC stated that application of ASME Code Case N-770 is necessary because the inspections currently required by the ASME Code, Section XI, were not written to address stress corrosion cracking of Alloy 82/182 butt welds, and the safety consequences of inadequate inspections can be significant. The NRC expects to issue the final rule incorporating this Code Case into its regulations toward the middle of 2011. This paper discusses the new examination requirements, the conditions that NRC proposed to impose, and potential areas of concern with implementation of the new Code Case.