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Risk-Informed Extension of the Reactor Vessel Nozzle Inservice Inspection Interval

[+] Author Affiliations
Stephen M. Parker, Nathan A. Palm, Paul R. Stevenson, Bruce A. Bishop

Westinghouse Electric Company LLC, Cranberry Township, PA

Paper No. PVP2011-57971, pp. 199-210; 12 pages
doi:10.1115/PVP2011-57971
From:
  • ASME 2011 Pressure Vessels and Piping Conference
  • Volume 5: High-Pressure Technology; Nondestructive Evaluation; Nuclear Engineering
  • Baltimore, Maryland, USA, July 17–21, 2011
  • Conference Sponsors: Pressure Vessels and Piping Division
  • ISBN: 978-0-7918-4455-7
  • Copyright © 2011 by ASME

abstract

Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code specifies a 10-year interval between reactor vessel (RV) nozzle weld inspections. The industry has expended significant cost and man-rem exposure performing inspections that have found no service-induced flaws in ASME Section XI Category B-F or B-J RV nozzle welds that do not contain Alloy 82/182. Furthermore, many plants have implemented a 20-year inspection interval for the RV shell-to-shell and shell-to-nozzle welds in accordance with WCAP-16168-NP-A, Revision 2. For many of these plants, continuing to inspect the RV nozzle welds on a 10-year interval presents a significant hardship without a corresponding increase in safety from performing the inspections. This paper will provide a summary of the technical basis and methodology developed by Westinghouse for extending the Section XI inspection interval from the current 10 years to 20 years for Category B-F and B-J RV nozzle-to-safe-end and safe-end-to-pipe welds that are not fabricated with Alloy 82/182 materials. Bounding change-in-failure-frequency values have been calculated for use in plant-specific implementation of the extended inspection interval. Plant-specific pilot studies have been performed and the results show that the change in risk associated with extending the interval from 10 to 20 years after the initial 10-year inservice inspection (ISI) satisfies the guidelines specified in Regulatory Guide 1.174 for an acceptably low change in risk for core damage frequency (CDF) and large early release frequency (LERF). Further, the pilot-plant results show that the effect of the extended inspection interval on the plant’s risk-informed inservice inspection (RI-ISI) program for piping, if any, would also be acceptable.

Copyright © 2011 by ASME

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