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Effects of Weld Repairs on Calculated Failure Probabilities for Reactor Pressure Vessels and Piping

[+] Author Affiliations
F. A. Simonen, G. J. Schuster, S. R. Doctor

Pacific Northwest National Laboratory, Richland, WA

Paper No. PVP2003-2034, pp. 121-127; 7 pages
doi:10.1115/PVP2003-2034
From:
  • ASME 2003 Pressure Vessels and Piping Conference
  • Flaw Evaluation, Service Experience, and Reliability
  • Cleveland, Ohio, USA, July 20–24, 2003
  • Conference Sponsors: Pressure Vessels and Piping Division
  • ISBN: 0-7918-4154-5
  • Copyright © 2003 by ASME

abstract

Section III of the ASME Boiler and Pressure Vessel Code for nuclear power plant components requires radiographic examinations (RT) of welds and repairs for RT indications that exceed code acceptable sizes. This paper describes research that generated data on welding flaws, which indicated that the largest flaws occur in repaired welds. Evidence suggests that repairs are often for small and benign RT indications at locations buried within the vessel or pipe wall. Probabilistic fracture mechanics calculations are described in this paper to predict the increases in vessel and piping failure probabilities caused by the repair-induced flaws. Calculations address failures of embrittled vessel welds for pressurized thermal shock (PTS) transients. In this case small flaws, which are relatively common, can cause brittle fracture such that the rarely encountered repair flaws of large sizes gave only modestly increased failure probabilities. The piping calculations simulated fatigue crack growth of fabrication flaws. These calculations showed that only relatively large fabrication flaws can fail piping because of the ductile nature of the piping material. The large repair flaws significantly increased the failure probabilities. The paper recommends the use of more discriminating UT examinations in place of RT examinations along with repair criteria based on a fitness-for-purpose approach that minimizes the number of unjustified repairs.

Copyright © 2003 by ASME

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