Methodology for Estimating Fabrication Flaw Density and Distribution – Reactor Pressure Vessel Welds (NUREG/CR-6989)

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Publication Information

Manuscript Completed: December 2006
Date Published: May 2009

Prepared by:
G.J. Schuster, M. Morra, S.R. Doctor

Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352

W.E. Norris, NRC Project Manager

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Codes Y6604/N6398

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Abstract

The Pacific Northwest National Laboratory has developed a generalized fabrication flaw distribution for the population of U.S. nuclear reactor pressure vessels in operating nuclear power plants. The U.S. Nuclear Regulatory Commission (NRC) will use the generalized flaw distribution to predict component-specific flaw rates. The estimates of fabrication flaws are intended for use in fracture mechanics structural integrity assessments. Structural integrity assessments, such as estimating the frequency of loss-of-coolant accidents, are performed by computer codes that require, as input, accurate estimates of flaw rates. Machine-made welds using the submerged arc method were destructively examined to measure and characterize fabrication flaws to provide empirical estimates of the density.

This report describes the fabrication flaw distribution and characterization in the submerged arc weld of four vessels. The work indicates that flaw density changed over the years of vessel construction. Furthermore, the results show that flaw distributions differ by manufacturer. Parametric analysis using an exponential fit is performed on the data.

This report describes the validation methods employed on the detected fabrication flaws. Nondestructive examination (NDE) measurements were applied to the clad surface of the vessel, large and small weld segments, and cubes of material. Ultrasonic, radiographic, and metallographic testing methods and results are described in the report. The implementation and application of high-resolution ultrasonic methods for imaging the fabrication flaws is also reported.