Characterization of Flaws in U.S. Reactor Pressure Vessels: Density and Distribution in Flaw Indication in PVRUF (NUREG/CR-6471, Volume 1, PNNL-11143)

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Publication Information

Manuscript Completed: September 1998
Date Published: November 1998

Prepared by:
G. J. Schuster, S. R. Doctor, P. G. Heasler

Pacific Northwest National Laboratory
Richland, WA 99352

D.A. Jackson, NRC Project Managerr

Prepared for:
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code L1099 and W6275

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Abstract

Characterization of Flaws in U.S. Reactor Pressure Vessels is a multi-volume report. Volume 1, this document, gives the results of a non-destructive examination conducted at the Oak Ridge National Laboratory's Pressure Vessel Research User Facility (PVRUF) on a vessel fabricated for a canceled nuclear power plant. Volume 2, in preparation, will document the results of Pacific Northwest National Laboratory's (PNNL's) destructive validation of the flaw rates in the PVRUF evaluation.

A nondestructive evaluation was made of the fabrication flaws in an unused U.S. Nuclear reactor pressure vessel. The examination was conducted at the Oak Ridge National Laboratory's Pressure Vessel Research User Facility (PVRUF) on a vessel fabricated for a canceled nuclear power plant of a pressurized water reactor (PWR) design. The inspections were made using the fieldable, real-time Synthetic Aperture Focusing Technique for Ultrasonic Testing (SAFT-UT) system developed by PNNL under sponsorship of the U.S. Nuclear Regulatory Commission. Twenty linear meters of weldment were inspected by SAFT-UT, including the entire circumferential beltline weld of the vessel. Ten different inspection modes were used, including five different modes specifically selected for the inspection of the inner 25 mm (1.0 in.) of the vessel wall.

There were 2500 detectable indications in the SAFT-UT inspections of the PVRUF vessel. The largest number of these, 982, were found at the clad-to-base metal interface, but 978 of these were less than 2 mm (0.08 in.) in size. In the near surface zone, the weld metal contained 98 detectable planar indications. The density of indications was four times higher in the weldment than in the base metal. The distribution of the empirical data provided enough information to apply a parametric model of the cumulative flaw rate to six different subsets of the data, and to obtain reasonable confidence bounds on the results. Recommendations are given for validating the indication rates by selective destructive analysis, to provide the necessary high quality flaw statistics for use in fracture mechanics calculations, such as those used in pressurized thermal shock (PTS) analysis.