Evaluating Flaw Detectability Under Limited-Coverage Conditions (NUREG/CR-7304, PNNL-34367)

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Publication Information

Manuscript Completed: June 2023
Published: October 2023

Prepared by:
J. Harrison
R. Jacob
M. Prowant
A. Holmes
C. Hutchinson
A. Diaz

Pacific Northwest National Laboratory
902 Battelle Blvd, Richland, WA 99354

Carol Nove, NRC Contracting Officer Representative (COR)

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

Assessing the structural integrity of welded piping components in the U.S. nuclear power fleet ensures safety to the public and environment. When conditions such as outside surface contours, physical obstructions from nearby structures or equipment, or different component metallurgical composition limit the ability to fully examine greater than 90% of a weldment's required examination volume, licensees must request regulatory relief from the Nuclear Regulatory Commission (NRC). Determining the impact on flaw detectability caused by an examination limitation can be complicated for both the licensee in evaluating the extent of examination coverage and for the NRC in determining the potential impact of the area unable to be examined.

To address the limited-coverage condition, the NRC Office of Nuclear Regulatory Research (RES) directed the Pacific Northwest National Laboratory (PNNL) to assess the ability of nondestructive examination (NDE) techniques to detect flaws when part of a weld is uninspectable due to limited-coverage conditions. The assessment included a review of submitted relief requests to better understand how incomplete weld coverage affects flaw detectability as well as the examination of select piping specimens with actual and simulated limiting conditions. The specimens contained flaws with varying lengths and through-wall depths and contained geometric conditions and flaw orientations known to complicate detectability. In addition, to support an understanding of limited coverage, selected empirical data were modeled and compared to simulation results.

The empirical and theoretical assessments in this report were conducted to aid development of a technical foundation that can be used to assist NRC Staff in the process of reviewing licensee requests for relief related to limited coverage.

This study identified that shallow flaws were the most susceptible to missed detection. The exact definition of "shallow" is difficult to specifically define, although in most cases these flaws were less than 30% through-wall depth. Additionally, a flaw may extend from an uninspectable region into an inspectable region and therefore be only partially insonified during a limitedcoverage exam. Definitively determining how far a flaw must extend into an inspectable region to be detected is not possible due to several variables, including the extent of the limiting condition, the propagating characteristics of the flaw (such as the length and orientation), and human factors (such as the complexity of the exam, examiner knowledge and experience, and time pressure).

This research identifies key considerations for limited-coverage UT examinations that may be implemented to increase the probability of detection. Phased-array techniques that use multiple beam angles and an imaging display of the results provide a significant analysis advantage over conventional techniques and equipment. When forward probe movement is restricted, removal of the weld crown is the most important action that can be taken as well as fully investigating any UT response that may appear outside the Code-required examination volume to discriminate between embedded responses and surface-connected flaws.