Risk-Informed Assessment of Degraded Buried Piping Systems in Nuclear Power Plants (NUREG/CR-6876, BNL-NUREG-74000-2005)
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Manuscript Completed: April 2005
Date Published: June 2005
J.I. Braverman, G. DeGrassi, G. Martinez-Guridi
R.J. Morante, C.H. Hofmayer
Brookhaven National Laboratory
Upton, New York 11973-5000
T.Y. Chang, NRC Project Manager
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report describes the research performed to assess the effects of age-related degradation of buried piping at nuclear power plants (NPPs). The evaluation of buried piping was conducted in order to develop analytical methods and degradation acceptance criteria (DAC) that can be used to assess the condition of degraded buried piping. The research focused on a risk-informed approach to evaluate the most common aging effects in buried piping consisting of general wall thinning and localized loss of material/pitting. The effects of degradation over time were included in the methodology developed to assess buried piping.
To achieve the goals of this research effort, fragility modeling procedures for degraded buried piping have been developed and the effect of degradation on fragility and plant risk has been determined. The measure used for plant risk was based on the change in core damage frequency (ΔCDF) due to internal events during full power operation. The analytical approach provides the technical basis for evaluating the structural adequacy of degraded buried piping and for developing guidelines for assessing the effect of degraded conditions on plant risk.
The results of this research demonstrate that, for a buried pipe meeting the conditions of the DAC, a pipe thickness loss less than approximately 45% of the original nominal pipe wall thickness, identified at the time of inspection, is not expected to have an immediate significant effect on plant risk. The effects of degradation over time were considered in developing the DAC in a manner that provides the number of years required for the buried pipe to reach a degradation level that would potentially have a significant effect on plant risk. The types of buried piping systems, configurations, materials, applicable pipe loads (e.g., pressure, surcharge, live load, etc.) and other conditions that must be satisfied to use the DAC have been developed and presented in this report. The results obtained are based on the service conditions that buried piping is designed for (e.g., pressure induced stresses less than ¼ of the minimum ultimate strength of the material and relatively low temperatures) and recognizing that seismic induced stresses in buried piping are self-limiting since deformations or strains are limited by seismic motion of the surrounding media. In addition, the DAC were developed from probabilistic risk assessments which accounted for the contribution to risk of the postulated degradation of buried piping systems at NPPs. It should be noted that even if a degraded buried pipe meets the DAC, it is expected that the licensee will evaluate the conditions that led to the degradation and may need to repair the degraded pipe based on the evaluation findings, the level of degradation, and the plant’s current licensing basis.
The methodology and degradation acceptance criteria (DAC) developed in this report are intended to provide guidance to the NRC staff for making an assessment in a timely manner whether degraded conditions, identified at a plant site, potentially have an immediate significant effect on plant risk. This knowledge is important in order to provide input that can help determine whether immediate repairs are warranted, or whether the appropriate investigation, inspection, aging management, or other actions can be determined in the normal course of evaluating the condition. The methodology and DAC cannot be used by the industry as a design tool to justify existing degraded conditions; licensees are still required to meet their commitments regarding their current licensing basis.