In May 2007, EPRI conducted a Nuclear Power Plants Piping Integrity Workshop in which the integrity of buried pipes was identified as one of the top priorities. In October 2007, EPRI conducted a follow-up Workshop on Buried Pipe attended by more than 40 representatives from utilities and EPRI. At the conclusion of the October 2007 meeting, the utility attendees unanimously recommended that EPRI sponsor the development of a recommendations document for buried pipe to help plant engineers prevent and mitigate degradation and leaks in buried pipes. This report has been prepared to address that need.
Degradation of buried piping is a significant issue facing nuclear power plant owners. Unlike aboveground piping systems, buried pipes corrode and foul from the fluid side (inside diameter) and corrode or experience mechanical damage from the soil side (outside diameter). This continuing degradation is challenging to assess since the pipes are difficult to reach for inspection. When buried pipes leak, the source of leakage is difficult to locate, access, and repair in a timely manner. As a result, several plants have experienced costly leaks and repairs of their buried pipes.
The integrity of buried pipes has received increased interest from the Nuclear Regulatory Commission (NRC) via Inspection Procedure 62002 and license renewal reviews. The Institute of Nuclear Power Operations (INPO) has also included buried pipe integrity in several reviews. Finally, buried pipe integrity is of interest for new plants, to ensure that new designs build upon lessons learned from operation of current plants.
Although the mechanisms of corrosion are generally slow, they are cumulative. As plants age, buried systems experience more frequent leaks. As buried pipe coatings approach their end of life, they tend to disbond or become brittle, leaving more areas of the pipe directly exposed to the soil. Cathodic protection systems, where installed, also require more frequent checks and maintenance to ensure their continued adequacy.
To provide a set of recommendations for nuclear power plants to use in implementing an effective program to detect and mitigate life-limiting degradation that may occur in buried piping systems.
Experience from the power industry as well as the experience and standards from other industries that have large quantities of buried pipe (such as waterworks, oil and gas transmission pipelines, and process plants) and utilities engineers’ reviews were used to develop the recommendations presented in this report.
This document provides guidance and recommendations for a programmatic approach to help plant owners prioritize inspections of buried pipes, evaluate the inspection results, make run-or-repair decisions, select a repair technique, where required, and take preventive measures to reduce the likelihood and consequence of failures. The activities are organized in six elements:
- Develop a corporate program including training, implementing procedures, documentation, and performance indicators.
- Prioritize buried pipe systems and locations to be inspected based on risk of failure (likelihood and consequence of failure).
- Perform direct inspections to quantify the degree of degradation and damage.
- Evaluate the fitness-for-service of degraded buried pipes.
- Select the appropriate repair technique where required, including both non-welded and welded repairs.
- Take preventive actions to reduce the risk (likelihood and consequence) of future leaks or failures.
Two options for performing risk ranking of a given piping location (segment) can be used. The first option utilizes risk analysis, where the risk is equal to a quantified likelihood of the failure times the quantified consequences of the failure. The second option, detailed herein, places each piping location (segment) into a risk matrix based on a non-quantified likelihood of failure (for example, low, medium, and high) versus the non-quantified consequences of failure (for example, none, low, medium, and high). However, since both approaches require inspection of a prioritized sample of risk-ranked locations, it must be recognized that it will not be possible to prevent all leaks and failures of buried piping systems.
Buried pipes in nuclear power plants are susceptible to degradation that can cause leaks and failures. Small leaks can be difficult to locate and all types of leaks can be expensive to repair due to accessibility issues. Some leaks also require that the plant be shut down in order to repair them. A broad-based and comprehensive program needs to be implemented at nuclear plants to reduce the probability and consequences of failure to an acceptable level. This report should be viewed as a living document that is subject to periodic revision as further data, application experience, and technology become available.