NACE 41013

Abstract

Introduction Recent operating experience from nuclear power plants, particularly those located in the United States, many of which are between 30 and 40 years old, indicates that degradation of buried piping is occurring in at least some plants and represents an issue requiring the attention of the nuclear industry. Degradation in a variety of forms has been observed on both the external and internal surfaces of the pipe. In most cases to date, the degradation was not detected until a leak was identified. Some of these leaks have resulted in the release of radioisotopes (normally tritium) and materials that may be harmful to the environment (environmentally sensitive material). These events: <br>(a) have not resulted in the loss of the intended safety function of any component or system; <br>(b) have not resulted in off-site exposure to radiation in excess of regulatory limits; <br>(c) constitute only about 10% of the leaks of tritium containing water into the environment;1 and <br>(d) have attracted significant public attention.¹ <br>Buried piping in nuclear power plants includes safety-related piping (required to attain or maintain plant shutdown), generally ASME International (ASME)(1) Class 3, and nonsafety-related piping. Although the number of piping systems that are wholly or partially buried varies from plant to plant, more than 30 different systems have been identified as being at least partially buried at several plants. <br>Typical systems that are at least partially buried include service water (generally raw water used to cool safety and nonsafety-related components), fuel oil, lube oil, compressed air, circulating water, and fire protection. Additionally, other systems that carry gases, such as oxygen, hydrogen, and nitrogen, may be buried. <br>The following is a list of events from the U.S. Nuclear Regulatory Commission (NRC)(2) NUREG-1801, Generic Aging Lessons Learned (GALL) Report Revision ², Aging Management Program (AMP) XI.M41,2 that provides some indication of the range of recent occurrences. <br>(a) In February 2005, a leak was detected in a 4 in (102 mm) condensate storage supply line. The cause of the leak was microbiologically influenced corrosion (MIC) or under-deposit corrosion. The leak was repaired in accordance with the ASME Boiler & Pressure Vessel Code (BPVC), Section XI, "Inservice Inspection of Nuclear Power Plant Components."³ <br>(b) In September 2005, a service water leak was discovered in a buried service water header. The header had been in service for 38 years. The cause of the leak was either failure of the external coating or damage caused by improper backfill. The service water header was relocated above ground. <br>(c) In February 2009, a leak was discovered on the return line to a condensate storage tank. The cause of the leak was coating degradation, probably as a result of the installation specification not containing restrictions on the type of backfill, allowing rocks in the backfill. The leaking piping was also located close to the water table. <br>(d) In June 2009, an active leak was discovered in buried piping associated with a condensate storage tank. The leak was discovered because elevated levels of tritium were detected. The cause of the through-wall leaks was determined to be the degradation of the protective moisture barrier wrap that allowed moisture to come in contact with the piping, resulting in external corrosion. <br>A similar list of events from international nuclear power plants is not available. Based on a lack of international information, this report has been written from the perspective of power plants located in the United States but may be viewed in an international context. <br>⁽¹⁾ ASME International (ASME), Three Park Ave., New York, NY 10016-5990. <br>⁽²⁾ U.S. Nuclear Regulatory Commission (NRC), Washington, DC 20555-0001. <br>⁽³⁾ ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.