Abstract

Degradation of buried metallic piping from corrosion is a significant issue facing owners and operators of nuclear power plants. This paper discusses the results of a major cathodic protection (CP) system upgrade project for buried piping and underground storage tanks at a nuclear power plant in South Carolina. To provide for effective CP, the design engineer needed to take into account the requirement for higher current demand, the location of electrical grounding, reinforced concrete foundations, bare or poorly coated structures, uniform current distribution, depth to bedrock, anode bed configuration, structure connections, test station type and location, system balancing, close interval survey results, rectifier design, operation and maintenance. The database for the system is incorporated into a mapping visualization program, so the system engineer can effectively monitor and trend the CP system performance. Criteria for effective CP in grounded mixed-metal piping systems are also discussed.

Introduction

The Catawba Nuclear Station (CNS) is a Pressurized Water Reactor (PWR) generating station having a total capacity of 2,258 megawatts. CNS is located on a 391 acre peninsula reaching into Lake Wylie, near the town of York, South Carolina. Unit 1 began commercial operation in 1985, followed by Unit 2 in 1986. The U.S. Nuclear Regulatory Commission (NRC) issued a renewed license to Duke Energy for Units 1 and 2 in 2003, which expires in 2043. Therefore the plant has been in operation for over 30 years, and the design life of any supplemental CP should be for at least 28 years.

The original CP system that was installed at CNS during plant construction consisted of ten (10) rectifiers, 43 semi deep anode beds, 22 distributed anodes and several test stations with permanent reference electrodes. The original anode wells consisted of three (3) 3 in (7.6 cm) dia. × 60 in (1.5 m) long graphite anodes spaced vertically in coke breeze backfill and encased in an 8 in (20.3 cm) dia. schedule 80 steel pipe. A review of historical records indicated that with the exception of the underground storage tanks and piping for the diesel generator (FD) system, the CP systems at Catawba have experienced various levels of intermittent operation since 1985. Shortly after commissioning, the entire system (excluding the FD system) was reportedly shut down for a period of approximately five (5) years. This was apparently due to low potentials and reported stray current interference problems. In 1993 rectifiers 8 and 9 (transformer yard and switchyard) were abandoned in place due to ineffective operation. Numerous other maintenance problems, including anode well depletion, rectifier problems and cable breaks, have also been reported since initial inception.

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