Cathodic Protection Design Considerations for Buried Nuclear Piping

The design and installation of cathodic protection (CP) systems for buried piping at nuclear generating stations can be extremely intricate due to the complexity of the structures and the many variables that exist. Since the piping is connected directly to station ground (copper grid), current demands for cathodic protection can be extremely high. This can result in a significant number of anodes, direct buried cable, rectifiers, AC power supplies and monitoring sites. Furthermore the design of the CP systems can be adversely influenced by poor current distribution, shielding, dissimilar metals, limted space and varying depths of pipe. This paper will discuss the various aspects associated with CP system design, methods for prioritizing system installation and options for construction. Corrosion of buried nuclear piping will be discussed as it relates to galvanic couplings, coating damage, soil resistivity and sulfate reducing bacteria. Case histories will be presented giving examples of failure mechanisms, along with methods for implementing effective corrosion control. Cathodic protection criteria in mixed metal piping systems will be discussed, along with the use of coupon test stations for monitoring system performance.