Abstract
ABSTRACT
Pitting produced by microbially induced corrosion (MIC) was discovered in stainless steel service water lines at TVA’s Sequoyah nuclear power plant. These pits were characterized by both nondestructive and destructive examinations of 6 in. (15.2 cm) nominal diameter pipe. The pitting was evaluated to assure the structural integrity of the piping, especially under seismic loading, and to protect adjacent engineered safety features from potential damage due to leakage. The evaluation concluded that the required ASME Code design margins remained intact for the flawed pipe, since applied stresses are significantly less than Code allowable stresses for the unflawed piping. The piping structural integrity was determined to be acceptable by performing Code stress calculations with reduced pipe cross—sections due to MIC. These calculations were further supported by net section collapse and elastic—plastic fracture mechanics methods. Furthermore, it was shown that the tearing of pipe ligaments between adjacent MIC pits and between MIC pits and the surface is unlikely during seismic events. The leakage resulting from through—wall MIC pits was found by field experience and by analysis to be on the order of "weeping" or "drops per minute" and would not impinge adjacent equipment. An inspection and water treatment program is planned to further mitigate MIC in these lines.
