Wall thinning in nuclear piping

Wall thinning due to wet steam is a well-known phenomenon in both fossil and nuclear power plants; however, the widely reported wall thinning and failure of a feedwater line in the Surry Power Plant was quite unexpected because it occurred in single-phase liquid water. Since the Surry failure, wall thinning had been reported on other nuclear plants, demonstrating single-phase erosion-corrosion to be a generic issue. Critical parameters influencing wall thinning in single-phase liquid water are: alloy composition of the pipe; degree of turbulence in a given location; pH level; oxygen content, linear flow velocity; and temperature. Other factors may influence rate of attack and type of attack; e.g., uniform wall thinning, slot-type attack, or highly localized wall thinning. The USNRC has requested action by the ASME Codes, specifically Section XI. This paper discusses the wall-thinning problem and current program, particularly at EPRI, aimed at its definition and resolution and the Section XI resolution. The Section XI plan is based on the maximum utilization of the available data generated by EPRI, by the USNRC and by the utilities. Section XI covers nondestructive examination (NDE) related to where and how often to examine and with what tools, e.g., ultrasonic or radiography. In addition, the issue of wall-thinning evaluation analogous to flaw evaluation is covered. It is hoped that there will be reduction in Section XI levels of weld examinations to partially compensate for added requirements of NDE to detect wall thinning. 9 refs., 15 figs., 2 tabs