An evaluation by Mitsubishi Heavy Industries (MHI) made public today cites ineffective tube supports, dry steam and high steam flow velocity as causes of excessive wear in the steam generators MHI supplied to Southern California Edison’s (SCE) San Onofre Nuclear Generating Station.
SCE previously disclosed these same causes based on its own investigation, and the Nuclear Regulatory Commission’s (NRC) augmented inspection team report last July found that MHI’s use of faulty computer modeling in the design process caused MHI engineers to inadequately predict the dryness of the steam, measured by void fraction, in the replacement steam generators.
MHI repeatedly reassured SCE of the efficacy of the design. During the design phase of the project, MHI advised SCE that, based on its own review and analysis, the maximum void fraction that MHI expected to occur was acceptable, did not require additional design changes or measures, and that the replacement steam generators would perform as warranted.
“SCE’s own oversight of MHI’s design review complied with industry standards and best practices,” said Pete Dietrich, SCE senior vice president and chief nuclear officer. “SCE would never, and did not, install steam generators that it believed would impact public safety or impair reliability.”In fact, MHI states in its root cause report (page 41), that its analysis of conditions in the steam generator during the design phase (which calculated void fraction and steam flow velocity) concluded that the thermal hydraulic conditions in the San Onofre steam generators were acceptable, and specifically that there was no need to reduce void fraction. 1 Additionally, SCE never rejected a proposed design change to address void fraction based on its impact on compliance with 10 CFR 50.59. “At no time was SCE informed that the maximum void fraction or flow velocities estimated by MHI could contribute to the failure of steam generator tubes,” said Dietrich. “At the time, the design was considered sound.”