Faulting software errors by Mitsubishi Heavy Industries, federal regulators have determined the steam generator design flaws that ultimately shuttered the San Onofre nuclear plant likely amounted to violations of its license.
Southern California Edison closed the two-unit plant in June, confronted by mounting outage and repair costs for steam generators exhibiting rapid tube wear. On Friday, the Nuclear Regulatory Commission notified SCE of two preliminary findings related to the potentially unsafe conditions caused by the equipment's engineering. A green finding for unit 2 will be considered a non-cited violation, but a white finding for unit 3 will be considered for escalated enforcement action, according to NRC correspondence.
The agency also issued a notice of nonconformance to MHI related to its thermal hydraulic and vibration analyses. According to that document, equipment at three additional reactors was also designed using faulty modeling assumptions. As explained by the NRC:
"The MHI Three-Dimensional Thermal and Hydraulic Analysis computer code (FIT-III) was initially developed in 1978 for use in modifying Westinghouse designed square pitch tube steam generators. In 1992, the MHI Takasago Research and Development Center (Takasago) modified FIT-III for use on triangular pitch tube steam generators. At the time of the conversion to triangular pitch, the output of FIT-III was the wide gap velocity versus the narrow gap velocity required to be input into the flow induced vibration analysis software (FIVATS) vibration analysis code. The MHI Steam Generator Design Section (SGDS) thought the output of FIT-III was the narrow gap velocity. All MHI designed triangular pitch steam generators up through SONGS (total of five nuclear power plants) used the incorrect gap velocities. However, no other operating steam generator has developed fluid elastic instability as found at SONGS."
The NRC has studied the steam generators intensely since their flaws were identified in early 2012. Further technical details can be found in the agency's root cause analysis released earlier this year.