San Onofre Nuclear Generating Station (SONGS) has four steam generators. The two steam generators installed in San Onofre Unit 3 contained a minor manufacturing feature that resulted in a “perfect pitch” harmonic. At just the wrong condition – 100% steam flow – a combination of relatively dry steam, precisely manufactured anti-vibration bars (AVB), and densely packed u-tubes resulted in a few hundred (out of nearly 10,000) tubes vibrating with a large enough amplitude to make contact. The unexpected vibration and contact resulted in accelerated wear and caused one tube to fail while the steam generator was operating.
If Unit 3 had remained in operation, other tubes in the same area of the steam generator would have likely failed. The same problem does not exist in Unit 2.
On January 31, 2012, San Onofre Unit 3 operators received indications that one of the two steam generators of the plant they were running was leaking and causing a tiny, but measurable, increase in the radioactivity of the normally non-radioactive water in the secondary (steam) side of the steam generators.
When they recognized the indication, the operators took the conservative course of action and shut down the nuclear plant, even though it turns out that the leak rate (84 gallons of coolant per day) was below the plant’s allowed technical specification (150 gallons of coolant per day per steam generator). The affected steam generator was isolated and the unit was brought to cold shutdown on February 1, 2012.
The small u-tube leak never represented any risk to the public. Unit 2 was not operating at the time and never experienced any steam generator u-tube leaks. Neither unit was in violation of its operating license. According to information obtained several months later, the maximum potential does of radiation to anyone was 5.2E-5 millirem (0.000000052 rem), which is one billion times lower than the annual limit for radiation workers and a very small fraction of the 100 mrem (1 mSv) per year regulatory limit to a member of the public.
Pre-planned testing of 100 percent of the steam generator tubes at Unit 2 was already in progress as part of a regularly scheduled outage when the event occurred in Unit 3. Testing results from Unit 2 also revealed unexpected tube wear at the retainer bars. Additional analysis and testing ultimately resulted in identifying two tubes with tube-to-tube wear similar to that observed in Unit 3. For both Units 2 and 3, this was the first cycle of operation with new replacement steam generators. SCE replaced the Unit 2 steam generators in January 2010 and Unit 3 steam generators in January 2011. All of the replacement steam generators were manufactured by Mitsubishi Heavy Industries. (Atomic Insights, San Onofre steam generators – honest error driven by search for perfection,' March 9, 2013 By Rod Adams, NRC Memorandum, 'REVIEW OF LESSONS LEARNED FROM THE SAN ONOFRE STEAM GENERATOR TUBE DEGRADATION EVENT,' March 6, 2016)