Last Tuesday evening, the San Onofre Nuclear Generating Station (Songs) had a non-emergency event. Southern California Edison (SCE), a unit of Edison International (EIX), manually shut down Unit 3 after a water leak was discovered in one of its steam generators. A miniscule amount of radioactive gas may have been released inside the facility; but any radioactive release would be the type and level normally associated with hospitals and doctors' offices.
SCE is the facility's operator and owns 78.2% of Songs. Sempra Energy's (SRE) San Diego Gas & Electric subsidiary owns 20%. A municipal utility owns the remaining 1.8%.
The issue isn't a radioactive release; it's about Songs' steam generators. North County Times reports that Songs' technical problems are beginning to pop up in reactors owned by other utilities.
Steam generators are one of the key components that differentiate pressurized water reactors (PWRs) from boiling water reactors (BWRs); PWRs have them, and BWRs don't. Steam generators are located inside the reactor building and can be thought of as giant heat exchangers or radiators. On one side of the exchanger is the primary loop, which channels energy, in the form of heated water, from the reactor. On the other side is the secondary loop, which channels converted energy in the form of stream, to drive the turbine-generator in an adjacent building. The water in the primary loop never touches the steam in the secondary loop. (If Fukushima Daiichi had steam generators, it would have avoided many of their turbine building problems.)
The reactor's energy is transported in the primary loop and transferred to the secondary side through approximately 10,000 radiator-like tubes located inside the steam generator casing. Occasionally, one or two of the tubes ages and begins to leak.
According to Will Davis, editor of Atomic Power Review, tube failure is not commonplace but it does happen. The nuclear power industry developed standardized procedures, which includes plugging a leaking tube or tubes. Since each generator has thousands of tubes, plugging several or even dozens of tubes does not reduce the heat transfer rate nor does it alter the safety profile of the facility. Many nuclear plants operate with dozens of plugged steam generator tubes for years.
According to the Nuclear Regulatory Commission (NRC), SCE discovered and monitored the leak for about an hour. When the leak grew to approximately three gallons of water an hour, SCE decided to remove Songs from service on a non-emergency basis to undertake detailed inspections.
The leak was not a surprise. At the time, Songs' other reactor was offline and under inspection. Plant engineers discovered similar tube problems. According to NRC's Scott Burnell, inspections showed greater-than-expected wear, with 871 out of 10,000 tubes showing a decrease in thickness greater than 10%. "Our working hypothesis here is that what we're seeing at San Onofre is the same sort of phenomenon that we're investigating at other plants."
While the NRC has not finished compiling its list of plants, Burnell suggested NextEra Energy's (NEE) St. Lucie Nuclear Power Plant, Entergy's (ETR) Arkansas Nuclear One plant, and Exelon's (EXC) Three Mile Island Unit 1 have experienced heightened steam generator wear, though not to the extent seen at Songs.
The question is why. Why are steam generator's tubes wearing out so quickly? The answer could be as simple as water chemistry or a material science issue.
While this is not a safety issue, it could be an economic challenge for the nuclear power industry. There are approximately 70 PWRs in the nation's fleet. While only a few are experiencing steam generator issues, the financial impact could be significant.
It costs over $250 million to replace a power plant's steam generators. According to Nuclear News, Songs' Unit 2 and Unit 3 last replaced their steam generators in 2009 and 2010. The cost for each unit was approximately $340 million, for a total of $680 million. That investment should have lasted 15 to 20 years.
The likely case is that most utilities will monitor their steam generators. When the root cause is identified, owners will implement an engineered solution that is pre-approved by the NRC. In all likelihood, that solution will avert costly replacements.