Southern California's idled nuclear power plant will need additional work before it can return to service, says the U.S. Nuclear Regulatory Commission. The two-unit San Onofre Nuclear Generating Station (SONGS) has been shut for more than five months because of premature wear of steam generator tubes.
SONGS is a 2,150-megawatt nuclear power station located halfway between Los Angeles and San Diego. It's owned by Edison International (EIX) (78.2%), Sempra Energy (SRE) (20%) and the City of Riverside (1.8%).
There's nothing wrong with SONGS' reactors, and there are no nuclear safety concerns for workers or for the public. The technical issue pertains to peripheral equipment called steam generators.
Steam generators serve a similar function as radiators in motor vehicles (trucks, cars and cycles). As the radiator removes excess heat from a motor vehicle's engine, the steam generator removes heat from the power plant's reactor. The difference is that the vehicles' radiator dumps the engine's heat into the atmosphere while the steam generator moves the reactor's heat to the plant's turbine-generator. The turbine-generator uses the heat to produce electricity.
Technically, steam generators are not a necessary piece of equipment. Most commercial reactors designed and manufactured by General Electric (GE) lack steam generators. The heat from GE's reactors moves directly to the turbine without anything in between.
The Fukushima units in Japan did not have steam generators. That's how radioactive water found its way into the turbine building. But those turbine buildings were designed to manage nominal loss of coolant incidences.
There are good reasons to use or not use steam generators. The decision comes down to a financial tradeoff between operating, maintenance and capital expenses. Power plants using steam generators are operationally less efficient than their counterparts. Less efficient plants have relatively higher production costs.
Plants lacking steam generators may be more efficient and enjoy lower production costs, but they must invest additional capital into their turbine systems, including upgrading their turbine building. This upfront investment is required to assure that the facility meets all safety standards.
However, SONGS' experience illustrates another challenge: maintenance expenses. From time to time, steam generators need to be replaced. If the process is carefully planned and managed, replacing steam generators can take up to four months, during which time the power plant is in cold shutdown and not producing any revenue. It's also expensive: The cost to replace SONGS's steam generators in both units in 2010 and 2011 was about $700 million.
After the owners invested the $700 million, SONGS' new steam generators were supposed to last 20 or more years. Instead, they didn't even make 20 months. The villain was premature tube degradation.
The NRC identified 10 issues, mostly related to the design of components in the steam generators. These issues allowed the excessive vibration between tubes inside the steam generators. In addition, the NRC's inspection report suggests that there was inadequate computer testing, which also led to the excessive vibration.
Short of replacing the steam generators again, it's not clear what modifications or repairs will be needed and undertaken. Once the vibrations are eliminated, nuclear operators can plug some of the tens of thousands of tubes and prevent minor leaks. Nevertheless, Edison International's Chairman and CEO Ted Craver said safety, not timelines, would determine when SONGS restarts. Also, SONGS' owners must seek the NRC's approval before they can restart either reactor.
Financial losses associated with SONGS's defective steam generators may be borne by their manufacturer, Mitsubishi, and possibly by the private insurance company Nuclear Electric Insurance. Any balance may become a risk for Edison International and Sempra's shareholders and ratepayers.
While SONGS' steam generator issues have garnered attention, its owners were also busy replacing the facility's turbines. According to the North County Times of San Diego, the older turbines, which have been in use since the early 1980s, were replaced with new aerodynamic designs that produce more power in the same amount of space that the old models occupied. The two new high-pressure steam turbines cost about $280 million, and they generate about 48 more megawatts of power, enough to support about 31,000 average-sized homes.
Additional power is needed. SONGS is a power plant designed to meet a region's base load, a load that's present 24 hours a day, seven days a week. Removing such a large source of continuous energy from the grid has system-wide consequences.
California's grid operator, California ISO, warned that a prolonged shutdown increases the possibility of outages.
SONGS is expected to operate for at least another decade. Both licenses expire in 2022. While no decision has been reached, owners are considering an application for a license extension.