By Scott DiSavino
(Reuters) - About 18,800 megawatts (MW) of power capacity at U.S. nuclear operators are expected to be offline at the peak of the 2013 spring refueling season, down roughly a third from a year earlier, Reuters data showed.
That compares with 27,500 MW of nuclear capacity shut in mid-April 2012, according to the data.
The data assumes units currently on extended outages, such as the two at Southern California Edison's (SCE) 2,150-MW San Onofre plant in California, Progress Energy Florida's 860-MW Crystal River in Florida and Omaha Public Power District's 478-MW Fort Calhoun in Nebraska, will still be shut.
Nuclear outages over the past five years have averaged about 23,200 MW in spring (2008-2012) and 22,200 MW in the autumn (2008-2012).
Since 1999, spring outages have peaked near 32,800 MW in 2011 and bottomed at 16,100 MW in 2004. Autumn outages peaked near 27,200 MW in 2009 and bottomed at about 12,300 MW in 2004.
The 104 U.S. nuclear reactors are capable of generating almost 101,200 MW, enough to power about 80 million homes.
Nuclear reactors produce about 20 percent of the country's generation and operate around the clock as baseload facilities, providing some of the lowest-cost power.
Natural gas traders follow nuclear outages closely because plants burning gas usually make up much of the missing nuclear generation, especially in periods of high demand.
It takes roughly 200 million cubic feet of natural gas per day to generate about 1,000 MW.
AUTUMN OUTAGE CALL
In September, Reuters forecast there would be about 18,700 MW of nuclear capacity shut in mid-October. There was 24,600 MW offline on Tuesday, compared with about 20,200 MW shut this time last year.
Reuters missed the October call in part because some reactors such as Exelon Corp's 1,122-MW Peach Bottom 2 and Tennessee Valley Authority's 1,123-MW Watts Bar 1 took longer than forecast to exit refueling outages. Other reactors such as Dominion's 903-MW North Anna 2 and PG&E Corp's 1,118-MW Diablo Canyon 2 shut unexpectedly.
The two reactors at SCE's San Onofre plant shut in January after the discovery of premature tube wear in the steam generators.
SCE, a unit of California power company Edison International, this month announced a plan to restart the 1,070-MW Unit 2 at San Onofre and run it at up to 70 percent power for about five months.
The U.S. Nuclear Regulatory Commission has said approval of the restart, if granted, could take months.
Sempra Energy's San Diego Gas and Electric unit also owns a stake in San Onofre.
Omaha Public Power District said it hoped to restart Fort Calhoun by the end of 2012 or early 2013. Fort Calhoun shut for refueling in April 2011 and has remained down due to damage caused by flooding of the Missouri River.
Crystal River shut in September 2009 for refueling and steam generator replacement. The unit has remained down since workers at Progress Energy, now a unit of North Carolina power company Duke Energy Corp, found a gap in the concrete after they opened the plant's containment dome to install the generators.
Duke has not yet decided whether to fix the Crystal River dome, which could cost more than $3 billion, or retire the reactor. If the company fixes the dome, it would likely not restart until 2014 or later.
(Reporting by Scott DiSavino in New York; Editing by Dale Hudson and Gerald E. McCormick)