The extended difficulties at the San Onofre Nuclear Reactor in San Diego plus a couple of other high-profile incidents have created the impression that the nation’s aging fleet of nuclear reactors is experiencing unprecedented difficulties and may be reaching the end of its life cycle. Except for a brief period in late February and March, however, the number of reactors off-line for various reasons has not been above its historical level and is now back in the midrange of traditional outages.

The graph shows the pattern of closings through the months of the year for the nation’s 104 nuclear reactors, which provide 20 percent of our electricity. Reactors regularly close every 18-24 months for a month-long period of refueling and maintenance. These refueling are generally scheduled for the spring and fall, since these are the periods of lowest demand. The wide shaded area represents the number of gigawatts in capacity closed at any one time from January to December over the years 2007 to 2011. The blue line represents the record in 2011, which had a high rate of closings in the spring and summer right after the Fukushima accident. The yellow line represents the 2012 record so far. Closings did rise above historical highs in February and March but are now back in the midrange.

The nation has a total of 106 gigawatts of nuclear capacity. Since 2000 the fleet has been operating at a remarkable capacity level of above 90 percent – meaning that on average more than 90 percent of the capacity is online throughout the year. The peaks of 30 gigawatt outage are somewhat deceptive because it happens only over a short period of time. The average for the year is below 10 percent.

The closing of San Onofre Units 1 and 2 for unexpected wear in the cooling pipes has made headlines and threatened a long, hot summer for San Diego. Pacific Gas & Electric’s Diablo Canyon Unit 2 also shut down unexpectedly on April 27 when an influx of jellyfish blocked the cooling water intake. But these high profile instances have obscured that the rest of the fleet is running well. The only other long-term outages have been Florida’s Crystal River Unit 4, offline since September 2009 because of cracks in the containment structure and Omaha Public Power’s Fort Calhoun reactor, which has been offline since April 2011 because of flooding in the Missouri River. The Nuclear Regulatory Commission is still evaluating the reactors ability to reopen.