Global power generated by nuclear reactors fell about 3.6 percent in 2007 from the 2.8 billion megawatt-hours (MWh) recorded in 2006, according to data released by Nucleonics Week, a publication of Platts, a provider of energy and commodities information.

In the otherwise lackluster 2007, U.S. reactors set a record for nuclear power generation, with output surging to 843 million gross MWh and using an average 91 percent of reactor capacity.

“The 2007 nuclear plant performance means about 20 percent of U.S. electricity was once again generated without the carbon emissions that would otherwise contribute to global warming,” said Margaret Ryan, Platts global nuclear editorial director.

World performance was led by two U.S. reactors. In terms of output, South Texas Project’s South Texas-1 in Bay City, generated 12.36 million MWh, the largest output of any reactor in the world. Of the 15 reactors generating the most in 2007, three were from the United States, eight from Germany and four from France. In terms of capacity factor, which measures how well a plant performed against the output level its manufacturer promised, the best worldwide was Constellation Energy’s Calvert Cliffs-1, south of Annapolis, Md., which actually exceeded its rated capacity level all year. Of the top 15 best performing reactors in 2007 measured by capacity, 11 were from the United States, and four were from Japan.

The slowdown in world nuclear power generation was in large part attributable to developments ranging from an earthquake in Japan to persistent aging of facilities in the United Kingdom to unplanned outages in Germany. Elsewhere, nuclear generation sustained good performance or went up or down by smaller amounts.

In terms of world nuclear power plant generating capacity, the median utilization rate was 84.2 percent with the top quarter of units all performing above 91.5 percent. This suggests many reactors performed very well during 2007. But with a total nominal operating capacity of 395 gross gigawatts, the 439 nuclear units worldwide could, if they had operated at an average 85 percent capacity factor, have generated approximately 400 million more megawatt-hours to meet growing world electricity demand than they did in 2007. That amount would supply the entire United States, which uses more electricity than any other country in the world.