Nuclear energy provides 11 percent of the world's electricity, but most reactors are now decades old. Many are approaching—or have surpassed—their initial 30- to 40-year lifespans (though upgrades can extend their lives to 60 years). Early nuclear adopters like France and Germany are curtailing their programs, even though analysts—including the authors of a recent report from the Intergovernmental Panel on Climate Change—say nuclear is necessary to keep worldwide carbon emissions in check. Emerging economies may take up the mantle: Planned reactors in China and Russia could keep the world's inventory stable.

Since late 2012, the U.S. has announced the retirement of five reactors, and several planned units are currently on hold.

China is one of the only nations investing heavily in nuclear power—28 of its 49 reactors are under construction, and 35 more have been planned. It still has a long way to go: Nuclear accounted for only two percent of the country's electricity in 2012.