PENNSYLVANIA has played a big role in the history of American energy. Coal has been mined there since the 1760s (Pennsylvania is sometimes called “the coal state”). In 1859 Edwin Drake drilled a rickety well that set off America’s first oil rush. More recently it has produced more natural gas than any other state except Texas, thanks to the vast Marcellus shale that runs beneath it. And though it barely advertises the fact, Pennsylvania is also America’s second-largest provider of nuclear energy—despite the near-disaster at Three Mile Island, a nuclear plant that suffered a partial meltdown of one of its reactors in 1979 (pictured, right), killing no one but scaring millions. Today Pennsylvania is again at the centre of a shift in the energy industry and Three Mile Island is caught up in it. An abundance of natural gas piped out of the Marcellus shale in recent years has helped push power prices down so sharply that nuclear energy has struggled to compete in some parts of America. Three Mile Island’s remaining reactor, Unit One, is one of many fighting to survive—but this time its troubles are about costs, not safety. In America and Europe slumping commodity prices are adding to the burden on nuclear power that was already growing after the 2011 Fukushima Dai-ichi nuclear disaster in Japan. America’s shale revolution, Europe’s growing supply of subsidised renewable energy and sluggish electricity demand in both markets have sharply cut wholesale power prices. That makes it harder for many nuclear plants to cover their running costs, leading their owners to shut them down. Perversely, at a time when countries around the world are pledging to cut carbon emissions, such closures often lead to the burning of more fossil fuels. Adding renewable-energy capacity does not solve the problem: when the wind doesn’t blow and the sun doesn’t shine, nuclear energy still provides the best low-carbon source of reliable “baseload” electricity. It is not all gloom in the nuclear industry. China plans almost to triple its nuclear generating capacity by 2020, and other emerging markets are also building new plants. But since Fukushima, all but two of Japan’s 43 reactors have been suspended, Germany is phasing out nuclear power and France intends to scale it back. Mycle Schneider, co-author of the World Nuclear Industry Status Report, says that a global total of 394 nuclear-power plants are operating, down from 431 in the pre-Fukushima year of 2010 (see chart 1). More closures are coming, particularly of older single-reactor plants that need lots of workers even though they generate only modest amounts of power. In America the most vulnerable plants are in deregulated markets such as the Northeast and Midwest, where nuclear-power providers must compete to provide the cheapest electricity against rivals that use other fuels. Those in more regulated southern markets, such as Georgia, fare better because electricity prices are guaranteed to cover their costs. With such an assurance, on October 22nd the Watts Bar plant in Tennessee became the first new nuclear facility to be licensed in America in 20 years. (Similarly, the operator of Britain’s proposed Hinkley Point C nuclear plant will sell electricity at high fixed prices.) Where markets are freer, it is harder for nuclear-power operators to make money, and too risky for them to build plants from scratch. Exelon, based in Chicago and the largest operator of nuclear plants in America, says that five of its 14 plants are vulnerable because of economic factors, including Three Mile Island’s Unit One, which it owns. “It’s ironic. People ask why we still operate a reactor there. But if gas prices were not [so low], it would be making money,” says David Brown of Exelon. On October 13th Entergy, Exelon’s rival, which is based in New Orleans, said it would close its Pilgrim nuclear plant in Massachusetts, partly because its costs, at about $50 a megawatt-hour (MWh), are higher than electricity prices in the state, which have fallen to about $45/MWh. As The Economist went to press, it was due to decide whether to close a third, Fitzpatrick, in New York State. In December it closed one in Vermont, the fourth American nuclear plant to shut in the past two years. The Nuclear Energy Institute, an industry body, says that last year generating electricity from a nuclear plant in America cost on average 2.40 cents per kilowatt-hour ($24/MWh). That is still cheaper than gas- or coal-fired power (see chart 2), but the nuclear average disguises wide variations. The least efficient nuclear plants have higher operating costs per unit of electricity than either coal or gas. Since the main cost of nuclear power is building the reactor in the first place, the narrowing gap in operating costs is ominous for the industry. And American gas prices are still plunging.