America's fleet of nuclear power plants is facing an existential crisis: Despite producing about 60 percent of the nation's carbon-free electricity, nuclear reactors increasingly struggle to compete with power produced from cheap natural gas.

Exelon Corp., the nation's biggest nuclear-plant operator, announced last week that it would give early retirements to two money-losing nuclear plants in Illinois, even though they are top producers.

Its Three Mile Island Unit 1 near Harrisburg is also facing financial challenges after failing for the second straight year to submit a successful bid in an auction to meet energy needs in PJM, the regional power grid serving 61 million people in Mid-Atlantic and Midwestern states.

In a statement, Exelon would not commit to continuing to operate TMI after May 2018. It said it is exploring "all options" to make the plant profitable.

"In TMI's case, we remain hopeful that power markets recover, but if we do not see a long-term path to sustainable profitability for a particular unit, we will consider all options, including unit shutdown," Ralph DeSantis, the plant's communications manager, said in an email Friday.

Troubles are not unique to Exelon's reactors.

In upstate New York, Entergy Corp.'s James A. FitzPatrick plant is set to close later this year or in early 2017. In New England, the Vermont Yankee Nuclear Power Plant shut down in 2014, and the Pilgrim Nuclear Generating Station in Massachusetts may close in 2019.

Exelon's Oyster Creek reactor in Ocean County, N.J., is scheduled to retire in 2019.

"Many, if not most, of the 99 operating [nuclear] units are facing significant economic challenges," Robert F. Powelson, a member of the Pennsylvania Public Utility Commission, told the Nuclear Energy Institute at a May conference in Miami.

At the heart of nuclear energy's woes these days is not long-standing discomfort about safety, aging reactors, or nuclear-waste disposal. It's the sustained low cost of natural gas, which sets most electricity prices in competitive markets.

Powelson and others say the power markets need to develop a means to reward the reliability, fuel diversity, and carbon-free emissions nuclear energy provides.

The unexpected dominance of natural gas in power markets is a recent phenomenon, tied to the emergence of shale gas derived from hydraulic fracturing. Though consumers benefit from low energy prices, an increasing lack of fuel diversity has raised fears an upset in gas supplies could cause power shortages.

The North American Electric Reliability Corp., which monitors wholesale power flows between generators and local utilities, issued a report in May prompted by concerns that the recent leak of gas stored underground in California could cause summer power outages.

"Overdependence on a single fuel type increases the risk of common-mode or single-point-of-failure disruptions as experienced during recent extreme weather events, like the 2014 Polar Vortex," NERC said.

Nuclear advocates also say that without reactors, some states will be unable to meet targets to cut greenhouse-gas emissions under the Obama administration's proposed Clean Power Plan.

Exelon said it decided to pull the plug on its Clinton and Quad Cities plants in Illinois after state lawmakers declined to approve legislation that included subsidies for nuclear power as a zero-carbon energy source.

No similar legislation is in the works in Pennsylvania.

In New Jersey, Ralph Izzo, chief executive of the parent company of Public Service Electric & Gas Co., has begun to push for a "zero emission credit" for nuclear power similar to renewable-energy credits. His company operates the Salem and Hope Creek reactors in Salem County.

"Our plants are being relied upon for their clean, reliable and low-cost output, which provides 50 percent of the energy in New Jersey, but are not receiving value for the same carbon-free attributes that renewables bring," Izzo said in a statement quoted by Politico.