The last few years have seen all sorts of drastic upheavals in the U.S. energy sector. Cheap natural gas is dominating. Wind and solar are growing. Coal is dwindling.

The Crystal River nuclear plant control room (Will Vragovic/AP)

Now we can add another trend to the list: Nuclear power is starting to decline. Since 2010, the amount of electricity generated from America's nuclear reactors has fallen about 3 percent, or 29 billion kilowatt-hours. That's a sizable drop: As John Hanger points out, we'd need to quadruple the number of solar installations in the United States just to make up the loss of that carbon-free electricity.

So why is nuclear on the wane? Part of the story here is simply that America's fleet of reactors is aging and are being taken offline more frequently for repairs. The San Onofre plant near San Diego, for instance, has been out since January 2012. Some of the big outages seen recently could just be a temporary blip.

But another growing factor here is competition from cheap shale gas. This month, Duke Energy decided to close its Crystal River nuclear plant in Florida, offline since 2009, rather than pay $1.5 billion to repair a cracked dome. The reason? It was easier to build new natural-gas turbines to replace the lost electricity than repair the reactor. Last fall, Dominion Power announced that it would close its Kewaunee reactor in Wisconsin, also citing pressures from cheap gas.

That could be the start of a trend: One energy analyst told Bloomberg that at least 4 of the 102 remaining U.S. reactors are now at risk of early retirement "due to new power market economics." Perhaps more significantly, the cheap gas boom could be killing off some future reactors, too: Back in 2011, NRG scrapped plans for two new nuclear units in Texas. Why? There were cheaper options available, including gas and wind.

Over at Scientific American, David Biello ponders the climate implications of this potential shift. As long as natural gas was edging out dirtier coal, the shale-gas boom was helping to drive down U.S. carbon emissions. (Natural gas emits about half the carbon that coal does when burned for electricity.) But if cheap shale gas is starting to elbow out carbon-free nuclear power as well, that makes the carbon story more complicated.

So is there any hope for new nukes? Perhaps. Construction is underway on two large new AP-1000 reactors at the Vogtle power plant near Augusta, Georgia — the first new reactors approved since the 1970s. Meanwhile, two other new nuclear units have been approved for the Virgil C. Summer plant in South Carolina. Although critics have raised questions about the rising costs of these projects, the new reactors would help offset recent losses if they're finished.

It's also worth checking out Matthew Wald's report in the New York Times Wednesday about how the Tennessee Valley Authority is pouring money into smaller, modular reactors that the industry hopes can compete with gas:

The reactor is intended as a direct challenge to natural gas generators, and it is intended to share two characteristics that make gas attractive.

First, the builders say they can be built quickly and be added onto later, so there is less risk of building too much capacity or running short. Second, they are meant to do something that is difficult for existing nuclear plants but easy for gas: change power output rapidly.

The idea's still in the early stages, but as Wald notes, "in an era of low natural gas prices, the small reactor concept is the most popular one in the field."

*Update: Added mention of plans afoot for four new reactors in Georgia and South Carolina.