Right now, the biggest source of clean energy in the United States is nuclear power. The country's 99 commercial reactors provide 20 percent of our electricity, all without emitting carbon dioxide. Compare that with wind at 4.9 percent or solar at 0.6 percent.

But that dominance may not last forever. Nuclear power has been facing serious headwinds over the past few years. And, ironically, it's possible that under Obama's Clean Power Plan — aimed at reducing CO2 emissions from the power sector — nuclear will decline even further.

Up to five new reactors are expected to come online in the next decade, in Georgia, South Carolina, and Tennessee. But an even larger number of existing reactors are facing pressure to close in places like New Jersey and Illinois. Obama's new climate plan won't necessarily help avert those closures — in fact, it might even create some incentives at the margins to shut down more plants.

So here's a closer look at what the future might hold for US nuclear power.

Obama's climate plan offers incentives to build (a few) new reactors

Nuclear power in the United States faces two distinct challenges. First, very few people are building new nuclear plants. There are just five reactors under construction at existing sites in Tennessee, Georgia, and South Carolina.

The high price tag is a huge deterrent here: Building a nuclear reactor is a slow, complex project involving large upfront costs and strict safety specifications. Cost overruns are common. In many regions, it's simply not economical to build new reactors when natural gas turbines and wind farms are so much cheaper. It's not a coincidence that the reactors being built today are located in heavily regulated states where utilities can recoup their costs by raising electricity rates.

Now along comes Obama's Clean Power Plan, under which every state gets an individualized target for curtailing electricity emissions by 2030 and has to come up with a plan for getting there. States can do so by reducing emissions from their existing coal and gas plants (considered in total), by improving energy efficiency, or by building new sources of clean energy.

That third part is mildly good news for new nuclear. Tennessee, Georgia, and South Carolina will be able to make headway on their state emissions goals by bringing online the five new reactors they're already building, so they now have extra incentive to finish. (This was a big change from the original draft version of EPA's rule, which basically assumed these plants were as good as built.)

But aside from those five reactors, industry observers say it's unlikely we'll see many more built elsewhere, at least in the near term. Even with EPA's new emission goals, it's easier for most states to switch from coal to natural gas or to build wind farms than it is to build a brand new reactor. (Arguably, we may need new nuclear reactors in the longer term if we want to go totally carbon-free, but that would require a new set of policies beyond the Clean Power Plan.)

But even more existing reactors could close in the years ahead...

The second issue is what happens with our existing nuclear plants. Again, these reactors still supply 20 percent of the nation's electricity. But a number of them are dealing with sharp economic pressures.

The precise challenges vary. Some plants, like Indian Point in upstate New York or Diablo Canyon in California, face significant local opposition. Some may need costly upgrades to deal with wear and tear, environmental concerns, or other regulatory requirements. Others, like Exelon's Quad Cities, Byron, and Clinton plants in Illinois, face intense competition from cheap natural gas and subsidized wind power.

We've already seen five reactors close since 2013 and more retirements are likely on the way. All existing reactors were originally licensed to operate for 40 years; many operators are currently seeking permission from the Nuclear Regulatory Commission to extend that to 60 years. And the industry is hoping to extend at least half of its plants' licenses even further — through the 2040s and beyond. That relicensing process can be expensive, potentially costing $500 million to $1.5 billion per reactor.

Some reactors simply won't survive the process, particularly if they require major upgrades and face continued competition from wind and gas. Case in point: the Oyster Creek nuclear plant in New Jersey, which Exelon is planning to close in 2019 rather than shell out for an expensive new cooling tower required by local regulators. (Here's an informal list of eight more reactors that are "at risk.")

Obama's climate plan doesn't help these "at risk" nuclear plants

The Clean Power Plan doesn't really do anything to forestall potential closures. It doesn't give states any extra credit for extending the life of their nuclear plants beyond their 40- or 60-year lifespans, nor does it penalize states for shutting down nuclear plants early. (This is because existing nuclear isn't included in the baseline for setting state emissions goals.)

At the margins, this could create odd incentives. Say a state's power sector was faced with the choice of a) spending $500 million to extend the life of an existing reactor for 20 years or b) closing the reactor and spending $1.5 billion to replace it with new renewables. The latter would be costlier. But under the Clean Power Plan's rate-based goals, the state would only get credit for reducing emissions if it did the latter, not the former. (These numbers are just meant to be illustrative.)

Even more perversely, because of quirks in how the Clean Power Plan's rate-based emissions goals work, some states might even be able to close a nuclear plant, replace it with natural gas generation, and still get credit for cutting emissions, even though overall emissions would have actually gone up, not down. It's not clear that states will do this, but it's something to watch out for. (If states adopt mass-based targets for compliance, however, this would be less of a concern.)

"States were basically given no incentive to look at extending these assets," says Steve Skutnik, an assistant professor of nuclear engineering at the University of Tennessee Knoxville who has studied the effects of the Clean Power Plan on nuclear power. "EPA essentially said [in its rule], 'Well, we can't predict which plants will close, so we won't deal with that.' I was surprised by that."

There is one final twist here. Under the Clean Power Plan, states can get credit against their emission goals if existing nuclear plants increase their overall output, a process known as "uprating." Since 1977, nuclear plants have added about 6.5 gigawatts of capacity through uprates. Skutnik estimates that further uprating could probably add another 2 or 3 gigawatts of nuclear capacity going forward. But, he says, the plants most likely to do this are the ones that are already financially healthy. It isn't likely to save the plants in trouble.

So how many nuclear plants will be left by 2030?

It's difficult to predict exactly how many plants will close in the coming years. For its part, the EPA expects overall nuclear capacity to decline only slightly between now and 2030 (from 104 gigawatts down to around 99), as retirements are partly offset by the 5.5 gigawatts' worth of new nuclear reactors currently being built.

But that projection might be wrong. Even bigger declines in nuclear are certainly possible — it all depends on what states and utilities decide to do. Will state regulators in places like California and New York require plants to install expensive new cooling towers to address environmental concerns? Will the NRC require major new upgrades as part of the relicensing process? Could further growth in wind power put increased financial pressure on (some) nuclear plants? Those things could hasten retirements.

That said, there are also potential developments that could help the existing nuclear fleet and slow the pace of retirements. Could Illinois's legislature pass a measure, now being considered, to include nuclear in its state clean energy standard? Could a new federal reliability rule save some of Exelon's at-risk reactors in the PJM region? Could Congress revamp tax incentives for wind power, giving nuclear a leg up? So many variables.

Does it matter if more nuclear plants close?

From a climate change perspective, there are a couple of ways to look at this. If states closed some of their nuclear plants but replaced them with carbon-free energy (wind, solar, geothermal, hydro, efficiency savings), then the overall effect might be neutral. As long as states reduce overall emissions at the lowest cost, who cares how they do it?

On the other hand, if states shut down their nuclear plants only to replace them with natural gas generation — again, a move that might be allowable under certain circumstances — then additional closures would have a negative climate impact.

Right now the EPA is projecting that power plant emissions will fall 32 percent below 2005 levels by 2030 under the Clean Power Plan. This isn't a hard-and-fast requirement. It's just EPA's estimate of what it thinks states will do under the rule — they'll retire more of their coal plants, they'll build more wind and solar, and (as noted above) they'll mostly maintain their existing nuclear capacity. Add it up, and emissions will end up falling 32 percent below 2005 levels.

But what if those assumptions are wrong? What if even more nuclear plants retire than EPA expects and, say, get replaced by natural gas? A new paper put out Wednesday by Samuel Brinton and Josh Freed of Third Way suggests this wasn't out of the question. That study, which relies on modeling by MIT researchers Jesse Jenkins and Fernando de Sisternes, finds that additional nuclear retirements in the short and medium term would more likely be replaced by gas, not just clean renewables. And as a result, emissions could end up higher than EPA expects.

"In our models," the authors write, "any [additional nuclear] retirements would make it extremely difficult for the Clean Power Plan to reach its emissions targets of 32% below 2005 levels."

We'll see what ends up happening. So much depends on what states decide to do in their plans to comply with EPA's rule. But the US nuclear industry could end up facing a lot of big changes in the years ahead.

Read more: How Obama's Clean Power Plan actually works — step by step