(tense music) - [Narrator] Mounting concerns about climate change are driving a new push to keep nuclear reactors open.

The plants are the largest source of carbon free electricity in the US, far outpacing renewable sources.

Across the country, about a third of existing nuclear plants are either unprofitable or scheduled to close.

Nuclear advocates are now pitching their carbon free electricity as a solution to global warming, but critics don't think they should be classified as clean energy.

- Virtually any time the subject of nuclear comes up in a group of people who understand the problem and are looking for solutions, they end up in an argument.

And it's because there really are good arguments in many different ways.

- [Narrator] In Pennsylvania, the debate reminds people of Three Mile Island.

40 years ago, it was the site of the most serious nuclear accident in US history, and sparked a backlash against the industry that halted its growth for decades.

On March 28th, 1979, the plant's unit two reactor near Harrisburg partially melted down.

Fearing for their safety, an estimated 80,000 people fled the region.

- I took our marriage certificate and I took our children's birth certificates, I was concerned that if in the confusion things really got bad, that I could prove those were my children and that we could at least be together.

- [Narrator] Joyce Corradi was a young mother of four, running a daycare out of her home a few miles from the plant.

- My strongest memory is pulling out of this driveway and wondering if I would ever come back.

- [Narrator] Public officials, even the governor, didn't seem to know what was going on.

(background chatter) - We're getting conflicting reports too, what we're trying to do is give you our best estimate of what the accurate facts are.

- [Narrator] After about a week, many people, including Corradi, returned home.

The plant eventually re-opened, minus the one damaged reactor.

That legacy left it hobbled, it means Three Mile Island can't produce as much electricity as other facilities, most have two working reactors.

- There is only a single unit now at TMI, so it's one of these single unit plants that's having a hard time competing in the market.

- [Narrator] Three Mile Island's unit one reactor, which is owned by Exelon, closed this fall, 15 years before its operating license was to expire.

Long time anti-nuclear activist Eric Epstein sees the renewed attention on nuclear power as just a new marketing ploy for a failed technology.

- I think a lot of environmentalists are making Faustian bargains, and signing up to bail out nuclear without understanding that it's really going to do little to nothing to combat climate change.

The time it takes to get a new nuclear power plant up and running, presuming you have the resources, is 10 to 15 years, we don't have that kind of time.

We've got to deal with climate change now.

- [Narrator] In recent years, states including New York, Illinois, New Jersey, Ohio and Connecticut have given billions of dollars to prop up their nuclear plants, recognizing them as a source of carbon free power.

- I want people to know that nuclear technology is safe, the tremendous contribution it makes to combating climate change because it emits no carbon, the tremendous economic role that it plays in the state and in the community.

- [Narrator] The nuclear industry has faced economic headwinds from the rapid growth of natural gas plants and renewable sources like wind and solar.

- This meant some plants have become less competitive in the market, and that's true certainly for some nuclear power plants as well, certainly not all nuclear power plants, but certainly some.

- [Narrator] Nuclear energy advocates have been pushing Pennsylvania to look to the ways other states have recently moved to prop up their own unprofitable plants, but legislation to reclassify nuclear plants as clean energy stalled in Pennsylvania earlier this year.

(distant speaking) Senator Ryan Aument, a sponsor of one of the nuclear subsidy bills says it was hard to talk about the issue without people remembering the past.

- When you mention Three Mile Island, you talk about nuclear energy, their mind immediately flashes back to the events of the Spring of 1979, and, without a doubt, that's been a real hurdle to overcome in terms of being able to educate folks on the benefits of the nuclear energy industry, and, frankly, a tremendous record of safety that the industry has had over its existence in the United States.

- [Narrator] For people who lived through the accident, like Joyce Corradi, its legacy still looms large.

- Nuclear power is an entity that, if you run it 100% of the time, 100% perfect, we're okay, but nobody does anything 100% of the time, 100% perfect.

- [Narrator] Her most vivid memory is evacuating, leaving behind her home and wondering if her life would ever be the same.

- I find that, really, 40 years down the road, I'm still sitting on top of a plant that has all the waste, a plant that cannot sell its electricity, and there's still no real answers.

- [Narrator] To this day, she avoids driving by the plants cooling towers.

- It's kind of like living with a giant in your neighborhood.

You know it's there, you know it could cause you problems, but you live in an uneasy compromise.

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