Former heads of nuclear regulatory bodies across Europe and the US put out a statement this week voicing their opposition to nuclear energy as a climate solution.

The debate over the benefits and risks of nuclear energy has been polarizing for years, but it’s escalating as world leaders work to transition from fossil fuels to clean energy. On one side of the debate, some argue that renewables alone are too dependent on the weather to provide a consistent power supply. Nuclear technology, which today provides about half of America’s carbon-free electricity, can reliably back it up, they say. And new nuclear technology is unlikely to trigger disasters like those at Chernobyl and Fukushima that have frightened the public in the past, proponents argue. But not everyone is convinced.

Nuclear energy is still too costly and risky to be a viable clean energy source, the authors of the statement write. They include Gregory Jaczko, former chairman of the US Nuclear Regulatory Commission, and the former leads of similar agencies in Germany, France, and the UK.

To learn more about why some nuclear power experts oppose the energy source as a climate fix, The Verge spoke with Jaczko, who chaired the NRC from 2009 to 2012 and, since then, has been outspoken about his concerns.

This interview has been edited for length and clarity.

The debate swirling around whether nuclear energy should play a role in climate action has been going on for years. What prompted you to issue a statement this week? Why now?

I think there’s been a lot of misinformation about the role that nuclear power can play in any climate strategy. A lot of attention has been put on nuclear as somehow the technology that’s going to solve a lot of problems when it comes to dealing with climate change. I just think that’s not true. And it’s taking the debate and discussion away from the areas that can have a role and that do need focus and attention.

I’ve certainly seen nuclear power making a lot of headlines recently. There was a leaked draft of European Commission plans to label nuclear power as a green investment. And here in the US, the infrastructure law is set to funnel billions into propping up nuclear energy. What goes through your head when you see this?

I think it’s money that’s not well spent. Nuclear has shown time and time again that it cannot deliver on promises about deployment and costs. And that’s really the most important factor when it comes to climate.

What I find kind of a little bit head-scratching is why, all of the sudden, this is getting attention when in fact, what’s actually happening is really, really negative for nuclear. You’re seeing nuclear power plants that, when I was chairman of the Nuclear Regulatory Commission, were supposed to be coming online — those plants have not come online all across the world. There are some new plants that have come online at a much later time. Then you have the complete fiasco that is the new build of nuclear reactors in the United States. You had four new design reactors that were licensed when I was chairman, which were supposed to be starting production in 2016 and 2017.

Two of those reactors were canceled, and that involved federal indictments for fraud among the heads of the company running that reactor development. Then the other two reactors are in Georgia, and those reactors continue to be pushed back and now are scheduled to start in 2022 or 2023. And they’re looking at a price tag that’s over $30 billion, which is more than double the initial estimate for the cost of that reactor.

[Editor’s note: Federal and state grand juries have charged the developers of an expansion project at South Carolina’s V.C. Summer Nuclear Station with fraud. They were charged with lying about progress on plans to build two new nuclear reactors at the site, which were abandoned in 2017 after ballooning costs that left utility customers to foot the bill.

The same company that was contracted to build the reactors in South Carolina, Westinghouse Electric Company, was also hired to build an additional two new reactors at the Vogtle Electric Generating Plant in Georgia. Costs for the Vogtle project similarly skyrocketed, and Westinghouse declared bankruptcy in 2017.]

Speaking of that Georgia project, the Vogtle nuclear power plant, you cast the lone dissenting vote on the NRC on it back in 2012. Looking back, has there been anything that surprised you?

I opposed that particular plant for a very specific reason: I thought that the agency that I ran, the Nuclear Regulatory Commission, should have put in requirements that part of its development process should adopt any reforms that were made to deal with the Fukushima nuclear accident. If you’d asked me at that time that I expected the plant would be five years over its time and more than double its budget, I would have said no. The reason I would have said that is because the industry at that time was assuring me and everyone else associated with nuclear policy and anyone in the financial community that the industry had control of costs, construction. They have fumbled it beyond even my expectations for how they could fumble it. [Editor’s note: costs for the Vogtle expansion were initially estimated at $14 billion.]

People like to characterize me as a very ardent opponent of nuclear power. I don’t really consider myself that. I consider myself kind of a realist. But even if you’d asked me at that time, I would not have said that that would be so far over budget and so delayed in its completion. And you know, I think right now, it’s certainly questionable whether it will ever get completed.

What should be done about older reactors? Some experts argue that premature nuclear plant closures lead to natural gas and coal plants filling in the gaps.

We have to get the facts right. And the premise of your question is not true. There’s no such thing as a direct one-to-one replacement, first of all.

Renewables and the amount of renewables that are in the pipeline far exceed any one closure of a nuclear power plant in the US. So nuclear is simply not being replaced by fossil fuels. We’re still seeing natural gas play too large of a role in our electricity sector. That is an issue by itself that has nothing to do with whether nuclear power plants shut down or not. So this is where I say so much of this discussion around nuclear focuses on the wrong thing. The right thing we need to focus on is what are we doing to get rid of gas?

What are your concerns with next-generation nuclear reactors, given that they’re very different than the older technology that we have?

It simply comes down to the need. I do not see a place in which these reactors will play a role because they do not meet the demands of the electricity space right now.

We have to stop believing the hype. Nuclear has never delivered on the hype, and to somehow hinge the future of the planet on unproven design is simply, I think, irresponsible, and we have to recognize that or we’re going to be throwing money at the technologies that are simply never going to deliver.

The window in which nuclear technology could deliver on a climate pledge closed a year ago or five years ago, realistically. It closed when the V.C. Summer plant decided to shut down. It closed when Vogtle was years and years and years over budget. And everyone has decided to try and knock a hole in the house and try and build a new window.

That’s what they’re trying to do today and say, well, that’s going to be the solution. It’s simply not. None of these designs are going to be ready for deployment, even as a prototype, for 2030. You need by 2030 the decarbonization of the electricity sector, not getting some brand new technology is going to build its first at the time and then you’re going to have to wait another five to 15 years before you can deploy that technology at scale. We have to deploy at scale today. And it’s simply not going to come from these advanced reactor designs.

Your statement says that nuclear energy as a climate strategy is “[m]ilitarily hazardous since newly promoted reactor designs increase the risk of nuclear weapons proliferation.” Can you explain?

The simplest way to think about it is the difference between a nuclear weapons program and a nuclear power program is really intent. So much of the technology that is used for nuclear power production can be used to make the material that you need for nuclear weapons. For a long time, one of the promises of advanced reactors was that they would somehow be more resistant to proliferation concerns, namely that they would be harder to make that transition from strictly a power production technology to a technology that could be used for weapons production. And as technologies have developed, those issues have not really panned out again in the way that many different reactor designs were touted. So it will always be there as a concern.

And there are some interesting new players stepping into the arena — private companies looking into small modular reactors for their own operations. I think the latest I’ve seen is Rolls Royce. What do you make of that trend?

I think it comes back to the same issues, which is I am skeptical that that will ever materialize because you don’t generate electricity at prices that are well above market rates just to prove a point.

Rolls Royce is looking to develop their own design for small modular reactors. You know, I think it still suffers from the same problem, which is that those designs don’t meet the needs of the electricity market — namely price, deployment, operational flexibility, and they have the potential hazards for accidents, although small modular reactors have a lower consequence than certainly a large reactor. There’s nothing about the benefits that outweighs any of those risks.