Last month, he took on a new role as chief executive of the Nuclear Threat Initiative, bringing his experience helping to negotiate the Iran nuclear deal and reviewing the global weapons stockpile to an organization dedicated to preventing attacks with weapons of mass destruction. He also recently joined the board of the fusion startup Tri Alpha Energy and helped established the Energy Futures Initiative, a nonprofit promoting clean-energy innovation and policies.

In a recent interview with MIT Technology Review, Moniz discussed the impact of the administration’s policies on U.S. leadership, how it feels to have his legacy come under attack, and what’s next for nuclear power.

Below is an edited excerpt of the interview. Premium MIT Technology Review subscribers can listen to the full interview.

Q: You responded fairly sharply to President Trump’s decision to withdraw from the Paris climate deal. What are the biggest or most immediate dangers of that decision, in your view?

A: First of all, withdrawing from the Paris accord is a pattern of many statements and actions that have called into question our leadership, and frankly our reliability, in terms of carrying out various commitments. The pattern has been to kind of then circle back and try to mend fences every now and then. But the reality is the uncertainty in our reliability is a very serious issue for our geopolitical leadership.

At the next level, withdrawing from the Paris accord obviously undercuts the leadership on that specific issue, an issue of tremendous global concern.

The United States demonstrated leadership in many ways, but in particular in two ways that I’ll mention. One was President Obama working with the Chinese to have the joint announcement with President Xi Jinping in November of 2014, which was a sea change on the path to Paris. There was also leadership in the area of clean-energy innovation. The United States Department of Energy was, obviously, in the middle of this, getting 20 countries at that time—plus 28 major international investors—to come forward in Mission Innovation, saying that we need to pick up the pace in innovation to meet our long-term goals (see “Paris Isn’t the Only Clean-Energy Pact the U.S. Is Fleeing”).

That leadership and its results were hard-earned, and an announcement to withdraw from the Paris accord obviously weakens that. We will see leadership enhanced from China, from Europe, from India, but I don’t think American leadership is very easy to replace.

We will also see leadership from the United States now at a subnational level, with mayors and governors and universities and businesses all stepping forward. I think that is tremendous, and very, very important. But you can’t be Pollyannaish about it. There’s no way that we aren’t compromised when the United States federal government is not exercising leadership.

In addition, the president’s budget request would also undercut the very programs that support American innovation, if the Congress goes along with that—which I hope they don’t, and I suspect they won’t. But if they were to go along with that, that would compromise our own economic opportunities to engage in that international-community market down the road. So, obviously, I find it difficult to tease out the redeeming features of having announced the withdrawal from the Paris accord.

Q: How do you react to seeing a big part of your and President Obama’s climate and clean-energy legacies come under deliberate attack?

A: The underlying reality is there’s virtually no one who believes that [rolling back clean-energy initiatives] is going to lead to a resurgence, let’s say, with coal. It’s unlikely to lead to new coal plants being built, because—again—businesses fully anticipate that in the end we’re not going back from a low-carbon trajectory. And any investments in new plants are 40-, 50-year investments, which would have a very good chance of getting stranded a few years down the road.

But again, I don’t want to be Pollyannaish. There’s no doubt that if the federal government is not rowing in the same direction as the governors and the mayors, etc., it obviously impedes progress and it will just be more costly, and more difficult, for us to get back on track.

Q: You’ve stressed that nuclear should be a part of the energy mix. So what can or should the U.S. be doing to bolster that sector at this stage?

A: In a number of states, in one way or another, without calling it exactly that, nuclear is being recognized as a zero-carbon source. And so you’ve seen states, like New York and Illinois in particular, try to sustain the existing plants because of their low-carbon qualities.

Having said that, I do believe we will still see more premature closures. The question is how many, and at what pace. The combination of efficiency, rapid growth in wind and solar, and continuing low-cost natural gas makes it tough in deregulated markets.

As far as new construction goes, obviously the whole situation with Toshiba Westinghouse, the cost overruns in the current build, is tough. I would just say that a direction that I remain very interested in is that of a new generation of small, modular reactors.

And the first of those is in the licensing process.

Q: NuScale?

A: NuScale, right. And that would be aimed for deployment in maybe seven years or so, for a first reactor. I think this is a very interesting direction, both because the designs are attractive and, secondly, the financial engineering is very, very different for a 50-megawatt plant versus a 1,200-megawatt plant. The financial risk equation is much changed. Financing is probably easier and less expensive, and that can be very, very important to the overall project cost.

Again, I don’t want to be Pollyannaish about it, but I think this is a very, very interesting direction, and could present a whole pathway for new nuclear builds going forward.