For decades, the U.S. has had a plutonium problem. Around 100 tons of the stuff was made during the Cold War to go into powerful atomic bombs. But as nuclear stockpiles were dismantled, the government had to store the radioactive material in high-security facilities.

Now it wants startups to help get rid of some of it.

The Department of Energy said Tuesday it has selected five nuclear startups to enter into negotiations with the government to receive a portion of the plutonium, which could potentially be used to power a new generation of nuclear reactors. The Department of Energy previously identified 34 tons of plutonium for disposal.

The five startups include Oklo, Standard Nuclear, Shine Technologies, Flibe Energy, and Exodys Energy.

Energy Secretary Chris Wright was previously on Oklo’s board, but he resigned when he joined the administration and said he has divested his shares. Sam Altman was Oklo’s board chair following its merger with his acquisition company, AltC; Altman resigned the position last year.

While plutonium does exist in nature, it is more typically a by-product of bombarding non-fissile uranium with neutrons. Once formed, that isotope of plutonium has a half-life of 24,000 years, meaning the government can’t just wait it out.

Oklo is developing a reactor that can run on traditional uranium fuel as well as plutonium. The plutonium would help the company fuel its first reactors. Exodys Energy is also developing a reactor that can operate using some plutonium as part of mixed oxide fuel, or MOX, which blends uranium with plutonium. Flibe Energy is working toward a reactor that would run on plutonium and other by-products of fission reactors.

MOX is currently produced in France, and while the U.S. had plans to make it in South Carolina, the first Trump administration canceled the project after it blew through budgets and timelines. One of Oklo’s partners in the project, U.K.-based Newcleo, said it intends to build its own MOX fuel fabrication facility nearby.

Not everyone is thrilled with the plan, though. Since the plutonium came from nuclear weapons, the security concerns are significant. “Countries have tried this before, and they concluded that, as nice as it would be to use that plutonium as fuel, it’s really just a liability and we need to dispose of it permanently,” Scott Roecker, a vice president at the Nuclear Threat Initiative, told the New York Times.

For the startups, the next step is to enter into advanced negotiations with the government over security and the transportation of the plutonium.