The Trump administration’s plan to offer plutonium from dismantled Cold War–era nuclear weapons to private energy companies is drawing criticism from experts who say it makes little economic sense and presents a national security threat.

There are currently no operational nuclear reactors in the country that are built to use plutonium-derived fuel. Instead nuclear power plants in the U.S. are powered by a mixture of two uranium isotopes. A small portion, usually around 5 percent, of that fuel is uranium 235, which can also be used to make nuclear weapons. The majority is uranium 238, which cannot sustain a nuclear fission reaction on its own. Because of that balance, if some of this fuel were to fall into the wrong hands, it would be enormously difficult to weaponize, says Scott Roecker, vice president of nuclear materials security at the Nuclear Threat Initiative, a nonprofit dedicated to preventing nuclear catastrophe.

“The most difficult step in getting a nuclear weapon is having enough of that material,” he explains. “The U.S. government has spent probably billions of dollars over the last several decades to remove highly-enriched uranium and separated plutonium from countries that don't need it.”

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Plutonium, meanwhile, is considered a human-made element and is a by-product of the reactions that take place inside nuclear reactors. As uranium 238 is bombarded with neutrons inside the reactor, the molecules absorb some of these particles and become the heavier uranium 239, which rapidly decays and eventually becomes extremely radioactive plutonium.

That plutonium can be mixed back with uranium to be used as fuel in specific nuclear reactors called mixed oxide reactors. The U.S. abandoned mixed oxide reactors in the 1970s because they were both difficult and expensive to run. These kinds of reactors do exist elsewhere, though—in Japan, Russia and France—but those countries have encountered their own problems with the reactors, Roecker says.

“In France, the government’s subsidizing that process,” he says. “Only I think 1 percent of the uranium that’s actually reprocessed is being reused. And in Japan, it’s cost the country billions of dollars and has still not started operation, and who knows if it actually ever will.”

The U.S. Department of Energy has defended the plan, saying the private sector could play a vital role in advancing U.S. nuclear power infrastructure. Ted Garrish, assistant secretary for the Office of Nuclear Energy, said in April that decommissioned nuclear fuel “represents an immense, untapped energy resource for the United States.”

“The Surplus Plutonium Utilization Program is anticipated to help companies unlock the next level of private funding to broaden domestic nuclear fuel supplies, spur innovation on American recycling technologies, and unlock private sector funding to fuel the nation’s nuclear renaissance,” said a DOE spokesperson in a statement, adding that five companies have been selected to take part in the program.

Aside from the concern over cost and feasibility, other experts point out that keeping plutonium secure is much more difficult than doing so with typical uranium-based nuclear fuel. Daniel Speyer, a professor of nuclear power plant systems at New York University, says he isn’t convinced that energy start-ups could properly store plutonium. Even if the material is mixed back with uranium, separating the two to isolate the highly fissile material isn’t so difficult as to be impossible—which introduces a clear security threat, he says.

“It’s not something that a small organization really probably could do, but if you give them plutonium in purer form, I think it’s almost a trivial act to make a bomb,” he says. “A simple atomic bomb is not difficult to make."

The DOE says that any company selected to receive the Cold War–era plutonium will have to show a deep understanding of the technology involved, as well as robust security plans and regulatory compliance. The plan has also met some pushback on Capitol Hill, however. Last September Democratic senator Edward Markey of Massachusetts and two Democratic congressional representatives sent a letter to President Donald Trump raising concerns over the risk to national security.

“The transfer of weapons-usable plutonium to private industry would increase the risk of nuclear weapons proliferation, including to rogue states or terrorists,” they wrote.