A Democratic senator is raising concerns that the Trump administration’s plans to let several private energy companies access weapons-grade plutonium could set a new and dangerous precedent globally.

The federal government announced last week it is pursuing a deal with five private companies that would, for the first time, allow them to use the federal government’s plutonium stockpile. The plutonium was originally used to create a Cold War-era fleet of weapons. Now, companies want it as a bridge fuel to power small modular nuclear reactors and generate electricity.

The Energy Department’s Office of Nuclear Energy announced last week it had selected advanced nuclear companies Oklo, Exodys Energy, SHINE, Standard Nuclear and Flibe Energy to begin “advanced negotiations” over whether the companies could access its Surplus Plutonium Utilization Program. The negotiations aren’t yet finalized.

But the move has rattled nuclear nonproliferation advocates like Sen. Ed Markey, a Democrat from Massachusetts. In a letter written to President Donald Trump and shared with CNN, Markey wrote of his concern that the move could set a dangerous example for the rest of the world and be extremely expensive.