Tricky element isolated from spent nuclear fuel

An electrical kick is all it takes to isolate one hazardous element from the rest of a nuclear power plant’s toxic waste.

New research published in the Nov. 6 Science demonstrates how to chemically manipulate the element americium so that it can be easily extracted from used nuclear fuel. While americium may not have the name recognition of uranium or plutonium, it’s highly radioactive and emits enough heat to complicate fuel storage. And until now, it was one of the trickiest elements to isolate and remove from nuclear waste.

The new technique could lead to improved methods of handling nuclear waste, reprocessing it in a way that eliminates the most dangerous elements (including americium) and ensures that the remaining waste is safer to store.

Scientists can already recycle portions of nuclear waste by dissolving solid fuel in acid and extracting uranium, plutonium and some other elements that are suspended in the solution. Americium, though, poses a challenge because it masquerades as other elements. In the acidic soup, neutral americium atoms tend to give up three of their electrons. That upsets the atoms’ even electron-proton balance, and they become ions with a charge of +3, that is, with three more protons than electrons. But other elements in the fuel also prefer to float around as ions with a +3 charge, and they are similar in size to americium. So techniques for extracting americium often end up grabbing those elements, too.

“It’s really difficult to selectively remove one and not the others,” says Christopher Dares, a nuclear chemist at the University of North Carolina at Chapel Hill. Dares and colleagues set out to coax americium ions to oxidize, or lose even more electrons. Americium ions with a charge of +5 or +6, unlike ones at +3, stand out from the other elements.

The researchers designed electrodes coated with metal-bonding chemicals, dipped them into an acidic solution with americium and applied a small voltage. The electricity nudged the americium atoms to lose a fourth electron, oxidizing into a more desirable but also very unstable +4 state. Eager to oxidize further, the +4 americium then forfeited more electrons, acquiring a +5 or +6 charge. Both of those states are sufficient for efficiently extracting americium from a jumble of elements.

“They came up with a slick method,” says Chuck Soderquist, a nuclear chemist at the Pacific Northwest National Laboratory in Richland, Wash. He says that previous techniques for oxidizing americium required adding new compounds that weren’t always easy to remove from the acidic hodgepodge later.

Dares says that recently he and his colleagues have gone beyond the scope of the paper and shown that they can actually extract the oxidized americium. For now, though, the technique’s usefulness is limited because many nuclear power plant operators, including all those in the United States, store their waste rather than recycle it. Still, the approach could bolster proposed protocols for extracting the most menacing elements individually from used fuel. Pure americium could then be placed in a reactor and split into safer, more stable elements.

The challenge, Soderquist says, is “trying to convince people it’s worthwhile to pursue this chemistry.”