Nuclear fusion fuel could be made greener with new way to supply lithium

Limitless power from nuclear fusion may be a step closer following the accidental discovery of a new process to supply the isotope lithium-6, which is vital to providing fuel for a sustainable fusion reactor.

The least challenging fusion process involves combining two isotopes of hydrogen, deuterium and tritium, to yield helium, a neutron and a lot of energy. Tritium, a rare, radioactive isotope of hydrogen, is difficult and expensive to source. “Breeder” reactors seek to manufacture tritium by bombarding lithium with neutrons.

Lithium atoms exist as two stable isotopes: lithium-7 makes up 92.5 per cent of the element in nature and the rest is lithium-6. The rarer isotope reacts much more efficiently with neutrons to produce tritium in a fusion reaction.

However, the two lithium isotopes are extremely difficult to separate. Until now, this has only been achieved at a large scale using a highly toxic process reliant on mercury. Due to the environmental impact, this process has not been employed in Western countries since the 1960s and researchers are forced to rely on dwindling stockpiles of lithium-6 produced before the ban.

Sarbajit Banerjee at ETH Zurich in Switzerland and his colleagues have now discovered an alternative method serendipitously, while they were looking at ways to clean water contaminated by oil drilling.

The researchers noticed that the cement membranes they employed, containing a lab-made compound called zeta vanadium oxide, collected large quantities of lithium and seemed to disproportionately isolate lithium-6.

Zeta vanadium oxide contains tunnels surrounded by oxygen atoms, says Banerjee. “Lithium ions move through these tunnels, which happen to be just the right size [to bind lithium-6],” he says. “We found that lithium-6 ions are bound more strongly and are retained within the tunnels.”

The researchers don’t fully understand why lithium-6 is preferentially retained, but based on simulations, they believe it has to do with the interactions between the ions and the atoms at the edges of the tunnels, says Banerjee.

He says they have only isolated less than a gram of lithium-6 so far, but they hope to scale up the process so it can produce tens of kilograms of the isotope. A commercial fusion reactor is expected to need tonnes of the element every day.

“However, these challenges pale in comparison to the bigger challenges with plasma reactors and laser ignition for fusion,” says Banerjee.