Controversial as it is, nuclear power remains a viable source of energy as we transition away from fossil fuels toward methods with far smaller carbon footprints. Underground uranium reserves may be on the decline, but the oceans contain billions of tons of the metal, just waiting for us to find a practical way to extract it. To that end, a Stanford team has developed a technique that improves the capacity, rate and reusability of materials that harvest uranium from seawater.

Despite the ongoing issues of radioactive waste disposal and the occasional Fukushima-scale disaster, nuclear energy can be more efficient and relatively cleaner than fossil fuel sources.

"For much of this century, some fraction of our electricity will need to come from sources that we can turn on and off," says Steven Chu, co-author of the study. "I believe nuclear power should be part of that mix, and assuring access to uranium is part of the solution to carbon-free energy."

Australia, Canada and Kazakhstan together account for about 70 percent of the world's uranium production, but for countries that aren't perched atop a rich mine, extracting it from the sea could be an alternative. Unfortunately, the concentrations are far too small to be viable, but the Stanford team is working on improving that.

"Concentrations are tiny, on the order of a single grain of salt dissolved in a liter of water," says Yi Cui, co-author of the study. "But the oceans are so vast that if we can extract these trace amounts cost effectively, the supply would be endless."

Chong Liu, one of the researchers on the study, with a carbon-amidoxime electrode, used for electrifying the material to improve its efficiency at capturing uranium from seawater(Credit: L.A. Cicero)

In the past, Oak Ridge researchers demonstrated a material that could pull uranium, in the form of uranyl ions, out of the water like a sponge. It did so with the help of plastic fibers coated in a chemical compound called amidoxime, which attracts the ions and holds them to the surface of the fiber. Once the fiber is saturated, the uranyl can be released by chemically treating the plastic, and then refined for use in reactors.

Using a similar system, the Stanford researchers created their own conductive fiber made of carbon and amidoxime, which allowed them to send jolts of electricity through the material to attract more uranyl to each strand. The method improved on the previous system in three key areas: the capacity for how much uranyl the fibers can hold, the speed of ion capture, and how many times each strand can be reused.

During side-by-side tests, in the time it took the existing fibers to become fully saturated, the new conductive material had already captured nine times as much uranyl, and still had room for more. Over an 11 hour period, Stanford's new fibers managed to soak up three times as much uranyl as the previous fibers, and were three times as reusable.

While it's still a long way from making the process practical on a commercial scale, the researchers say this marks a big step forward for that possible future.

"We need nuclear power as a bridge toward a post-fossil-fuel future," says Chu. "Seawater extraction gives countries that don't have land-based uranium the security that comes from knowing they'll have the raw material to meet their energy needs."

The research was published in the journal Nature Energy.

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