Trace amounts of uranium exist in seawater, but efforts to extract it for nuclear power haven’t produced enough to make it a viable source for countries that don’t have uranium mines.
A practical method for extracting that uranium, which produces higher quantities in less time, could help make nuclear power a viable part of the quest for a carbon-free energy future.
“Concentrations are tiny, on the order of a single grain of salt dissolved in a liter of water,” says Yi Cui, a professor of materials science and engineering and of photon science at Stanford University and coauthor of a new paper in Nature Energy. “But the oceans are so vast that if we can extract these trace amounts cost effectively, the supply would be endless.”
Bridging the gap
Alternative forms of energy like wind or solar are critical for reducing the world’s carbon emissions. While wind and solar costs are plunging, some experts argue that nuclear power remains important because it can be turned on and off to match peaks and valleys in demand without carbon emissions.
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“We need nuclear power as a bridge toward a post-fossil-fuel future,” says coauthor Steven Chu, professor of molecular and cellular physiology and a Nobel Prize-winning physicist. “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.”
Chu, a former US secretary of energy who encouraged seawater extraction research before he left the Department of Energy, says that nuclear power currently generates 20 percent of US electricity and 13 percent worldwide. Even as researchers work to improve reactor safety and solve the waste disposal issues, he believes that a practical way to extract uranium from seawater is needed to reduce the energy insecurity of nations that depend on nuclear power but lack uranium within their own borders.
The new findings build on years of research in Japan and China, as well as by DOE scientists at Oak Ridge National Laboratory and the Pacific Northwest National Laboratory.
Dipping plastic fibers into seawater
Scientists have long known that uranium dissolved in seawater combines chemically with oxygen to form uranyl ions with a positive charge. Extracting these uranyl ions involves dipping plastic fibers containing a compound called amidoxime into seawater. The uranyl ions essentially stick to the amidoxime. When the strands become saturated, the plastic is chemically treated to free the uranyl, which then has to be refined for use in reactors just like ore from a mine.
How practical this approach is depends on three main variables: how much uranyl sticks to the fibers; how quickly ions can be captured; and how many times the fibers can be reused.
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In the recent work, researchers improved on all three variables: capacity, rate, and reuse. The key advance was to create a conductive hybrid fiber incorporating carbon and amidoxime. By sending pulses of electricity down the fiber, they altered the properties of the hybrid fiber so that more uranyl ions could be collected.
Postdoctoral scholar Chong Liu oversaw the lab tests that compared Stanford’s amidoxime-carbon hybrid fibers with today’s amidoxime fibers. First she tested how much uranyl each type of fiber could hold before reaching saturation. The findings show that by the time the standard amidoxime fiber had become saturated, the amidoxime-carbon hybrid fibers had already adsorbed nine times as much uranyl and were still not saturated.
What’s more, the electrified fiber captured three times as much uranyl during an 11-hour test using seawater from Half Moon Bay, about an hour from Stanford, and had three times the useful lifespan of the standard amidoxime.
“We have a lot of work to do still but these are big steps toward practicality,” Cui says.
Research on seawater extraction has to proceed in parallel with reactor safety and waste disposal challenges, Chu says. “For much of this century, some fraction of our electricity will need to come from sources that we can turn on and off. I believe nuclear power should be part of that mix, and assuring access to uranium is part of the solution to carbon-free energy.”
Source: Stanford University