Researchers at the U.S. Energy Department’s Pacific Northwest National Laboratory (PNNL) and LCW Supercritical Technologies made use of readily available acrylic fibers to pull 5 grams of yellowcake—a powdered form of uranium used to produce fuel for nuclear power reactors—from seawater.
The milestone, announced in mid-June, follows seven years of work and a roughly US $25 million investment by the federal energy agency. Another $1.15 million is being channeled to LCW as it attempts to scale up the technique for commercial use. The effort builds on work by Japanese researchers in the late 1990s and was prompted by interest in finding alternative sources of uranium for a future time when terrestrial sources are depleted.
Nuclear power plant operators increasingly want their facilities to run for up to a century, says Gary Gill, a researcher at PNNL who led the seawater extraction effort. But within decades, he says, terrestrial sources of uranium could be depleted or prove to be too expensive for use in commercial reactors.
A 2017 assessment by the World Nuclear Association says that roughly 445 reactors worldwide, with a combined 390 gigawatts of generating capacity, require around 75,000 tons of uranium oxide concentrate each year to operate.
In the United States, an expected surge of demand for uranium to fuel a fleet of new reactors largely dried up after the 2011 accident at Japan’s Fukushima nuclear power plant. More recently, low natural-gas prices have hurt the economic case for nuclear power plants, leading some developers to scrap plans for new units and a number of utility operators to consider closing existing ones.
Before it stopped reporting on U.S. uranium reserves a decade ago, the DOE’s Energy Information Administration said terrestrial uranium reserves could meet anywhere from 10 to 23 years’ worth of demand, depending on market prices. EIA pointed out at the time that domestic U.S. uranium mining supplied around 10 percent of U.S. requirements for nuclear fuel.
Unlike terrestrial sources that can be mined at specific locations, uranium in seawater shows up in concentrations of around 3.3 parts per billion. With a total volume estimated at more than 4 billion tons, there is around 500 times more uranium in seawater than in land-based sources. As a result, the widely dispersed sea-based resource could last for thousands of years, Gill says.
The Japan Atomic Energy Agency focused its work starting in 1999 on adsorbent fabric materials that could be suspended on a floating frame hung around 20 meters below the ocean’s surface. Using the technique, they collected about 1 kilogram (kg) of uranium as yellowcake.
In the PNNL-LCW collaboration, researchers developed an acrylic fiber to attract and then hold on to uranium that was dissolved in seawater. The uranium binding agent (called a “ligand” or, more generally, a functional group) is called amidoxime.
The adsorbent has a second functional group called a carboxylate. This group does not bind uranium, but instead makes the adsorbent hydrophilic, meaning it’s attractive to water. Fiber can be readily obtained from a local craft store, helping with the technique’s overall economics, says Chien Wai of LCW, which developed the extraction fabric and brought it to the lab for testing.
The process involves putting the fiber into a container with reagents such as hydroxylamine, then cooking it for up to 24 hours. Once cooked, the treated fiber is ready for use.
PNNL researchers have conducted three separate tests of the adsorbent's performance, exposing it to large volumes of seawater from Sequim Bay next to its Marine Sciences Laboratory.
For each test, the research team put 2 pounds of fiber into a tank about the size of a hot tub, pumped seawater through it to mimic ocean conditions, and waited a month. Wai’s team at LCW then extracted the uranium from the adsorbent. From these first three tests, they produced about 5 grams of yellowcake, which is roughly how much a nickel weighs.
Based on its work so far, LCW has received $1.15 million in DOE grant money to help it scale up the technology for commercial use.
For that to happen, the technology needs to be able to compete with conventional mining operations. Over the past 12 years, the market price of 1 kg of uranium has ranged from more than $330 to around $40, Gill says. Recent prices have been closer to the low end. The LCW-developed extraction method will need a price of $180 to $280/kg to be economical.
The economics could improve, Wai says, if the technique is used to extract other marketable metals and even to filter water to remove toxic metals such as lead. The researchers next plan to test the extraction technique in the Gulf of Mexico or off the coast of Florida. Warm water in either location will help the extraction process, compared with the colder waters of the Pacific Northwest where initial tests were carried out.
“The ability of the material to adsorb uranium depends on the water temperature,” Gill says. A water temperature increase of 20 to 30 degrees Fahrenheit should double the amount of uranium that can be extracted.
The extraction is unlikely to have any adverse environmental impacts, Gill says. No known biological need exists for uranium in seawater, so no animal or plant life is likely to be harmed through its removal.
“What we’re proposing,” Gill says, “is far greener than land-based mining operations.”