

LCW Supercritical Technologies

Pacific Northwest National Laboratory

Take some regular old acrylic knitting yarn, modify it with a special kind of adsorbent chemical, and wave it around in the ocean. After some time, the yarn will pick up enough molecules of uranium that grams of yellowcake, the precursor to fuel used in nuclear reactors, can be made.

Of course, that description is a little reductive. The tricky part is the second step: finding an affordable adsorbent material that will attract uranium in a marine environment and then relinquish it so that it can be processed into nuclear fuel. Researchers from the Pacific Northwest National Laboratory (PNNL) in Washington state say they've done that, and they suspect that their method might be approaching an economically viable point.

But why should we even try to pull uranium from the sea? Well, the sea is full of uranium—it's estimated to be 500 times more plentiful on Earth than gold, according to the World Nuclear Association. But on land, it's pretty labor- and carbon-intensive to recover. Penn State's Department of Earth and Mineral Engineering says that in the US, for every 2,000 pounds of uranium ore that's mined, just three to 10 pounds of that will contain uranium. Uranium milled from ore is called yellowcake, technically known as uranium oxide (U 3 O 8 ). The uranium oxide has to undergo further refinement to be used as a fuel, but once it's properly processed, it can generate a significant amount of greenhouse-gas-free energy.

If collecting uranium from the sea reaches cost-parity with mining uranium ore, we could potentially unlock a significant resource whose extraction is theoretically less environmentally damaging than uranium mining on land.

Heading for warmer waters

This latest advancement in the technology was announced by PNNL last week. Although the chemical that adsorbs the uranyl molecules from the ocean is proprietary, Gary Gill, a researcher on the project, said it was an amidoxime-based polymeric material.

The latest test was done at the Sequim Marine Sciences Laboratory, where researchers put two pounds of treated yarn in a tub through which seawater was circulated for about a month. From that small test, the team was able to recover around five grams (about the weight of a nickel) of yellowcake.

Of course, the safety of the material is a major concern: nanoparticles in sunscreen have recently been implicated in creating toxic environments for sea life, and it's easy to imagine that some other kind of adsorbent material could do the same. But Gill said the researchers conducted previous tests with this uranium-collecting material to make sure it wasn't toxic to ocean life, and the researchers have not yet found any negative interactions. Gill and others published a 2016 paper on the topic , which also touched on whether removal of uranium from an ocean environment would create negative consequences for sea life. So far, the results are encouraging.

The process of modifying yarn with this uranium-selective material was spearheaded by LCW Supercritical Technologies, a company founded by University of Idaho professor Chien Wai and funded by the Department of Energy. In a press release from PNNL, Wai said that waste yarn could be used (many of us have whole drawers we could volunteer) and once the adsorbent material is joined with the yarn, the net can be reused—that is, caught uranium can be discharged from the netting.

Although the process can only catch very small amounts of uranium at a time, it's inexpensive enough that, according to PNNL, "an analysis of the technology suggests that it could be competitive with the cost of uranium produced through land-based mining." More specifically, Gill said that at seawater temperatures of 20°C (68°F), they believed this method could cost $168 ±41 per pound. But in much warmer water, the adsorbent works more effectively, so trawling for uranium in, say, the Gulf of Mexico could cost more like $105 ±23 per pound.

Incidentally, the team is currently applying for funding for another test, which it hopes to conduct in the Gulf of Mexico.

While the cost of producing yellowcake on land is generally not made public by companies, and uranium is not traded on an open market, but independent market consultants have published very low spot prices for uranium lately. But if a nuclear renaissance occurs due to efforts to respond to climate change, and uranium prices rise, then recovering uranium from the sea might not be a bad option.

Gill also noted that the system could be deployed in most places with sea access. "The adsorbent process relies on currents in the surface ocean to deliver uranium to the adsorbent," Gill said. "Only very mild currents are required, which exist in most ocean areas. Envision a field of kelp-like structures, with currents slowly passing through them."

"Uranium is fairly evenly distributed in the ocean" and "varies in direct proportion to the salinity of the ocean," Gill added. "Since salinity in the ocean varies only marginally, so does the uranium concentration."

In PNNL's press release, Wai said that the treated yarn can gather other heavy metals as well: it can be used to extract vanadium for flow batteries, and it could be used to remove less-desirable metals that poison some waterways.