The best method works like this: A polymer substrate—basically, plastic—is irradiated, and then chemicals with an affinity for uranium are grafted onto it. The material is woven into 60-meter-long braids, and these are then brought out by boat to water at least 100 meters deep. The braids are chained to the ocean floor and allowed to float passively in the water, like an artificial kelp forest. After about 60 days, the boat returns and pulls in the adsorbent materials—now sporting a healthy yellow tint from the uranium. The plastic is then brought back to shore, and the uranium is eluted off.
“You get between 2 and 4 grams of uranium sticking to this stuff per kilogram of plastic,” says Erich Schneider, a nuclear engineer at the University of Texas at Austin. “That doesn’t sound like a lot, but it all adds up.”
Schneider presented a promising economic analysis of this system at the recent American Chemical Society conference, in Philadelphia. If the adsorbent can manage only 2 grams of uranium per kilogram of plastic, and each braid is reused six times (with a 5 percent drop in performance each time)—parameters that have been achieved in the real world by Japanese researchers—then the cost is US $1230 per kilogram of uranium, about a factor of 10 more expensive than traditional mining.
Costas Tsouris, a chemical engineer at Oak Ridge National Laboratory, in Tennessee, does marine testing of the newest uranium extraction materials and says his group has already seen a doubling of the 2 grams per kilogram of adsorbent in real-world trials. That rate “is the maximum observed so far,” he says.
Schneider adds that 6 grams per kilogram is well within reach, and reusing the braids 18 times instead of six is likely also on the horizon. The easiest way to bring down costs would be to find cheaper chemicals for the adsorbent preparation process. For example, dimethylformamide is used to wash the polymers before they go into the ocean and represents as much as 10 percent of the total system cost. Researchers are looking for a cheaper but similarly effective chemical. Increasing the surface area of the polymer to allow more uranium to stick to it would also bring costs down, as would harvesting and selling some of the other elements—such as vanadium—that inevitably join uranium on the braids. The result of such improvements would mean lowering the cost to about $300 per kilogram of nuclear fuel, which puts it in the upper range of mined uranium spot prices over the past decade.
To get 5500 kilograms of uranium from this process in a year, Schneider says “you would need—get ready for this—a million tons of plastic per year. That’s a lot of plastic.” The eventual goal is to make the polymers completely recyclable, but a million tons of plastic is daunting nonetheless. And previous ideas, such as pumping the seawater past an adsorbent and genetically engineering seaweed to absorb uranium, have all stalled.
Japan had been looking at using cloth dipped in persimmon juice to lower costs of extracting uranium and vanadium from seawater.
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