Next time you go to the beach, think about this: You’re swimming in nuclear fuel. Our oceans contain an estimated 4.5 billion metric tons of uranium, diluted down to a minuscule 3.3 parts per billion. The idea of extracting uranium from seawater has been kicking around for decades now, but the materials and processes to do so may finally be economically viable.
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.
Schneider also estimated an energy return on investment (EROI). This asks the simple question, For every unit of energy we put into harvesting this material, how many units of energy do we get out? The answer, again with those basic parameters and a standard nuclear reactor in use today, is 22. This is promising but far from competitive. Schneider says traditional uranium mining and milling has an EROI of about 490.
“We’re not intending to develop a technology that will compete with conventional uranium mining and milling as it is done today,” Schneider says. “The purpose is really to establish the technology as an economic backstop. There’s a thousand times more uranium in seawater than in all the known reserves of conventional uranium, so it’s a huge resource. The idea here is to take some of the uncertainty out of the picture.”