14 September 2012—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.”
And there is uncertainty. The most recent joint study of uranium supplies from the Nuclear Energy Agency and the International Atomic Energy Agency found that we have enough for about 100 years of nuclear power, “based on current requirements.” The cost of uranium production is increasing, however, and nuclear power could expand as much as 99 percent by 2035, an obvious strain on fuel supplies.
“Pushing the seawater-extraction methods into the viable range could basically solve that problem. And that $1230 per kilogram number is probably overstated, even with today’s technology. 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.
Importantly, though, there would be substantial challenges to actually scaling up seawater uranium production toward usable levels. 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.
Still, the progress toward viable economic and energy returns is intriguing, and such measures don’t even account for the substantial environmental impact of traditional uranium mining. Controversy is always present when uranium mining is nearby: It has a long history of harm to Navajo lands in the southwestern United States, and President Obama recently made a disputed decision to shut down uranium mining near the Grand Canyon. Putting plastic into the ocean is not without its environmental impact either, of course, but it is hard to imagine how seawater extraction wouldn’t be a cleaner alternative to mining nuclear fuel. With nuclear power set to expand rapidly, and terrestrial fuel resources on the decline, the oceans may well end up as our uranium supplier of the future.
About the Author
Dave Levitan is a science journalist who contributes regularly to IEEE Spectrum’s Energywise blog. Earlier this month, he reported on two new measures of the limits of wind power.