U.S. scientists say they're making progress towards a 40-year-old dream of extracting uranium for nuclear power from seawater.

"Estimates indicate that the oceans are a mother lode of uranium, with far more uranium dissolved in seawater than in all the known terrestrial deposits that can be mined," researcher Robin D. Rogers of the University of Alabama told a meeting of the American Chemical Society in Philadelphia.

"The difficulty has always been that the concentration is just very, very low, making the cost of extraction high. But we are gaining on that challenge."

An economic analysis done for the U.S. Department of Energy comparing seawater extraction of uranium to traditional ore mining shows DOE-funded technology now can extract about twice as much uranium from seawater as the first methods, developed in Japan in the late 1990s.

That brings production costs down to around $300 per pound of uranium, from a cost of $560 per pound using the Japanese technology, although extraction from seawater remains about five times more expensive than uranium mined from the ground, researchers said.

However, the researchers said, the current goal is not to make seawater extraction as economical as terrestrial mining but to establish uranium from the ocean as a sort of "economic backstop" that will ensure there will be enough uranium to sustain nuclear power through the 21st century and beyond.

"This uncertainty around whether there's enough terrestrial uranium is impacting the decision-making in the industry, because it's hard to make long-term research and development or deployment decisions in the face of big uncertainties about the resource," Erich A Schneider of the University of Texas at Austin said. "So if we can tap into uranium from seawater, we can remove that uncertainty."