Extracting Uranium

With the signing of the Paris Agreement in April 2016, world leaders have begun to tackle climate change by pledging to invest in alternative energy sources, with the United States alone pledging three billion dollars to the Green Climate Fund. Organizations like the American Council On Renewable Energy have lobbied for the expansion of solar, wind, hydroelectric, and geothermal power, but there is one potent source of energy that has flown under the radar. Nuclear energy, you may be surprised to know, provides over 11% of the world’s electricity, and 65 nuclear power plants are currently under construction worldwide.

However, one of the issues with nuclear energy is that it requires a radioactive material source, such as uranium or plutonium. The issue with uranium, and nuclear energy as a whole, is that power plants need to be closely monitored, because small malfunctions can have catastrophic effects (take the nuclear meltdown in Chernobyl in the 1980s as an extreme but all-too-real example).

Nuclear power plants work by taking pellets of a radioactive isotope, such as Uranium-235, and bombarding them with neutrons. When neutrons collide with atoms of Uranium-235, the uranium atom breaks down in a process called nuclear fission. Fission produces a large amount of heat, and this heat is used in the power plant to boil large quantities of water in order to power a steam turbine. Nuclear energy is extremely efficient – since only a single, one-inch pellet of uranium can provide the same amount of energy as a ton of coal.

Only a single, one-inch pellet of uranium can provide the same amount of energy as a ton of coal.

With current quantities of terrestrial uranium, we would be able to generate nuclear power for about 100 years. Moreover, scientists have discovered another source that is rich in uranium, and chances are you’ve probably gone swimming in it. That’s right, oceans are the largest source of uranium on this planet, containing about four billion tons of it. That would be enough uranium to power the world for the next 10,000 years.

Extracting this uranium, however, was easier said than done until the 1990s, when the Japan Atomic Energy Association synthesized materials that would absorb uranium in seawater. The researchers used a coated polymer that binds to uranium, however, the original polymer was not very cost-effective. Consequently, scientists in the U.S. worked on developing a cheaper polymer that would extract uranium equally well or even more efficiently. Just this year, a team at Oak Ridge National Laboratory in Tennessee announced that they had developed a material that absorbs uranium at a cost three to four times cheaper than before.

The material that the Oak Ridge scientists used to design the adsorbent was a polymer called polyethylene. The scientists synthesized polyethylene into long fibers, and then coated the braids with a chemical called amidoxime. When the polyethylene-amidoxime braids are deployed in seawater, the water flows through the fibers. Any molecule that is not uranium flows right past the fibers and doesn’t react, but uranium reacts with amidoxime to produce uranium oxide, which binds to the polyethylene. Scientists then remove the uranium-oxide coated fibers and place them in an acid solution, which breaks up the uranium oxide into free-floating uranyl ions, which can be isolated and enriched. As an additional bonus, the fibers are reusable, so this process can be repeated many times without incurring the cost of continuously synthesizing new devices. The uranium is then ready to be sent to nuclear power plants.

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monofilament.nylon_
This monofilament polymer is similar in appearance to the polyethylene fiber used to extract uranium.

The scientists determined that one kilogram of polyethylene fiber is able to hold 5.2 g of uranium, and recent experiments with different absorbents have yielded up to 6 g of uranium per kilogram of fiber. This may not sound like much, but these quantities are much higher than the previous extraction rate of 1.5 g of uranium per kilogram of absorbent, and, as formerly noted, pellet of uranium can produce the same amount of energy as a ton of coal. However, one of the potential downsides of this process is that it took 56 days for the scientists to extract this amount of uranium. At that rate, it would be hard to compete with the current terrestrial extraction rate of around 60,000 tons of uranium per year.

While this seawater method is not yet ready to compete with current terrestrial extraction methods, it is important to note that current terrestrial sources of uranium will be depleted in around 100 years.

However, there is plenty of global opposition to nuclear power, especially considering the myriad safety risks involved. Due to the high amount of energy generated from fission reactions, nuclear reactor meltdowns are a possibility. Research focused on power plant safety may need to be done to convince the public that nuclear is a safe, viable alternative to nonrenewable fuels. Furthermore, another issue that researchers have found with the seawater extraction method is that marine life hinder uranium absorption. Researchers found that the presence of biomass and microbes decreased uranium adsorption by 30%. Additionally, the maximum absorption rates were observed when the fibers were placed blow the photic zone, where there is no sunlight – this could increase the cost of extraction, and may potentially impact marine ecosystems. More work is required in these areas.

Nonetheless, don’t be surprised if, 100 years from now, you see polymeric fibers floating in the ocean, helping to power the entire world.

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