While discussion of the world energy outlook often centers on oil, uranium should not be ignored. Although nuclear power only accounted for 5.5% of world electricity as of 2007, uranium's supply and demand dynamics could nevertheless have a significant effect on energy availability in the future. [1] According to the World Energy Council, nuclear power is the only non-greenhouse gas emitting source that can be justified economically, noting that if "the climate change threat becomes a reality, nuclear is the only existing power technology which could replace coal in baseload". [2] Attempting to project the supply and demand dynamics for uranium into the distant future, however, is largely speculative. Possible outcomes range from no deficits in a low demand case to extreme deficits in a high demand case, with key variables to consider including demand levels, uranium price levels, the discovery/confirmation of additional uranium supplies, and technological advances. [2] While current uranium supplies only come from terrestrial sources, additional supplies could come also come from another source: the earths oceans. Concentrations are estimated at only at 3 parts per billion, but the sheer volume of the Earth's oceans-1.37 x 109 cubic kilometers- implies a potential supply of 4.5 billion tons. [3] Much work has been done in recent decades to develop the technology necessary to take advantage of these supplies. One currently promising line of research attempts to use amidoxime, a chemical functional group, to capture uranium from seawater.
Although efforts to develop amidoxime methods for uranium extraction from seawater have largely occurred in Japan, the first study on the extraction of uranium from seawater occurred in Great Britain in 1953. [4] Efforts began spreading to Japan in the 1960's, with activity in China, Germany, France, the United States, and USSR beginning the 1970's. Efforts to approach the challenge had to deal with several serious problems. First, the incredibly low concentration of uranium in seawater (3 ppb) meant that extremely large amounts of water had to be processed to get meaningful amounts of uranium concentrate. [3] Thus, many small-scale laboratory methods of chemical separation were rendered impractical. On the other hand, appropriately scalable methods had to ensure that the equipment and solvents involved were durable enough to handle processing such large volumes of water. Second, extraction methods had to ensure that other ions present in seawater at exponentially higher concentrations, such as sodium, magnesium, calcium, and bromine, did not interfere with the process. [5] As early as 1964, it was found that hydrous titanium oxide had particularly absorbent properties. However, prototypes of this technology built in Japan during the 1980's failed to produce practical results. Absorption did occur, but not at rates economical enough for mass production. A search ensued for a better absorbent material, which ultimately uncovered a chemical functional group called amidoxime that showed promise. [3] However, finding an actual form within which to deploy amidoxime proved difficult. Likewise, an amidoxime fiber developed through chemical reactions failed to prove practical; the fiber lacked the intrinsic strength necessary to operate in ocean or real-world conditions. To overcome this challenge, a technique called "graft polymerization" was used. By using an electron beam and chemical processes, scientists were able to attach amidoxime groups to polyethylene fabric, a mechanically strong polymer. By selectively attaching amidoxime groups, the new fabric was able to possess both the absorbent properties of amidoxime and the strength of polyethylene that would enable it to survive real-world conditions. [3] The search then began for a delivery system. One initial iteration attempted to use "absorbent stacks." In this method, the amidoxime-infused fabric was cut into thin sheets, which were then stacked 120 at a time into absorbent stacks. Then, groups of 144 absorbent stacks would be assembled into absorption beds, which would then be suspended, 3 at a time, from a floating frame anchored in the middle of the ocean. These experiments, finished in 2001, demonstrated that over a 30 day period, this method could capture 0.5 grams of uranium per kilogram of absorbent, a 5-fold improvement over titanium oxide methods of earlier decades. However, additional analysis revealed that 40% of the cost of the collections stemmed from the mechanical elements of the collection system, including the frame and absorption beds. [3] In a subsequent design, seaweed-like braids were constructed from the amidoxime-infused polyethylene fabric. Rather than being suspended from the surface, these braids were attached to anchors on the ocean floor, in a configuration resembling seaweed. Over 30 days, this method captured 1.5 grams of uranium per kilogram of absorbent. However, increased water temperatures in the location where this testing was conducted skewed results upwards; adjusting for this, aborption rates in this case were twice as good as using the 'stacks' method. [3]
For the braid method, Japanese scientists were able to perform cost estimates, using assumed absorption rates of 4 grams per kilogram of absorbent over a 60-day period, which would be consistent with the maximum levels of absorption witnessed during testing. If braids can be reused 8 times, as is currently possible, costs would amount roughly to $177/pound of uranium (32,000 Yen per kilogram). [3] Assuming that braid construction will allow for 18 reuses, this would allow for costs to fall to $138/pound of uranium (25,000 Yen per kilogram). [3] As it currently stands, these costs are more than double current spot prices for uranium in U3O8 form, which stood at $66.50/pound as of March 2011. Hence, this method may be marginally viable at best currently. [6] For example, Dr. Masao Tanada of the Japanese Atomic Energy Agency has been attempting to secure funding for a 400 square mile 'uranium farm' that would supply a sixth of Japan''s uranium requirements each year, but has largely been unsuccessful. [7] However, these seawater extraction price estimates are notable improvements over past decades, when costs would have been orders of magnitude higher. Given continued technological improvements, it is not inconceivable that amidoxime extraction from seawater may become a meaningful source of uranium in the near future. It is notable that the amidoxime method, in its current form, can only be practically implemented in a small subset of locations. The absorption rates of uranium are extremely sensitive to seawater temperatures, and thus it would be unprofitable to use the method in waters colder than 15 degrees Celsius. Since water temperature drops as distance from the equator increases, and as depth increases, it is only practical to use current techniques at depths of less than 100 meters in the latitude band ranging from 35 degrees South and 45 degrees North. [8]
It is likely that future improvements could come from many directions. Optimizing real-world implementations of current techniques may yield to reasonable gains; as noted from the experiments involving both the absorbent stacks and braids, absorption rates are positively correlated with both sea water temperatures and volatile sea conditions, with an increase of 10 degrees Celsius alone resulting in absorbency increasing by a factor of 1.5. [3, 4] It is also possible that location choice can be optimized by selecting areas with maximum uranium concentration in the surrounding water; it has already been noted that higher seawater salinity levels have a positive correlation with higher uranium levels. [4] Decreasing costs may come from an improvement of the delivery system for amidoxime, that enables more efficient physical contact with seawater, or the discovery of a better chemical absorbent. Amidoxime can likely be improved upon, since it does not specialize in capturing Uranium, and instead just has a general affinity for any "any toxic metal". [9] Finally, decreased costs may also come from cheaper manufacturing of absorbents and associated equipment, as well as from cheaper processes for isolating uranium from the uranium compounds harvested from the ocean. Finally, while this discussion has focused on amidoxime methods for extraction, research is currently ongoing into other methods, and these may very well prove to be more cost-effective than even improved amidoxime techniques.
In conclusion, the most promising method for extracting uranium from seawater is to use braids constructed from polyethylene fabric infused with amidoxime absorbents. Currently, this technique result in costs that are more than double spot uranium prices, but it is not inconceivable that this technique may become economically viable in the near future. Given current research, there are many areas for possible future improvements to occur. It may ultimately even be surpassed by another completely different method for harvesting the ocean's vast uranium resources, but at the moment amidoxime extraction method appears to be very promising.
© Bryan Chan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "International Energy Outlook 2010," US Energy Information Administration.
[2] "Analysis of the Uranium Supply to 2050," International Atomic Energy Agency, STI/PUB/1104, May 2001.
[3] L. Rao, "Recent International R&D Activities in the Extraction of Uranium from Seawater," Lawrence Berkeley National Laboratory, 5 Jan 11.
[4] R. Khamizov, D. M. Muraview and A. Warshawsky, "Recovery of Valuable Mineral Components from Seawater by Ion-Exchange and Sorption Methods," in Ion Exchange and Solvent Extraction: A Series of Advances, Vol. 12, ed. by J. A. Marinsky and Y. Marcus (CRC Press, 1995).
[5] M. Kanno, "Present Status of Study on Extraction of Uranium from Sea Water," J. Nucl. Sci. Technol. 21, 1 (1984).
[6] D. Winning, "Rally in Uranium Will Resume," Barron's, 12 March 11.
[7] K. S. Parthasarathy, "Technologies to Harvest Uranium From the Sea," The Hindu, 12 Nov 09.
[8] A. M. Macfarlane and M. Miller, "Nuclear Energy and Uranium Resources," Elements, 3, 3 (2007).
[9] N. Seko, M. Tamada and F. Yoshii, "Current Status of Adsorbent for Metal Ions with Radiation Grafting and Crosslinking Techniques," in Nuclear Instruments and Methods in Physics Research 236 1-4 (2005).