Many people do not realize that seawater has a natural concentration of uranium. The percentage of uranium in seawater is quite low, as one may expect. It has been shown that the uranium concentration of seawater is only about 3 parts per billion, which is about 3 milligrams of uranium per cubic meter.  The total volume of the oceans is about 1.37 billion cubic kilometers, so there is a total of about 4.5 billion tons of uranium in seawater. Assuming we could recover half of this resource, this much uranium could support 6,500 years of nuclear capacity. 
Shortly after World War II, recovery of seawater by ion-exchange resins was being considered.  It was deemed more economically viable to focus on exploitation of known uranium ores, though. It was later determined that an economically acceptable method of uranium extraction from seawater may be found, which has prompted more research in the area.
To extract uranium from seawater, people are using organic and inorganic absorbents.  For extraction to be successful, the extractant must work efficiently at the normal pH level and ionic strength of seawater. The extractant must also be nearly insoluble. 
A group from Japan Atomic Energy Research Institute designed a fabric absorbent to extract uranium from seawater in 2002.  They prepared a polymeric absorbent in a nonwoven fabric, which contained an amidoxime group that was capable of forming a complex with uranyl tricarbonate ions, which is the type of group required to yield a maximum absorption rate of uranium.  They then submerged an absorption cage that had a 16 square meter cross-sectional area cage that was 16 centimeters in height into the Pacific Ocean. The cage had 144 stacks of the nonwoven fabric, with each stack consisting of 120 sheets of the fabric. Over the next two years, a total of 450 submersion days, the group extracted 1083 grams of uranium, with an average absorption rate of 0.00133 grams per day per stack. 
Is seawater extraction economically viable? Japanese research suggests the lowest possible cost to extract uranium is 25,000 Yen per kilogram of uranium.  At the current exchange rate (March 2012, 1USD ~ 81 JPY), that equates to about 300 USD per kilogram of uranium. This is about 3 times more than the current price of uranium, and it is expected that the actual recovery price would be about 10 times the current price of uranium. 
Seawater naturally has a concentration of about 3 milligrams of uranium per cubic meter, and some people have become very excited about the vast amount of uranium in the ocean. Methods of extracting this uranium has mostly been worked on by the Japanese, who have come up with a method of extraction which would cost about 300 USD per kilogram of uranium. This is about 3 times the current price of uranium, but we may one day need this uranium to solve the worlds energy needs.
The extraction of uranium from sea water has also been discussed Chan. 
© Ken Ferguson. 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.
 N. Seko et al, "Aquaculture of Uranium in Seawater by a Fabric-Adsorbent Submerged System." Nucl. Technol. 144, 274 (2003).
 N. F. Lane et al., "Powerful Partnerships: The Federal Role in International Cooperation on Energy Innovation," Office of the President of the United States, June 1999.
 R. V. Davies et al., "Extraction of Uranium from Sea Water" Nature 203, 1110 (1964).
 M. Tamada, et al., "Cost Estimation of Uranium Recovery from Seawater with System of Braid Type Adsorbent," Trans. Atomic Energy Soc. Jpn. 5, 358 (2006) [in Japanese].
 B. Chan, "Amidoxime Uranium Extraction From Seawater," Physics 241, Stanford University, Winter 2011.