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Uranium is a relatively common element in the earth's crust, about as common as tin or germanium and about 40 times more common than silver. Uranium is present as a trace element in many rocks, land and ocean water, but is generally only economically mined where it is present in high concentrations A growing number of higher-yield mines are remotely underground operations, such as the McArthur River mine in Canada, which alone accounts for 13% of global production.
Known global uranium resources, mineable at an arbitrary ceiling set at $130 per kg, were sufficient to last 70 to 100 years. In 2007, the OECD, assuming consumption for that period, estimated a total of 670 years of economically viable extraction for all conventional resources and phosphate ores.
Light water reactors burn fuel relatively inefficiently, using only uranium-235 which is a very rare isotope. Reprocessing can make waste reusable, and modern reactors can be more resource-efficient than older ones. With a pure fast reactor fuel cycle with consumption of all uranium and actinides, an estimated amount of uranium in contracted resources and phosphate ores for about 160 000 years at a price of $60–100/kg.
One analysis found that uranium prices could increase by two orders of magnitude between 2035 and 2100 and that there could be a shortage of uranium by the end of the century. A study by MIT and WHOI researchers found that at the current rate of consumption, the Earth's conventional global reserves of uranium could be depleted in just over a century.
The limited supply of uranium-235 could prevent substantial development with current technologies. One study found that it was uncertain whether the identified resources would be developed in time to supply continuously fuel to expanded nuclear facilities and various forms of extraction could conflict with ecological and economic barriers.
The researchers also report the considerable dependence on imported nuclear energy. However, there are also unconventional uranium resources. Uranium is naturally present in seawater at a concentration of approximately 3 micrograms per litre, with 4.4 billion tons of uranium considered to be present in seawater at any moment.
Like fossil fuels, over geological timescales, uranium extracted from seawater on a large scale would be replenished both by river erosion and by the natural process of uranium being dissolved from the surface of the ocean floor; both mechanisms keep the solubility equilibria of the seawater concentration at a stable level. Some commentators argue that this is a point in favor of considering nuclear energy as a renewable energy source.
