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Peak uranium is the point in time that the maximum global uranium production rate is reached. After that peak, according to Hubbert peak theory, the rate of production enters a terminal decline. While uranium is used in nuclear weapons, its primary use is for energy generation via nuclear fission of the uranium-235 isotope in a nuclear power reactor. Each kilogram of uranium-235 fissioned releases the energy equivalent of millions of times its mass in chemical reactants, as much energy as 2700 tons of coal, but uranium-235 accounts for only 0.7% of the mass of natural uranium. While Uranium-235 can be "bred" from U, a natural decay product of U present at 55 ppm in all natural uranium samples,Uranium-235 is ultimately a finite non-renewable resource. Due to the currently low price of uranium, the majority of commercial light water reactors operate on a "once through fuel cycle" which leaves virtually all the energy contained in the original U - which makes up over 99% of natural uranium - unused. Nuclear reprocessing is a technology currently used at industrial scale in France, Russia and Japan, which can recover part of that energy by producing MOX fuel or Remix Fuel for use in conventional power generating light water reactors. However, at current uranium prices, this is widely deemed uneconomical if only the "input" side is considered.
Advances in breeder reactor technology could allow the current reserves of uranium to provide power for humanity for billions of years, thus making nuclear power a sustainable energy. However, in 2010 the International Panel on Fissile Materials said "After six decades and the expenditure of the equivalent of tens of billions of dollars, the promise of breeder reactors remains largely unfulfilled and efforts to commercialize them have been steadily cut back in most countries." But in 2016, the Russian BN-800 fast-neutron breeder reactor started producing commercially at full power , joining the previous BN-600. As of 2020, the Chinese CFR-600 is under construction after the success of the China Experimental Fast Reactor, based on the BN-800. These reactors are currently generating mostly electricity rather than new fuel because the abundance and low price of mined and reprocessed uranium oxide makes breeding uneconomical, but they can switch to breed new fuel and close the cycle as needed.
The CANDU reactor which was designed to be fueled with natural uranium is capable of using spent fuel from Light Water Reactors as fuel, since it contains more fissile material than natural uranium. Research into "DUPIC" - direct use of PWR spent fuel in CANDU type reactors - is ongoing and could increase the usability of fuel without the need for reprocessing.
M. King Hubbert created his peak theory in 1956 for a variety of finite resources such as coal, oil, and natural gas. He and others since have argued that if the nuclear fuel cycle can be closed, uranium could become equivalent to renewable energy sources as concerns its availability. Breeding and nuclear reprocessing potentially would allow the extraction of the largest amount of energy from natural uranium. However, only a small amount of uranium is currently being bred into plutonium and only a small amount of fissile uranium and plutonium is being recovered from nuclear waste worldwide. Furthermore, the technologies to eliminate the waste in the nuclear fuel cycle do not yet exist. Since the nuclear fuel cycle is effectively not closed, Hubbert peak theory may be applicable.