With breeders, this is extended to 8,500 years. If one is willing to pay $300/kg for uranium, there is a vast quantity available in the ocean. It is worth noting that since fuel cost only amounts to a small fraction of nuclear energy total cost per kWh, and raw uranium price also constitutes a small fraction of total fuel costs, such an increase on uranium prices wouldn't involve a very significant increase in the total cost per kWh produced. In 1983, physicist Bernard Cohen proposed that uranium is effectively inexhaustible, and could therefore be considered a renewable source of energy. He claims that fast breeder reactors, fueled by naturally replenished uranium extracted from seawater, could supply energy at least as long as the sun's expected remaining lifespan of five billion years. While uranium is a finite mineral resource within the earth, the hydrogen in the sun is finite too – thus, if the resource of nuclear fuel can last over such time scales, as Cohen contends, then nuclear energy is every bit as sustainable as solar power or any other source of energy, in terms of sustainability over the time scale of life surviving on this planet. His paper assumes extraction of uranium from seawater at the rate of per year of uranium. The current demand for uranium is near per year; however, the use of breeder reactors means that uranium would be used at least 60 times more efficiently than today. A nuclear engineer writing for American Energy Independence in 2004 believes that there is a several hundred years' supply of recoverable uranium even for standard reactors. For breeder reactors, "it is essentially infinite". All the following references claim that the supply is far more than demand. Therefore, they believe that uranium will not deplete in the foreseeable future. The IAEA estimates that using only known reserves at the current rate of demand and assuming a once-through nuclear cycle that there is enough uranium for at least 100 years. However, if all primary known reserves, secondary reserves, undiscovered and unconventional sources of uranium are used, uranium will be depleted in 47,000 years. Kenneth S. Deffeyes estimates that if one can accept ore one tenth as rich then the supply of available uranium increased 300 times. His paper shows that uranium concentration in ores is log-normal distributed. There is relatively little high-grade uranium and a large supply of very low grade uranium. Ernest J. Moniz, a professor at the Massachusetts Institute of Technology and the former United States Secretary of Energy, testified in 2009 that an abundance of uranium had put into question plans to reprocess spent nuclear fuel.