From wikipedia: At the present use rate, there are 50 years left of known uranium-235 reserves at the current extraction price per kilogram. Given that the cost of fuel is a minor cost factor for fission power, more expensive, more difficult to extract sources of uranium could be used in the future. For example, doubling the price of uranium, which would have only little effect on the overall cost of nuclear power, would increase reserves to at least 200 years. To put this in perspective; a doubling in the cost of natural uranium would increase the total cost of nuclear power by 5%. On the other hand, if the price of natural gas was doubled, the cost of gas-fired power would increase by about 60%. Another alternative would be to use thorium as fission fuel. Thorium is three times more abundant in the Earth crust than uranium, and much more of the thorium can be used (or, more precisely, converted into Uranium-233 and then used).
Current light water reactors burn the nuclear fuel poorly, leading to energy waste. Nuclear reprocessing or burning the fuel better using different reactor designs would reduce the amount of waste material generated and allow better use the available resources.
there are many reactor designs. The molten salt reactor has been built in the 1960's and 1970's and would be 99% efficient with the nuclear fuel