The total abundance of Uranium in the Earth's crust is estimated to be approximately 40 trillion tonnes. The Rossing mine in Nambia mines Uranium at an Ore concentration of 300 ppm at an energy cost 500 times less than the energy it delivers with current thermal-spectrum reactors. If the energy cost increases in inverse proportion to the Ore concentration, shales and phosphates, with a Uranium abundance of 10 - 20 ppm, could be mined with an energy gain of 16 - 32. If deep burn reactors are developed and used where all of the nuclear fuel is used then 20 times more power would be generated from the same amount of metal.
Uranium Distribution in the Earth's Crust
The following table is from Deffeyes & MacGregor, "World Uranium resources" Scientific American, Vol 242, No 1, January 1980, pp. 66-76.
UPDATE: Corrected the calculations and estimates.
If all of the 2 ppm fuel was able to be mined for higher energy return then the energy cost of mining then about 20 trillion tons is accessible. And then about quadruple that by including thorium. The earth's crust has 6 ppm of Thorium and 2 ppm of Uranium. Some deep burn reactor approaches such as fusion/fission hybrids do not require any enrichment. Any uranium is usable not just uranium 235.
80 trillion tons times 950 gigawatt days/ton times 24 billion watt/hours per GWd.
1750 billion trillion kilowatthours.
World net electricity generation nearly doubles in the IEO2008 reference case, from about 17.3 trillion kilowatthours in 2005 to 24.4 trillion kilowatts in 2015 and 33.3 trillion kilowatthours in 2030.
100 times current world electricity usage for 1 billion years.
Advanced nuclear (deep burn 99.9% usage of fuel) can last for billions of years at 100 times the energy usage rate we have now.
This chart assumes only 10% of the Uranium in seawater is recoverable.
Power Plant: Fuel Burnup [GWd/t U]
Thermal energy produced in the nuclear power plant from 1 t metric tonne of enriched uranium contained in the nuclear fuel. It ranges between 40 and 43.4 GWd/t U for pressurized water reators (PWR), and 33 and 40 GWd/t U for boiling water reactors (BWR). GWd stands for Gigawatt-days, 1 GWd = 24 million kilowatt-hours.
If only 1% of the deuterium in world's oceans – equivalent to 1040 atoms of deuterium – is used to produce tritium, this would be equivalent to using up all the world's fossil fuel reserves 500,000 times. So fusion of all deuterium is 50 million times the world's fossil fuel reserves. The world's fossil fuel reserves are about 100-200 years of suppply. So nuclear fusion of all the ocean's deuterium is 5 billion to 10 billion years of energy. There are other kinds of fusion reactions that are possible.