Uranium ore is a mix of two chemically identical isotopes, U-238 (99.3%) and U-235 (0.7%). The main one, U-238, is faintly radioactive and very rarely an atom disintegrates or “fissions” spontaneously and spits out 2 or 3 hot neutrons as it does so. These cool off to be a seed, captured by an atom of the other isotope, U-235, which promptly fissions, releasing lots of energy and 2-3 more neutrons. With sufficient U-235 nearby, this leads to a chain reaction which can power a nuclear reactor.
Burning one tonne of U-235 gives enough energy to generate a 1000 MegaWatts of electricity for a year, a total output of 1 GigaWatt-year-electricity (1GWy-e). This is pretty much the output of a large modern PWR running with a 90% load factor. To find out how many GWy-e are possible we need only know the total mineable resource on the planet to calculate the number of tonnes of U-235. It is only 161,000t according to the IAEA.
It is not a good idea to separate out a whole tonne of U-235 in one place since the critical mass for a nuclear explosion is only 15kg. So, the PWR uses 20t of fuel rods enriched to only 5% U-235 and 95% U-238 as an annual fuel load.
Uranium-238 has its own trick. It too can absorb a cool neutron to become U-239. This is highly unstable and, in 23min, turns into an isotope of a new element, Plutonium-239. This is a more energetic fissile fuel than U-235, with a critical mass of 5kg. It is made and partly burned in every PWR, to be thrown away and buried in the Open cycle.
How many reactors do we need to match the primary energy from Coal? World Coal now delivers 2000 GWy-e of energy for electricity, steel, cement and chemical by-products [BP, 2014], but total demands are likely to grow by 40-50% as world population rises to 9Bn. A more appropriate target for nuclear power would be 3500 GWy-e by 2050. Coal generators typically produce ~500MW-e with a load factor of 50%, with a total ~6400 generators. Most of these may be swapped for nuclear as they retire before 2050. The coal industry will of course resist this. BP shows that the total world coal resources would last only 113 years if burned at the current rate, putting 2.7Tn tonnes of CO2 into the atmosphere.
BP 2014: Statistical Review of World Energy, 2014: http://www.bp.com/content/dam/bp/pdf/statistical-review/statistical-review-of-world-energy-full-report-2014.pdf
What are the 'Fission Fragments' and are they radioactive?
Can we make new U-235?
The Ap-1000 is a big reactor for base load. How might Small Modular Reactors (SMRs)?
Does breeding make any other Plutonium or Uranium isotopes?
Could CCS be deployed, if it worked, as fast as the IPCC predictions require?
FOR 1 GWy-e