The advantage of nuclear power in producing lower carbon emissions holds true only as long as supplies of rich uranium last. When the leaner ores are used - that is, ores consisting of less than 0.01 percent (for soft rocks such as sandstone) and 0.02 percent (for hard rocks such as granite), so much energy is required by the milling process that the total quantity of fossil fuels needed for nuclear fission is greater than would be needed if those fuels were used directly to generate electricity. In other words, when it is forced to use ore of around this quality or worse, nuclear power begins to slip into a negative energy balance: more energy goes in than comes out, and more carbon dioxide is produced by nuclear power than by the fossil-fuel alternatives.15
There is doubtless some rich uranium ore still to be discovered, and yet exhaustive worldwide exploration has been done, and the evaluation by Storm van Leeuwen and Smith of the energy balances at every stage of the nuclear cycle has given us a summary. There is enough usable uranium ore in the ground to sustain the present trivial rate of consumption - a mere 2 1/2 percent of all the world's final energy demand - and to fulfil its waste-management obligations, for around 45 years. However, to make a difference - to make a real contribution to postponing or mitigating the coming energy winter - nuclear energy would have to supply the energy needed for (say) the whole of the world's electricity supply. It could do so - but there are deep uncertainties as to how long this could be sustained. The best estimate (pretending for a moment that all the needed nuclear power stations could be built at the same time and without delay) is that the global demand for electricity could be supplied from nuclear power for about six years, with margins for error of about two years either way. Or perhaps it could be more ambitious than that: it could supply all the energy needed for an entire (hydrogen- fuelled) transport system. It could keep this up for some three years (with the same margin for error) before it ran out of rich ore and the energy balance turned negative.16
http://www.feasta.org/documents/energy/nuclear_power.htm