The way that nuclear power is used in the early twenty-first century can be compared to a person who burns all the matches in the house to get warm because he can't be bothered going out to get wood from the back shed. The time comes when he has plenty of wood but no way to set it alight.
Most nuclear power stations use only about 1% of the energy available from the uranium that they use as fuel. This is not only a waste of a finite resource; it is a betrayal of our responsibility to future generations.
If humanity mines all the easily available uranium, runs it through our terribly inefficient reactors, uses 1% of the available power and then disposes of the remainder, it means that the other 99% of the energy becomes unavailable. On the other hand, if we use it efficiently, get 100% of the available power out of it, the resource will last much longer.
Natural uranium consists mainly of two varieties, or isotopes; 99.3% is 238U and 0.7% is 235U. Current nuclear power reactors use (fission) the 235U and a very little of the 238U, but technology is available to get nearly 100% of the energy from the uranium; fast neutron reactors are capable of fissioning both isotopes. 235U is needed to start all nuclear reactors; it can be compared to the match that sets fire to the wood.
Another radioactive element, thorium, while not fissile itself, can be converted into fissile 233U in a nuclear reactor. (Uranium 233 does not occur in nature.) There is estimated to be three times as much thorium in the earth's crust than uranium. If Man 'burns' all the 235U without using it to convert the thorium to uranium that will also be a resource lost to future generations.
Summing up then, present reactors use 1% of the available energy from uranium and none of the energy from thorium, which is three times more abundant than uranium. We are using 0.25% of the available energy now, because it is less trouble than using the resources efficiently, and possibly denying the use of the remaining 99.75% of the energy available from uranium and thorium to future generations.
References and further readingWikipedia has an article on thorium.
Look up fast neutron reactors in Wikipedia. Also an article in Scientific American, Dec. 2005, "Smarter use of nuclear waste", described fast neutron reactors combined with pyrometallurgical recycling of the fuel.