Proven fossil and nuclear reserves in 1997 were as shown in table 2.3 [21].

Note that if nuclear energy were to rise to 40% of the total energy pro­duction, the reserves, which are estimated presently at 5 million tons of natural uranium at market rates,[2] would decrease to 20 years in the PWR case and to 1000 years in the breeding case. In this last case it would, however, become cost effective to exploit very low grade ores, such as oceanic uranium, so that reserves would be much larger. It would also be

possible to use oceanic uranium, which amounts to nearly 3 billion tons, in non-breeding reactors provided a 50% cost increase of the electricity produced were acceptable [22].

One should take the above reserve estimates with some caution since exploration efforts seem to level off when the estimated reserves amount to about 40 to 50 years. For example, in the case of oil, current reserves have constantly increased since, at least, 1940. However, a recent careful study [23] shows evidence for a decrease of the real estimated reserves starting around 1980. This study predicts a decrease in oil production by 2010.

Taking into account the large reserves of coal, unconventional gas and oil, it seems that fossil fuels could be available at an adequate level during the 21st century. The main limitation to their use will rather, probably, be related to the greenhouse effect.