Employing uranium in light-water reactors of conventional design in which essentially only 235U is used (up to 0.7% in natural uranium) and where about 1000 MWd/kg 235U are attained means that 4 x 106 tonnes uranium correspond to ca. 690 x 1012 kwhr. If this uranium were to be fully exploited using fast breeder reactors, then this value could be very considerably increased, namely to ca. 80000 x 1012 kwhr. An additional 44000 x 1012 kwhr could be obtained if the aforementioned thorium reserves were to be employed in breeder reactors. The significance of the fast breeder reactors can be readily appreciated from these figures. They operate by synthesizing the fissionable 239Pu from the nonfissionable nuclide 238U (main constituent of natural uranium, abundance 99.3%) by means of neutron capture. 238U is not fissionable using thermal neutrons. In the same way fissionable 233U can be synthesized from 232Th.