In contrast to fossil fuel the fuel in nuclear reactors cannot be converted “in one go” since the fuel undergoes changes during its use in the reactor which require the fuel elements to be exchanged. The fuel which has not been spent and the generated plutonium may be recovered by reprocessing the removed fuel elements. Light water reactors have a burnup of 45,000 to 50,000 MWd/t of uranium. This means that about 45 to 50 kg of fissionable material per tonne of nuclear fuel used have been fissioned and 360 to 400 million kWh of electricity have been generated at a nuclear power plant efficiency of 34%.