Nuclear reactors are of different types, based on the nature of technology deployed for extracting the nuclear energy. Though the first classification is based on whether the energy released is due to nuclear fission or fusion, at present, only fission reactors are found to be practically viable. Of the different types of fission reactors, one major classification is based on the type of coolant used: light water ( \(\text {H}_2\text {O}\) ), heavy water ( \(\text {D}_2\text {O}\) ), gas, or liquid metals. Fission reactors can also be classified based on the energy of the neutrons used for producing fission. Thermal reactorsThermal reactor are those which use a moderator to slow down the neutron speed to increase the probability of fission. The average neutron energy that causes the self-sustaining fission reactions is low in thermal reactorsThermal reactor (around 0.025 eV), whereas fast reactors use highly energetic neutrons (around 10 keV). It may be noted that the probability of fission reaction, known in the nuclear parlance as fission cross section and measured in barns ( \(1\,\text {barn} = 10^{-23}\,\text {cm}^2\) ), is higher in the case of less energetic neutrons. Those reactors which use sodium as the coolant operate in the fast region and are generally known as SFR (Sodium Cooled Fast Reactors). When the fission is caused by fast neutrons, the number of neutrons released per fission is high. Hence, more neutrons are available in the core over and above those required for maintaining the chain reaction. This amount of excess neutrons can be used for breeding more fuel (converting fertile nuclides to fissile nuclides) than is consumed. Hence the term breeder reactors. However fast reactor needs high enrichment of fissile material to overcome the low fission cross section due to fast neutrons.