A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain decisive advantages over the existing fleet of water cooled and water moderated reactors. These are: More neutrons are produced when a fission occurs, resulting from the absorption of a fast neutron, than the comparable process with slow (thermal, or moderated) neutrons. Thus, criticality is easier to attain than with slower neutrons. All fast reactor design built to this date use liquid metals as coolant, such as the sodium fast reactor and the Lead-cooled fast reactor. As the boiling points of these metals is very high, the pressure in the reactor can be maintained at a low level, which improves safety considerably. As temperatures in the core can also be substantially higher than in a water cooled design, such reactors have a greater thermodynamic efficiency; a larger percentage of the heat generated is turned into usable electricity. Atoms heavier than uranium have a much greater chance of fission with a fast neutron, than with a thermal one. This means that the inventory of heavier atoms in the nuclear waste stream, for example Curium, is greatly reduced, leading to a substantial lower waste management requirement. In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum for these reasons. In order to describe the properties of a fast reactor design, an overview of neutron moderated reactor properties is first needed. Fission processes Fast reactors operate by the fission of uranium and other heavy atoms, similar to thermal reactors. However, there are crucial differences, arising from the fact that by far most commercial nuclear reactors use a moderator, and fast reactors do not. Moderators in conventional nuclear reactors Natural uranium