More neutrons are produced when 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 designs built to this date use liquid metals as coolants, such as the sodium-cooled fast reactor and the lead-cooled fast reactor. As the boiling points of these metals are 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 substantially lower waste management requirement.[1]
In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum for these reasons.