Before about 1960 it was thought that a high breeding ratio was the most important quality of a fast reactor. To achieve this the mean energy of the neutrons has to be kept high, and this requires that extraneous materials, especially moderators, should be excluded as far as possible from the reactor core. As a result the early reactors had metal fuel, the metal being either enriched uranium or plutonium, alloyed in some cases with molybdenum to stabilise it to allow opera­tion at higher temperature.

The critical masses of these reactors were small and the cores were correspondingly small so that for high-power operation they had to be cooled by a high-density coolant (to avoid impossibly high coolant velocities). Hydrogenous substances were precluded because hydro­gen is a moderator, so liquid metals were used. In most cases the coolant was sodium or sodium-potassium alloy. Some early experi­mental reactors were cooled with mercury but this fell out of favour because of its toxicity, cost, and low boiling point.

The many neutrons that leaked from the small cores of these reactors were absorbed in surrounding regions of natural or depleted uranium where the majority of the breeding took place. These were known as breeders, or blankets.

The first generation of low-power experimental fast reactors were built in the late 1940s and early 1950s to demonstrate the principle of breeding and to obtain nuclear data. They included CLEMENTINE and EBR-I in the United States, BR-1 and BR-2 in the Union of Soviet Socialist Republics, and ZEPHYR and ZEUS in the United King­dom. CLEMENTINE, ZEPHYR, and BR-1 and 2 used plutonium fuel, which in the early years was more readily available than highly enriched uranium. Apart from ZEUS, which was a zero-power mock — up of the later DFR, they all had very small cores, the largest being EBR-I (6 litres), which was a small power reactor with an output of 1.2 MW.

When it came to higher powers, however, the volume of the core had to be increased to keep the heat fluxes down to reasonable levels and to allow for the extra coolant flow. The result was EBR-II and EFFBR (the Enrico Fermi Fast Breeder Reactor) in the United States, and DFR (the Dounreay Fast Reactor) in the United Kingdom. When they were designed they were seen as prototypes of the reactors to be used in power stations, but as they were built it began to be recognised that they would be the end-point of the development of metal-fuelled fast reactors, and the principal use to which DFR and, for many years, EBR-II were put was to test oxide fuel for the next generation.