thermal reactor. In practice, this means significantly higher enrichment: >20% enrichment in a fast reactor compared to <5% enrichment in typical thermal reactors. Alternatively, a mixture of plutonium from nuclear waste, combined with natural or depleted uranium could be used. History US interest in breeder reactors were muted by Jimmy Carter's April 1977 decision to defer construction of breeders in the US due to proliferation concerns, and the suboptimal operating record of France's Superphénix reactor. The French reactors also met with serious opposition of environmentalist groups, who regarded these as very dangerous. Despite such setbacks, a number of countries still invest in the fast reactor technology. Around 25 reactors have been built since the 1970s, accumulating over 400 reactor years of experience. A 2008 IAEA proposal for a Fast Reactor Knowledge Preservation System noted that: during the past 15 years there has been stagnation in the development of fast reactors in the industrialized countries that were involved, earlier, in intensive development of this area. All studies on fast reactors have been stopped in countries such as Germany, Italy, the United Kingdom and the United States of America and the only work being carried out is related to the decommissioning of fast reactors. Many specialists who were involved in the studies and development work in this area in these countries have already retired or are close to retirement. In countries such as France, Japan and the Russian Federation that are still actively pursuing the evolution of fast reactor technology, the situation is aggravated by the lack of young scientists and engineers moving into this branch of nuclear power. As of 2021, Russia operates two fast reactors on commercial scale. The GEN IV initiative, an international working group on new reactor designs has proposed six new reactor types, three of which would operate with a fast spectrum. List of fast reactors Decommissioned reactors United States Clementine was the first fast reactor, built in 1946 at Los Alamos National Laboratory. It used plutonium metal fuel, mercury coolant, achieved 25 kW thermal and used for research, especially as a fast neutron source. Experimental Breeder Reactor I (EBR-I) at Argonne West, now Idaho National Laboratory, near Arco, Idaho, in 1951 became the first reactor to generate significant amounts of power. Decommissioned in 1964. Fermi 1 near Detroit was a prototype fast breeder reactor that powered up in 1957 and shut down in 1972. Experimental Breeder