Reactors known as fast neutrons or breeders reactors do not need to slow down neutrons. With fuel enriched to 15 or 20% in uranium-235 or plutonium, it become no longer necessary to "thermalize " neutrons as in the common pressurized water reactors, for example. The major interest of fast neutrons is that they are able to extract all the fission energy contained in uranium of the earth’s crust.
Fast neutron reactors use mainly plutonium as nuclear fuel. Since they do not require any moderator, their core is 10 times more compact than those of conventional civilian reactors. Cooling is achieved with sodium or with molten lead. This cooling poses a safety problem, particularly concerning sodium, which reacts violently with air and water. But a breeder like PHENIX has been operating 40 years in France without accident, before its final shut down in 2011.
Their main interest is to generate under certain conditions more fissile material than consumed. Such reactors are called "breeders". Their principle is particularly attractive for power production, since breeders offer a virtually inexhaustible source of energy: one of the oldest fantasies of mankind , like a motor running with water ! It is estimated that one could extract with them 50 to 80 times more energy than with today ordinary reactors using slow neutrons to burn uranium-235.
Fast neutrons are also effective to burn the long-lived minor actinides of radioactive waste. With a breeder, the amount of plutonium remains in principle constant, because the fuel is regenerated. But regeneration is not mandatory. If regeneration is abandoned, it becomes also possible to burn plutonium partially or completely.
The only fast neutron reactor that supplied electricity at an industrial scale was the French breeder Superphenix. It was stopped in 1997 more for political than technical reasons. Various countries, including France, Japan and Russia are continuing research on this sector in the context of studies of fourth generation reactors.
As waste burners, fast neutron reactors may have to compete with hybrid reactors, whose principle was proposed by the Nobel Prize Carlo Rubbia in 1993. These hybrid reactors, still under consideration, which have the advantage or nuclear fuel regeneration, might be dedicated to the destruction of actinides. They would have the ability to destroy plutonium and would additionally be safe.
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