Design
In practice, all liquid metal cooled reactors are fast neutron reactors, and to date most fast neutron reactors have been liquid metal cooled fast breeder reactors (LMFBRs), or naval propulsion units. The liquid metals used typically need good heat transfer characteristics. Fast neutron reactor cores tend to generate a lot of heat in a small space when compared to reactors of other classes. A low neutron absorption is desirable in any reactor coolant, but especially important for a fast reactor, as the good neutron economy of a fast reactor is one of its main advantages. Since slower neutrons are more easily absorbed, the coolant should ideally have a low moderation of neutrons. It is also important that the coolant does not cause excessive corrosion of the structural materials, and that its melting and boiling points are suitable for the reactor's operating temperature.
Ideally the coolant should never boil as that would make it more likely to leak out of the system, resulting in a loss-of-coolant accident. Conversely, if the coolant can be prevented from boiling this allows the pressure in the cooling system to remain at neutral levels, and this dramatically reduces the probability of an accident. Some designs immerse the entire reactor and heat exchangers into a pool of coolant, virtually eliminating the risk that inner-loop cooling will be lost.
Read more about this topic: Liquid Metal Cooled Reactor
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