Some designs immerse the entire core and heat exchangers into a pool of coolant, virtually eliminating the risk that inner-loop cooling will be lost.
Sodium and NaK (a eutectic sodium-potassium alloy) do not corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials.
Sodium and NaK do, however, ignite spontaneously on contact with air and react violently with water, producing hydrogen gas.
There are two proposals for a sodium cooled Gen IV LMFR, one based on oxide fuel, the other on the metal-fueled integral fast reactor.
Lead has excellent neutron properties (reflection, low absorption) and is a very potent radiation shield against gamma rays.
The high boiling point of lead provides safety advantages as it can cool the reactor efficiently even if it reaches several hundred degrees Celsius above normal operating conditions.
The melting point can be lowered by alloying the lead with bismuth, but lead-bismuth eutectic is highly corrosive to most metals[6][7] used for structural materials.
In July 1959, the Sodium Reactor Experiment suffered a serious incident involving the partial melting of 13 of 43 fuel elements and a significant release of radioactive gases.
SRE was the prototype for the Hallam Nuclear Power Facility, another sodium-cooled graphite-moderated SGR that operated in Nebraska.
Fermi 1 in Monroe County, Michigan was an experimental, liquid sodium-cooled fast breeder reactor that operated from 1963 to 1972.
It was succeeded at the same site by PFR, the Prototype Fast Reactor, which operated from 1974 to 1994 and used liquid sodium as its coolant.