Sodium has great thermal conductivity and runs at high power density.
Fast reactors need a large load of fuel (often high enrichment) to attain a critical mass. High power density helps pay for the fuel. It also means the reactor is smaller and the capital cost goes down compared to, say, a lead cooled reactor.
If you get fuel damage the most biologically dangerous fission product is iodine. The iodine reacts with the coolant to form NI salt, that salt dissolves in the sodium. Dangerous iodine isotopes decay in a few weeks. An experimental reactor melted down in the suburbs of LA in the 1950s and they never saw the iodine because it stayed put and it decayed in place.
Sodium reactors can run at high temperatures compared to water reactors. In the 1970s it was assumed that sodium reactors were attached to steam turbines and it was assumed fast reactors would cost more than thermal reactors, even though the performance of the steam turbine improves at high temperature.
Modern thinking is that a closed-cycle gas turbine is 10% the size of a steam turbine and the same for the heat exchangers so a high temperature reactor could beat the LWR for capital cost and be competitive with other power sources. A sodium reactor is a good match for a CCGT.
I can tell you know this but just to clarify, sodium and lead don't moderate the neutrons like water does (i.e. slow them down), so you can have a fast reactor, which means you can fission your U238 and transuranics instead of throwing them away as nuclear waste.
You can run a water reactor with a much faster spectrum if you have more fuel and less water.
Shippingport was able to breed on the Thorium-U233 cycle.
Plutonium breeding could also be accomplished with a water reactor, possibly with two separate reactors in the fuel cycle to tune up the use of odd and even numbered isotopes. See
The trouble with it is that water has limited ability to remove heat so you are going to have a large amount of fuel tied up creating a critical mass producing relatively little water. That makes it hard to build up the fuel inventory for a fleet of breeders and economics are even worse than today's water reactors.
Lead does slow down sufficiently fast neutrons, by inelastic nuclear scattering. But this has a threshold (0.57 MeV); below that energy it hardly affects neutron energy at all.
Fast reactors need a large load of fuel (often high enrichment) to attain a critical mass. High power density helps pay for the fuel. It also means the reactor is smaller and the capital cost goes down compared to, say, a lead cooled reactor.
If you get fuel damage the most biologically dangerous fission product is iodine. The iodine reacts with the coolant to form NI salt, that salt dissolves in the sodium. Dangerous iodine isotopes decay in a few weeks. An experimental reactor melted down in the suburbs of LA in the 1950s and they never saw the iodine because it stayed put and it decayed in place.
Sodium reactors can run at high temperatures compared to water reactors. In the 1970s it was assumed that sodium reactors were attached to steam turbines and it was assumed fast reactors would cost more than thermal reactors, even though the performance of the steam turbine improves at high temperature.
Modern thinking is that a closed-cycle gas turbine is 10% the size of a steam turbine and the same for the heat exchangers so a high temperature reactor could beat the LWR for capital cost and be competitive with other power sources. A sodium reactor is a good match for a CCGT.