Sodium is used as coolant for Uranium to Plutonium breeder reactors.
Sodium is heavy enough that the neutrons released from fissions are not thermalized too quickly, thereby giving the Uranium 238 a better chance of absorbing a neutron and being converted to Plutonium 239. (These breeder reactors are referred to as Intermediate Reactors, vs. Thermal reactors - and both terms refer to neutron energy levels.)
Uranium fissioning releases, on average, 243 neutrons per 100 fissions - and when at power, steady state - you want every “life cycle” to have 100 fissions for every 100 fissions in the previous life cycle, and you want as many of the remaining 143 neutrons to be absorbed by U-238 to create Pu-239. Realistically, there is leakage, neutron capture by other materials - but to optimize reactor design - you want to maximize fuel breeding.
For this article - Thorium is an easier breeder at Thermal neutron energy levels - so water moderator is okay. Other fluids are okay, if they have suitable heat conduction, minimal corrosion characteristics, etc.
(US civilian reactors were never used for weapons production of Plutonium. Some Plutonium production reactors use Graphite as the moderator - not as efficient as sodium, but far better than water. The Soviets used graphite moderated reactors in the civilian realm, but diverted Plutonium produced to weapons use. Think Chernobyl!)
Mike
Good summary. Thank you.