One desperate snag is that sodium is solid at room temperature, so if you have to shut down the reactor and have no auxiiary heating system, you’re pretty well screwed with every pipe and pump jammed up solid. This happened to a Russian submarine.
The Chernobyl accident occurred while they were simulating a shut down of the electrical grid … Wikipedia’s article seems fairly correct, but take it with a grain of salt.
You are thinking of aqueous solutions of ammonia and water, or ammonium hydroxide. Often written as NH4OH, or NH4+(OH-), or simply NH3(aq). It can be a strong caustic with high enough ammonia concentration.
NH3 is a gas at room temperature and pressure. If you keep it dry, it will work as a good refrigerant (like Freon). It has an unfortunate characteristic of a bad smell, which is one reason Freon was developed; to compete with ammonia (early refrigerators and freezers were ammonia based).
Say rather that it has a very fortunate characteristic of a bad smell, since it smells bad enough to get people out of the area before they’re poisoned by it.
Cold warehouses use ammonia as a coolant. I was once at a supermarket chilled distribution centre when an alarm went off. We were escorted outside in double quick time and had to stand around for hours while guys in full haz kit did their stuff.
Has mercury ever been tried? Seems like it could work.
Water is still mainly used as best coolant
I think ammonia is used as refrigerant, which is slightly different. A coolant just carries heat from a hotter place to a cooler place. A refrigerant is something you compress, then allow it to expand or vaporize in order to cool something down.
I’m not sure what temperatures are used, but mercury has a much lower boiling point, and high vapor pressures below that point. I’ve accidentally boiled mercury. But never sodium.
There may be materials incompatibilities with both metals. Sodium likes to dump electrons into everything, and mercury will form liquid amalgams with some other metals.
What are the drawbacks to ammonia as a general coolant? Smells bad? I really don’t know.
It’s poisonous and it can explode.
Ammonia also boils at -33 C.
Apart from that though?
A number of automotive waterless coolants:
The engineers at work used to laugh about the effort going into the “terrorist” threat. A bad industrial accident in our refrigeration plant would have wiped out our entire suburb, and on the historical record seemed much more likely.
The ultimate answer is “you need to take a chemistry class”, since coolants are chosen on simple physics and complex chemistry.
The physical properties (melting point, boiling point, latent heat, specific heat, viscosity) you can get from a table in your engineering handbook (or now, from the www).
The chemical properties you get by using specialist cooling fluids sold by specialist cooling fluid companies.
Or you use water, which isn’t best, but it’s common.
It most certainly has: see: http://www.douglas-self.com/MUSEUM/POWER/mercury/mercury.htm
It is theoretically useful in a topping cycle to improve power station efficiency, but the cost, toxicity, and other problems make it a technological nightmare. It was however given a fair trial in at least three US power stations.