idobox wrote:And there aren't any simple compounds that are liquid in this range of temperature that contain a lot of carbon or hydrogen?
Sure, but radiation and temperature will unavoidably create some complexer, larger molecules, some of them solid or highly viscous. And some of those will foul your systems, get deposited everywhere, get into moving parts. A nuclear reactor is a silly amount of pipework, pumps and valves, and it's a pain to keep that running realiably. Even a water-based reactor already has lots of systems to keep the water clean and pure enough.
idobox wrote:Have people considered dissolving moderators in non-moderating fluid?
That's basically a molten salt reactor. The salt is a mixture of fluoride of the fuel, with berylliumfluoride or lithiumfluroide as moderator. The you create a geometry such that the mixture is only critical in the reactor zone, and you pump it around. There was an experimental twist on those in the Netherlands in the 1970s, with little spheres of thorium suspended in water. One big downside of such systems is that the operating fluid is highly radioactive and toxic, so it's an incredible pain to do maintenance on your systems.
An alternative is to use molten salt only as coolant. So less radioactivity otuside of the reactor zone, but you lose the attraction of the thorium-cycle. And the suitable salts are still very chemically reactive stuff, that eat away metals etc.
Or adding something in the water than increases the boiling temperature, to allow lower pressure operation?
Not sure about this. That would basically be a kind of thick salt water sludge, right? At temperatures of more than a few hundred degrees, I guess that would be basically molten salt again.
Oil is not expensive (compared to the cost of a nuclear power plant), and the better moderation implies smaller cores, so less coolant.
Yeah, cost of the material is a factor, but not an unsurmountable one. I've read that the heavy water in Canadian reactors costs something like 20% of the entire power plant.
idobox wrote:And why were so many early reactors or piles using graphite?
Again, it's a solid, even at very high temperatures. You can shape it, it adds to the structural integrity of the core. Often it simply is the structural material of the core. That's hardly something from the past, most high-temperature designs proposed for the future are also based on graphite moderation, with gas or salt to cool.