Some people like the idea of nuclear power plants fueled by uranium made by thorium, that use liquid fuel consisting of molten salt with fluoride in it.
This is a topic that I know a truly unnecessary amount about.
First, molten salt reactors and using thorium are almost completely separate issues. The only connection is that molten salt reactors are one type of reactor that could potentially produce enough uranium from thorium to be self-sustaining.
Molten salt reactors can use the same fuel as current water-cooled reactors. Thorium could be used with heavy water cooled or heavy water moderated CO2 cooled reactors.
The advantages of molten salt reactors are:
- fuel in the core can be continuously chemically processed
- the core could be drained to something with passive cooling in emergencies
- some molten salts absorb relatively few neutrons
But molten salt reactors have some issues:
- the molten salt can freeze and block pipes
- fissile stuff can precipitate out of the molten salt
- the hot molten salt is corrosive
The usual responses to those problems are:
- add a heater system where needed
- don't worry about it
- there are metals that are only corroded slowly by the molten salt, so it's fine
Heater systems for pipes aren't free, and the second problem is actually a serious safety issue that needs to be carefully considered, but the biggest issue is corrosion. Just because there are metals that resist corrosion by the molten salt doesn't mean the problem is solved. Heat exchangers for power plants are already expensive, and if you make them out of expensive metal that's hard to work with, they're too expensive.
With a molten salt reactor, the core can be smaller, but then the liquid fuel needs to go through a heat exchanger. Heat exchangers are more expensive with a pressure difference, and it's better to have a pressurized core than a big pressure difference in the heat exchanger. Having liquid fuel and an external heat exchanger isn't really any better than having the core act as a heat exchanger with the working fluid.
All things considered, if you want to use thorium, it seems better to use heavy water cooled or heavy water moderated CO2 cooled ("HWGCR") reactors than reactors with molten salt fuel. HWGCR plants fell out of favor partly because test reactors had some safety issues, but their problems came from bad design that, in my opinion, was basically unrelated to them being HWGCR plants.
Thorium is another source of nuclear fuel; using it doesn't really make reactors cheaper when the actual uranium used in current reactors is a fairly small part of their cost, which it currently is. Thorium-based reactors might be cheaper than U238 breeder reactors, but they wouldn't be cheaper than current reactors, and current nuclear plants are fairly expensive, and current designs for breeder reactors are too expensive and much less safe than current nuclear power plants.
A lot of the interest in thorium power has actually come from one guy, Kirk Sorensen. That's impressive, and I like people like that. But he's way too overconfident. For example, he thought that submarine nuclear power plants could be a good way to provide electricity for cities; silly stuff like that is generally a bad sign. And his understanding of thermodynamic cycles, molten salt chemistry, and the economics of power plant construction is a bit inadequate for what he's been trying to do. But still, I can appreciate him trying to do stuff that's normally considered impractical for one guy.
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