Not an expert, but one of the big benefits of molten salt reactors is that they aren't really under pressure - no big poof if containment is breached.
It's also really easy to halt meltdowns - since the fuel and coolant are a big liquid glob, you can have thermal plugs that will melt above a certain temperature and drain the coolant+fuel into tubes small enough that, even when the small tubes are full, the coolant+fuel in the tubes is below critical mass and the reaction stops. It fails safe and is immune to mechanical failure, unlike control rods in current reactor designs that have to be moved in and out.
It’s a good system, but the main problem comes from the same source, the fact that you’re dealing with a pool of molten salt. Maintenance on a reactor vessel is never without challenges, but the problems with servicing a molten salt (or something like a Pb-Bi eutectic) are still a financial and practical roadblock to widespread adoption.
This more than anything else is likely the biggest reason these reactors never took off, they're certainly safer from meltdowns, but now instead of piping water we have to pipe an incredibly caustic molten salt! The challenge hasn't gone away, it's just shifted.
I remember reading about issues with storage of the waste from a molten salt reactor. A problem discovered was the radiation was producing fluorine gas from the metal fluorides.
I'd prefer superheated chlorine gas to superheated fluorine gas, in the same sense that I'd rather have an angry weasel in my pants than a grizzly bear, but...
It's not superheated if it's just slow emissions from waste, as mentioned above.
A pipe leak in the reactor itself doesn't create a lot of gas. It's molten salt at atmospheric pressure. It drips out and solidifies as it cools. An advantage of MSRs is that the troublesome fission products (like iodine, cesium, and strontium) don't leak out as gases, like they do from conventional reactor cores; instead they are chemically bound in the salt.
That’s both an upside (as you’ve described), and a downside because the already challenging environment of molten salt becomes increasingly radioactive making servicing even harder than the chemical and thermal environment of the salt make it. You also have to carefully monitor the salt, and a lot of the probes and other means of monitoring it tend to rapidly degrade in the molten salt.
It’s another issue that feels like the answer involves new materials that have a much longer life in situ, so that the inevitable maintence is infrequent. It’s a very promising technology, but it isn’t mature yet.
Terrestrial Energy and Thorcon use small sealed reactor cores that get replaced every few years.
Moltex uses a pool design, where everything is immersed from above in a pool of coolant salt, and can be pulled out and replaced as necessary. The actual fuel is isolated in vertical rods.
At least three companies use chloride salts. According to a presenter from Elysium, in the absence of water the chloride salt is less corrosive to stainless steel than water is.
I certainly agree that the technology isn't mature, since we don't have any production reactors yet. But we're making good progress, especially in Canada where regulation of new nuclear technology is more rational than in the U.S.
Canada is such an amazing contrast to the US in that area, true. I wish people here had a better understand of just how necessary nuclear power is if we want to survive to ever reach the hoped-for “clean energy future.”
When your coolant can go up to over 700 °C without gassing then you might also be able to cool the reactor using convection only in emergencies, which would do away with the necessity of emergency coolant pumps.
On the other hand, working with a big pool of radioactive liquid that will literally explode and catch fire if it comes in contact with water and still catches fire if it comes in contact with air may also be ... challenging.
You're thinking of sodium. This reactor uses salt, which is very stable, just like the salt on your kitchen table.
The lack of any sort of driver for chemical explosions is one of the advantages of molten salt reactors over light water reactors, in which the water can split into oxygen and hydrogen and cause explosions (as we saw at Fukushima).
That's pretty cool but the guy's conclusion is that it's not a chemical explosion, it's just from the water turning to steam. But I'll concede that we shouldn't drop a molten salt reactor in a lake.
I was indeed thinking about the NaK (sodium+potassium) alloy (which is used in some molten salt reactors, but not this one). And indeed this reactor design uses LiF (Lithiumfluoride). However, LiF-based salts have the obvious drawback of their high melting points (pure LiF 840 °C, FLiBe 460 °C).
Do you have any examples of liquid-fueled reactors using NaK?
Some MSR companies are working on fast reactors using chloride salts, including Moltex, Elysium, and Terrapower (in a project separate from their better-known sodium-cooled fast reactor).
High temperature is a property of all MSRs I know of and does have some advantages, including better thermodynamic efficiency and usefulness for process heat.
It's also really easy to halt meltdowns - since the fuel and coolant are a big liquid glob, you can have thermal plugs that will melt above a certain temperature and drain the coolant+fuel into tubes small enough that, even when the small tubes are full, the coolant+fuel in the tubes is below critical mass and the reaction stops. It fails safe and is immune to mechanical failure, unlike control rods in current reactor designs that have to be moved in and out.