LFTR fuel is molten; “melt down” is standard operation. Fuel circulates through the reactor, and gets completely fissioned. No expensive fuel rod fabrication. No storing spent fuel rods in cooling ponds. No catastrophic fuel melting out of fuel rods. No risk of spewing radioactive material into the atmosphere if fuel rods overheat.
Fission byproducts are continuously and easily removed from molten fuel, for 99+% fuel consumption. (Byproducts trapped in fuel rods in a solid-fuel reactor, stop the nuclear reaction with <2% of the fuel used; the fuel rod then has to be replaced.)
In any emergency, a frozen plug melts and the fuel quickly drains out of the core into tanks where nuclear fission is physically impossible. Radiation is contained by materials that remain solid at temperatures much higher than inside the reactor, with passive air cooling. This safety feature is only possible with molten fuel. (In solid-fueled reactors, you have to override everything that normally happens in the core and bring in coolant.)
Even if an accident blocks the fuel from draining (e.g. earthquake or sabotage crimps the pipe), the reactor vessel materials can handle the hottest the fuel can possibly get, unlike with LWR.
In LWR, if there is a “loss of coolant accident”, the coolant is gone, the fuel rods melt, the careful spacing of the fuel is gone, the fuel is now more dense than the reactor was designed for. The fuel temperature gets hotter than normal, but the temperature in the fuel pellets is normally much hotter than the reactor vessel can handle.
In MSR, the fuel is molten in the salt, and chemically bound to the salt, the same temperature as the salt, and the salt can’t boil away — the fuel doesn’t get more dense except by thermal expansion/contraction (if it gets hotter it gets less dense), strongly regulating fission rate. No catastrophe possible.
LWR gets refueled about every 18 months (about 1/3 of fuel rods replaced). MSR would likely have fuel added hourly, daily, or weekly (except for designs with no refueling or chemical processing, seal it and replace in 30 years).
Since there is no need for a lot of “excess” fuel above what is needed to maintain fission, there is less fuel in the reactor when any accident occurs. Fission products that are gasses (e.g. krypton) get continuously removed and stored. There is far less radioactive material that could be released in an accident than in LWR, and there is no high pressure to push that material away from the reactor.
LFTRs Are Cooled by Stable Salts, Not Water
LFTRs are cooled by salts that remain liquid, even up to ~1400° C, so they operate at atmospheric pressure — no massive high-pressure containment structures are needed.
Almost all safety problems with current nuclear reactors can be traced to high-pressure water coolant. (Water-cooled reactors have up to 150 atmospheres pressure, to keep water a liquid under high temperatures, so need thick steel walls and massive reinforced-concrete buildings to contain water explosively converting to radioactive steam in an accident.)
LFTRs use No water, so can be built where water is scarce, using passive air cooling.
The salt used is very chemically stable, doesn’t react with materials in the reactor, or with air or water in an accident. The salt is essentially impervious to radiation damage, and doesn’t absorb neutrons (which would affect the rate of fission).
The fluoride salt chemically bonds with most of the more dangerous fission products, so in any accident they remain trapped in the salt. For example, radioactive cesium and iodine that were released in Fukushima-Daiichi would not be released in a LFTR accident.
There are a few sodium-cooled reactors. Sodium reacts violently to water. A broken pipe weld in a heat transfer unit heavily damaged the Monju Nuclear Power Plant. Molten Salt Reactors would not have this risk.