Current reactors (almost all Light Water Reactors, LWR) use solid fuel in complex fuel rod assemblies, most with water cooling. If water isn’t there to cool, fuel rods melt. LWR materials can’t handle the hottest the reactor could get if cooling fails, e.g. zirconium fuel rods oxidize at ~500°C, only 45% above normal temperature of ~350°C, releasing hydrogen from water, and further heating the fuel rods. At Fukushima, the hydrogen then exploded, spewing radioactive fission products into the air. If the LWR fuel rods get still hotter, the rods would melt, and the fuel could melt through the reactor vessel, letting fission products into the ground water and ocean.
Steel reactor vessel and water cooling weren’t the best materials and cooling, just the best known, for finding out if controlled fission for power generation would work. Also, the first reactors of this type were for submarine propulsion, where water and steel made a lot of sense (small, well below the limits of steel, simple design; scaling up to commercial scale and maximizing temperatures for more electricity production is what required the increased complexity). The nuclear engineers and reactor designers thought commercial reactors should have better materials, better design. (For example, the patent holder on PWR, the type of reactor that includes LWR, ran Oak Ridge National Laboratories during the Molten Salt Reactor Experiment.)
The fuel in a Molten Salt Reactor such as LFTR, is molten (a liquid, with no water), under normal operation. The fuel is strongly chemically bonded to a salt coolant, that doesn’t evaporate (the salt won’t boil below ~1400°C, the reactor operating temperature likely between 600°-950°C).
Uranium molten in liquid fluoride salt is chemically stable (unlike sodium coolant in some types of reactors, sodium reacts explosively with air or water). Molten Salt Reactors operate very close to 1 Atmosphere of pressure, there is no high pressure to make a pressure explosion.
Materials in LFTR or any other MSR, would be designed to safely handle the hottest the reactor could possibly get, in normal or emergency situations.
If a LFTR (or any Molten Salt Reactor) somehow overheats, a frozen plug melts and fuel drains harmlessly into passive cooling tanks, where further nuclear reaction is impossible, by the geometry of the tanks. (Later the fuel can be re-heated and pumped back into the reactor, and the nuclear reaction re-starts.) The cooling tanks also would be made of materials that handle the hottest the fuel could get, and cool quickly to air or earth, no water or electricity or operator action needed.
Beyond the obvious “the fuel is already molten”, the fuel can’t melt through the reactor vessel and put radioactive material in the environment.
Even if the reactor vessel is damaged (e.g. terrorist bomb, or earthquake) the fuel would simply spill out (no high pressures, so no pressure explosion like LWR) and the fuel would remain in the salt. The fuel density would remain the same (unlike LWR fuel that would melt out of the fuel rods and become more dense), and would spread out and rapidly cool to solid.
No fission. No water needed. No water spreading radioactive material. Too dense to be carried by air, doesn’t dissolve well in water, doesn’t interact with water.
Fuel and most fission products are chemically bound to the salt. Fission products that are gasses (krypton and xenon, mainly) get continuously collected, since they just bubble out of the molten salt. Therefore, there are minimal amounts of these in the reactor. The reactor would be continuously fueled, so there is just enough uranium in the reactor to maintain fission, much less than in LWR (which usually has about 1/3 of the fuel rods replaced every 18 months).
Everything dangerous in LWR with “loss of coolant” or “hydrogen explosion”, is simply not possible in any Molten Salt Reactor.
Thorium LFTR – a large quantity of melted material at circa 700 oC would exist in such apparatus when in operation, especially if there is a surrounding blanket region for transmuting Thorium to fissile Uranium 233 (wherein U233 is extractable via use of Fluorine gas); Fluorine is a very dangerous gas to have about, esepcially in accident conditions. [MSR would have very little flourine gas, just in equipment converting U to UF4, easy to contain in fluorine absorbing materials; fluoride salts are chemically very stable.] If a major leaks occurs, for example failure of a heat exchanger, fracture of a pipe, major corrosion leak (yes – these things do happen in practice !), such molten material is hardly benign, especially when it is emitting hard Gamma radiation. [MSR materials are selected to not corrode in fluoride salts. Leaks from whatever cause are not under pressure, cool quickly to stable solids. Gamma radiation inside a nuclear reactor isn’t a problem; multiple layers of radiation shielding.] To say that accidents will not occur with MSR and Thorium LFTR is just fairytale, as one would expect from someone who has never been involved in real engineering.
There are lots of Thorium LFTR “enthusiasts” about, who lack experience and who are making dangerous recommendations to politicians and planners. Fact is, as now being pursued by Germany and other countries, that renewables actually produce electrical power more cheaply than nukes [“nukes” meaning Light Water Reactors, and excluding the costs of the coal plants when the solar or wind plants don’t produce], and without the environmental risks. MSR and Thorium LFTR are really only useful for transmuting existing insane stockpiles of highly radioative nuclear waste into something more benign. [Why ignore MSR in locations where there aren’t LWR waste stockpiles? Use MSR if you don’t want to pay over 10x more for “renewable only” power (with no fossil fuel backup): solar/wind requires massive energy storage systems, much more land, mining, manufacturing, installation. Solar/wind for peak power is cheap when it is generated; solar/wind for 24/7/365 is very expensive.]
Failure of a heat exchanger: the reactor fission rate self-adjusts, by thermal expansion, to match the decreased heat removal, and the equipment the heat is no longer being transferred to stops producing. We replace the heat exchanger. (This isn’t LWR, where loss of heat exchange systems leads quickly to destruction of the reactor and breach of reactor vessel.)
Fluorine gas: There would not be pure fluorine, but UF4, UF6, HF, etc. Much less reactive. Yes, of course we protect against leaks. And this is standard chemical processes. We know how to build and operate this equipment safely, and the nuclear industry has a better safety record than other industries using this equipment. Enclose the fission product handling equipment each in an appropriate chemical containment vessel, with automatic sensors and reporting.
Major corrosion leak: Not with periodic inspections and maintenance; this isn’t the coal industry. Plus, for every salt that could be used in a molten salt reactor, there are metals that it doesn’t corrode; so you’re even less likely to get a “major corrosion” problem. And if there is a leak, all types of radiation are contained, by the equipment housing or reactor vessel, and an inner room radiation shield, and the reactor building, and several meters of cement or compacted dirt — it’s not hard to have more radiation protection than is needed. Remember, MSR has no high pressure, no flammable materials in most designs (graphite in some designs is not very flammable), no chemically explosive materials; leaks are easier to handle than in most industrial chemical plants.
You have been writing like you believe engineers haven’t thought of all these things, or engineers can’t protect against simple problems like this. The bigger risk is stupid management making cost-cutting inspection and maintenance reductions.
(The earthquake and tsunami didn’t destroy Fukushima-Daiichi; management not putting diesel backup power generators above the flood level, like all other nuclear operators had, destroyed it. Or they could have built adequate sea wall, like all other nuclear operators. Or they could have arranged for emergency generators to be flown in and connected any time there was an earthquake above 6.0; they didn’t. They could have quickly tested reactors at low power, found that at least one was completely undamaged, and run one to power the cooling systems. )
We can’t engineer against a really determined idiot. We can make MSR so the worst mistakes that can happen, have less impact than the worst mistakes of LWR, which are less than the worst mistakes of many other industries. One advantage of MSR is it can be fully assembled in factories where best practices are strictly followed, leaving fewer places for idiots to mess it up.
Keep thinking of things that could go wrong, and ways to prevent and to correct them. Communicate them to reactor designers, inspectors, management.
But you should give up your belief that everyone doing anything with Molten Salt Reactors is “just fairytale”, has “never been involved in real engineering”, “is quite irresponsible”, is with a “religious cult”. There are good engineers, looking how to make safer and less expensive power than any we have currently available.
>We can’t engineer against a really determined idiot.
The unfortunate truth is many determined idiots exist. You said yourself, had all the appropriate protocols been implemented, Fukushima-Daiichi would not have been destroyed.
Why assume that LFTRs are immune from mismanagement? The likelihood of failure in an ideal reactor is irrelevant. The consequences of failure in one inevitably faulty reactor need to be small.
[George’s comment: We need to keep watching all forms of power generation. Too many solar installers die; too many coal ash ponds pollute the water system. So look at what the realistic failures of MSR are — with no high pressure, and most fission products chemically bound to the salt, and the salt quickly cools to solid, and reactor materials that withstand the fuel salt highest temperature — and see if Molten Salt Reactors are much safer than Light Water Reactors, than coal plants, than natural gas pipelines, than trains with volatile oil that explodes.]
[This is not a Fukushima blog. “It’s so bad” comments deleted. — George] The Japanese government wants to restart many nuclear reactors in Japan based on BWR with solid fuel elements. This is utter insanity ! If the Japanese must have nuclear reactors, they must be LFTR or other MSR design for improved safety. Insanity reigns in the nuclear industry, that should have used Thorium LFTR from the outset, if nuclear power were to be employed.
However, experience in the future will show eventually that LENR would have been the very best route to safe power generation, but would not have acceptable to the military establishment as P239 is not generated by LENR.
[Restarting LWR in Japan isn’t “insanity”, TEPCO poor maintenance caused Fukushima, not the design of LWR. Other LWR were undamaged, or minor damage, and can be used.
Pu239 for weapons can’t be generated by LWR either. Power generation reactors create several isotopes of plutonium, that would make any bomb “tend to pre-detonate” (i.e. in their lab). Weapons grade plutonium needs to be made in specialty reactors, with strict timing. U238 to Pu239 breeding was preferred in power generation reactors because of the higher breeding ratio (vs Th to U233) in thermal spectrum, when making more fuel was essential. LENR didn’t have even theoretical possibility until decades later. — George]