Here’s what I wrote in response to “Nuclear Energy’s Future: Thorium”, by Zain Nayer, Matthew Liu, Gilbert Yang
Lastly, and most importantly, a thorium reactor is at no risk of meltdown: if the reaction goes critical, the reactant expands and the reaction slows down. Therefore, they don’t require active systems to stop meltdowns.
You aren’t using ‘critical’ correctly. In a nuclear reactor, ‘critical’ means “Every fission causes an average of one more fission, leading to a fission (and power) level that is constant.” (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
In a molten salt reactor, the fuel and coolant salt are molten. [Molten-salt cooled, solid fueled reactors are a very different technology.] The salt has a low viscosity (comparable to water), so with additional heat (for example from less being removed to produce electricity) the salt naturally expands, lowering the concentration of uranium in the reactor core, slowing fission. Removing extra heat increases the concentration of uranium. This happens quickly and automatically, producing a very stable reactor, capable of precisely following electric load demands.
Instead of “they don’t require active systems to stop meltdowns”, the fuel is molten during normal operation; “nuclear meltdown” doesn’t apply. Physical meltdown of the reactor materials won’t happen either. The material used in the 1960s to make the reactor, Hastelloy-N, was rated for 1050 degrees C (wouldn’t last as long under higher temperatures), but the melting point is much higher. Modern materials would need to be certified for reactor use, and would allow even higher temperatures. Industrial processes could use 950 C (for example break water and CO2 for making vehicle fuels) still within Hastelloy-N limits. Even running at 950 C, “meltdown” is physically impossible, there is no mechanism for suddenly adding all that heat. (Light water reactors are limited to about 350 C, by the pressure needed to keep water liquid at high temperature.)
Similarly, LFTRs (or other MSRs) can’t have “loss of coolant accidents”. The boiling point of FLiBe salt is about 1400 C. Uranium, and trans-uranic elements, and most fission products, are strongly chemically bound to the salt. The coolant remains liquid, at atmospheric pressure, so there is no chance of pressure explosions. No combustible materials, no hydrogen production, nothing that chemically reacts to air or water.
If the reactor gets too hot (turbine failure or heat transfer unit failure), or if electric power is cut (by power outage, operators, automatic sensors detecting some failure, or even remote sensors detecting an earthquake), frozen salt in a cooled pipe melts and the fuel quickly drains from the reactor into cooling tanks where fission is impossible (uranium density and shape have to be precise for fission to occur). Instead of needing power for emergency control systems, MSRs need power to Prevent reactor shutdown. In the 1960s, scientists turned off a FAN (which kept that pipe cold) on Fridays to shut down the reactor! Then they left for the weekend, came back on Monday, heated the fuel until it melted, pumped it out of the cooling tanks back into the reactor, and fission resumed.
Obviously LFTRs will have much greater safety while not needing a light water reactor’s “multiple redundant” engineered systems with high pressure pipes and valves, and will not need a LWR’s hugely expensive steam containment building (LFTRs have no steam, and no pressure), and so will cost much less than any LWR, even not counting the cost reduction (and quality control) of mass production.
Keep reading, at https://molten-salt-reactor.glerner.com/