I highly recommend HBO's Chernobyl. Explains a lot this stuff. Basically, as a nuclear reactor operates, it is turning the Uranium into lots of other stuff. One component of this other stuff is Xenon-135, which absorbs neutrons and basically gets in the way of nuclear reactions. This is not an issue if you keep the reactor on because there are enough neutrons to keep things going and overcome the Xenon absorption. But if you turn the reactor off, you now have no neutrons from the fission reactions and lots of neutron absorbers. There is so much absorption that it's impossible to start up the reactor for a couple of days. That's why nuclear reactors operate at constant power and only turn off to refuel. This MMR also operates at constant power, which would also have benefits for cycling issues, and uses the molten salt loop to store heat and then use it when it's needed.
Scott Manley made a more condensed form of the explanation of the nuclear physics involved here: https://www.youtube.com/watch?v=q3d3rzFTrLg (~21 minutes). It's an excellent explanation of the processes.
The HBO show is also an excellent drama and worth a watch.
When a reactor is "poisoned" with Xenon, the Xenon absorbs a large fraction of neutrons that otherwise would cause fission.
If you don't want to wait for the Xenon to decay you could construct a reactor that has a larger control swing than you would need otherwise -- for instance you could put in more and denser control rods. In that case, however, you need to have a lot of "excess reactivity" and also a lot of neutrons lost in the control rods under normal use, which in turn means the "neutron efficiency" is worse. (If you're not planning to breed Pu239 or U233 maybe you don't care)
Note the Xenon concentration can vary in different parts of the reactor so you have to manage the "oscillations" in space just as you do in time. Not a catastrophe, but definitely a hassle.
You can't shut it off on a whim, but lots of reactors can drop to half output or one third output without much trouble. It probably doesn't make a big difference on the current grid, but if you have a significant fraction of nuclear power then the ability becomes useful. (I don't know if those mechanisms are applicable to this design.)
My read is that it's somehow the inverse of a flywheel. A flywheel means consistent output for variable input whereas this is consistent input leading to variable output.
That's my understanding as well. Rather than ramping the power output up or down with demand, you keep it at a constant power level and basically use the molten salt loop as a battery -> store energy when you're outputting more than is required, draw energy from it when you need more that you are producing.
The multi-gigawatt nuclear installations are typically supplemented by natural gas "peaker" plants to deal with higher demand than the reactor output, but lose any extra power produced.
If I recall correctly, the naval reactors (submarine and surface vessels) are the only ones that are designed to rapidly shift their power output.
> The multi-gigawatt nuclear installations are typically supplemented by natural gas "peaker" plants to deal with higher demand than the reactor output, but lose any extra power produced.
In France, the load variation is directly handled by the nuclear plants themselves[1], even if there are also some natural gas running (and most of the time their output is more regular than the one of the nuclear plants, so their must be drawbacks to big load variations on a gas plant as well).
