[datenwolf]

There are also physical limitations with load following. In a fission reactor you're dealing with a steady state of influx of fission products (by the primary nuclear reaction) and the delayed decay of those products. In fact a large portion of the heat generated by a fission reactor comes from the delayed decay of the fission products. Furthermore some of the fission products are very efficient neutron catchers; notably Xe135 which is so efficient in catching neutrons, that its buildup in a reactor is called "xenon poisoning"; the accumulation of neutron catchers in a fission reaction is called "neutron poisoning".

What this comes down to is, when you ramp down a fission reactor, there's some internal inertia in its internal workings, that will actively prevent it for some time from being able to ramped up in a safely manner. The quicker the shutdown, the larger the amount of neutron poisoning and the longer you have to wait before ramping it up again. This leaves you with a nice second order differential equation coupling the power output modulation factor with the period of that power modulation.

The period of the power modulation is 24h, following the daily load swings, so for a given reactor that gives you only so much load following capacity to stay within safe margins.

[DennisP]

That's one advantage of molten salt reactors: the Xe135 bubbles right out, making load following a lot easier. The Transatomic white paper talks about this on page 44 (pdf): http://www.transatomicpower.com/wp-content/uploads/2015/04/t...