> No, it's an economic limitation.
I don't believe that I said it was not. Although, you should look at France as a special case.
> You can of course turn the output of a nuclear plant up and down, reasonably quickly.
Well, yes and no. Most contemporary reactor designs do have the physical capability to fluctuate between about 30%-100% power. But the ability to model such a core (accurately) has been a relatively recent thing, which has knock on effects for developing a core loading plan, doing a safety analysis, and performing monitoring/prediction. This was because most of these reactors were specifically built to satisfy base load requirements for their communities, there was no demand to develop the methodology initially. The whole economics reason came about much later when people started talking about actually developing said methodology. Its kind of a big deal because your AO bands are part of your licensing basis.