If you can add thermal storage to a nuclear plant, you can add thermal storage to a solar or wind farm and do it cheaper.

Thermal storage is harder to integrate with renewables because you need a recovery mechanism, i.e. a steam turbine. Because nuke plants already have the turbines, it’s cheaper to collocate the storage there. I could see a future where the nuke and fossil plants are converted into thermal storage facilities without the boilers to reuse the existing turbines (and permits to operate, and interconnection agreements).

renewables with a recovery mechanism is still simpler engineering than a nuclear plant.

Thanks for the info. I've asked people this question ("is there a fundamental reason why nuclear can't vary?") but nobody that I've asked yet has known the answer. So this is helpful!

I’m optimistic we could find a way to ramp solar/wind up and down. We don’t do it now because it’s easier to run it at full capacity for economic and safety reasons. Adding some thermal storage (which the SMR companies are trying) or varying the reaction rate are in the area of research but certainly seem feasible.

We can't ramp them up because they are dependent on natural forces (weather patterns and the rotation of the earth) that we can't control.