I keep earing that, but it is not totally clear for me. It seems the start and the stop of a nuclear plant takes a day if we want to be safe and efficient. A fast shutdown in a couple of seconds may cause difficulties to restart afterward. Maybe we could study procedures for safe quick start/stop even if the efficiency is not perfect.

If you had enough nuclear capacity to cover for when wind/solar aren’t outputting anything, then you don’t need wind/solar. You’d just use your nuke plant 100%.

France actually has such a high fraction of nuclear in their grid (70%) that their reactors load follow.

Granted, due to the cost structure of nuclear, it makes sense to run flat out if possible, but technically it's not necessary.