https://en.wikipedia.org/wiki/Load_following_power_plant#Nuc...
Modern nuclear plants with light water reactors are designed to have maneuvering capabilities in the 30-100% range with 5%/minute slope.
Still, I wonder if the economics don't favor running near 100%. You've already paid the high up-front cost of the equipment, and fuel costs are low, so I assume you're better off selling excess power when possible.
Future grids with more intermittent renewables will have increasingly volatile prices, but not necessarily lower on average. So generators that can produce during high price periods (e.g. if wind and solar aren't producing much) can make a lot of money then, compensating for less income during low price periods.