“Modern nuclear plants with light water reactors are designed to have maneuvering capabilities in the 30-100% range with 5%/minute slope, up to 140 MW/minute.[7] Nuclear power plants in France and in Germany operate in load-following mode and so participate in the primary and secondary frequency control. Some units follow a variable load program with one or two large power changes per day. Some designs allow for rapid changes of power level around rated power, a capability that is usable for frequency regulation.[8] A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration.[9]”

https://en.wikipedia.org/wiki/Load-following_power_plant#Nuc...