"PWR plants are very flexible at the beginning of their cycle, with fresh fuel and high reserve reactivity. An EdF reactor can reduce its power from 100% to 30% in 30 minutes. But when the fuel cycle is around 65% through these reactors are less flexible, and they take a rapidly diminishing part in the third, load-following, aspect above. When they are 90% through the fuel cycle, they only take part in frequency regulation, and essentially no power variation is allowed (unless necessary for safety)."

On the other hand it doesn't make economic sense to not utilize 100% of nuclear reactor output, because nuclear fuel is cheap.

https://world-nuclear.org/information-library/country-profil...