Nuclear power plants can vary power output quickly if planned during the design. As France has an installed capacity of more than 60 GW of nuclear power production, their power plants can quickly adapt their production. That's needed to keep the network balanced.
France has 80% nuclear. It works there not because French reactors can respond meaningfully to short-term meteorology, but because:
1) France is connected to a continent-sized grid and
2) All of France's neighbours are nowhere near 80% nuclear, so will buy this baseload power.
If Germany, NL, Denmark, Spain, UK et al. had 80% nuclear France's nuclear power would become uneconomic.
It's the grid and unique political considerations, not those plants' responsiveness that makes it work for France.
The French do vary the power of the reactors to follow the load [1] [2]. They don't purely rely on their neighbors, far from it. As a consequence, the usage factor of the plants is lower than nuclear plants in countries where they they purely use them for base load.
