France built this hydro because it could not have moved from fossil fuels to 95% nuclear.
You can't realistically bring nuclear up and down in line with a daily demand curve. Hydro provides the peaking. Pumped hydro is better for this, since it provides both positive and negative balancing, but any hydro will do if there is sufficient storage.
If you have a good balance of nuclear and hydro, the nuclear will run close to full capacity and the hydro at less than half capacity, averaged over a day or longer. This is by design.
This is an article about progress being made towards, in some circumstances, being able to get around this problem. The implication is that in general, using standard techniques and technology, you can't do so, or it is very difficult, or incurs disproportionate costs.
