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.