The issue is, that in a traditional electrical system, you need both base load and dispatchable power plants. And while nuclear power plants work as base load,

The issue is, that in a traditional electrical system, you need both base load and dispatchable power plants. And while nuclear power plants work as base load, they're really bad for dispatchable power, both economically and in how they work. So if you want to base an electrical grid solely on nuclear power without additional fossil fuel plants, you need either a lot of often unused (and thus very, very expensive) nuclear capacity or some kind of .. storage. It's similar to renewables in that regard, although much more predictable and reliable (with exceptions, such as france this winter).