It depends what you consider as costs. France power generation (and consumption as well) is one of the very few with reliably low carbon intensity. You can play around with https://app.electricitymaps.com/map and see how bad the emissions are in the rest of Europe.

So not cost-effectively in the universally understood meaning.

I think carbon should be taxed and the externality should be moved to balance sheets, but that's another discussion.

France didn't solve the problem of cost-effective nuclear power.