We don't actually know how much that cost France, sine it was mixed in with their military nuclear effort. French auditors threw up their hands trying to figure out the actual costs.

What we do know is their attempt to build more NPPs now has gone spectacularly tits up, with costs completely out of control. This should make one view their earlier efforts with great suspicion. Have they become much worse, or were earlier problems concealed?

Costs are out of control due to nuclear regulation.