They have had many years to research and plan, but apparently the
French nuke power experts themselves haven't learned or firsthand
experienced enough--see the Swedish anecdote

Here's what seemingly happens, which makes it a shammy show

The contractor bids low, apparently knowing there'll be supplememts if
he get the mega project contract

For instance:

Nuclear Plant Vogtle in the State of Georgia, 1970s-1880s, was
originally estimated at only "$600 million," though the over run
amounted to TEN BILLION DOLLARS

http://counterpunch.com/

Note: Pardon me for being so cynical and jaded, but we've now had
years and years to go solar, and ya wouldn't have to worry about solar
as ya do about (coal and) nuke power and the waste problem forever.