> Figuring out what's causing those changes involved diving into detailed accounting records on the construction of these nuclear plants; data on that was available for plants built after 1976.

My understanding was that the cost of nuclear started skyrocketing around the mid-late 70s, so around the period this study period begins to get data.

And I thought the nature of the cost increases was things like being stuck iterating on designs optized for being part of a weapons program rather than safety or economics, and making it difficult enough to get approval that only companies of the "win the contract, then hold the sunk costs to random for more money" sort would bother trying to build them.