https://greglewisinfo.com/2020/04/18/the-b-17-saved-by-a-miracle/
The ultimate source appears to be a guy's memoirs:
I came across this story in Elmer Bendiner’s marvellous 1980 memoir, The Fall of the Fortresses, while researching the lives of USAAF crews flying out of England during WW2.
I'm still very sceptical. This guy is not even the primary source actually:
Bohn said the shells had been sent to the armorers to be defused but had then been rushed away by an intelligence officer.
Bohn had tracked down the officer and had hounded him until eventually he had told Bohn the full story – before swearing him to secrecy.
Now, sabotage like this undoubtedly happened, although the scale is impossible to verify. However I think this specific story has just way too flimsy a chain of evidence to put any faith in. Good story though.
Another big factor is that every plant is effectively a completely custom design. Because of how few nuclear plants are constructed, every new one tends to incorporate technological advancements to enhance safety or efficiency. The design also has to be adapted to the local climate and land layout. This makes every single plant effectively one of a kind.
It also tends to be built by different contractors, involving different vendors and electric utilities every time. Other countries have done better here (e.g. China and France) mostly due to comprehensive government planning: plopping down lots of reactors of the same design, done by the same engineers. Although these countries are not fully escaping cost increases either.
You are completely correct that regulation is also a big factor. Quality assurance and documentation requirements are enormously onerous. This article does a pretty decent job explaining the difficulties.