(Neither Three Mile Island [0] nor Chernobyl [1] would have been so disastrous had it not been for cascading sequences of human error.)
Expecting nominal performance by people or machinery is ... unwise; as Admiral Rickover famously said, "you get what you INspect, not what you EXpect."
All that adds to costs.
Source: Former Navy nuclear engineering officer, qualified as [chief] engineer aboard the eight-reactor aircraft carrier USS Enterprise.
[0] https://www.energy.gov/ne/articles/5-facts-know-about-three-...