> They wanted to estimate the risk of accidental detonation of a nuclear weapon, and concluded that the risk is zero because it never happened so far.

Given it didn't happen so far, I'd say they were right on the money.

While mishandling and accidents did happen, the number of failsafes guarantees you have to purposefully activate it. Most nuclear weapons nowadays just don't have a way to reach criticality outside of nano-second controlled timing array.