Nuclear weapons can't detonate from being physically damaged. They might explode and scatter their contents about from the explosives in the core, but they won't reach criticality in anything other then an intentional, triggered detonation.
> “Until my death I will never forget hearing my sergeant say, 'Lieutenant, we found the arm/safe switch.' And I said, 'Great.' He said, 'Not great. It’s on arm.'” https://en.wikipedia.org/wiki/1961_Goldsboro_B-52_crash#Bomb...
Also what you are describing is "One Point Safety" in which an external explosion or fire can cause a nuclear detonation. This is mostly true for the current US arsenal, but wasn't in the past. It's one of the main reasons why the Little Boy design was so poor for safety, it only required one (preferably intentional) explosion
Source: https://www.amazon.com/Command-Control-Damascus-Accident-Ill...
The Hiroshima enriched uranium bomb was about as safe as an IED. It would have gone off if it hit the ground hard or was in a fire. The bomber crew had orders to dump the bomb in deep water if they had to abort the mission and return to base. Landing on Tinian with the bomb was considered far too risky.
It has happened before, but fortunately without the detonation.