There was some nervousness when the first A-bomb was detonated i.e. a chain reaction that might blow up the world.
There’s a great story about the first A-bomb, the one that was tested at Alomogordo in July, 1945.
The night before the test, a technician was making the final electrical connections inside the bomb casing. A particular cable ran from one end of the casing to the other, and he had to connect it to an assembly at either end. When he went to do so, he discovered, to his horror, that the cable had been put in backwards, so that the one end was female-to-female, and the other end was male-to-male.
Extracting the cable and reversing it would take many hours of disassembly and reassembly of the bomb casing, jeopardizing the test schedule.
He did what he had to, without asking anyone (because they would have had to say, not on your life!). He dragged an extension cord from outside into the bomb assembly room and warmed up a soldering iron.
(The room was air conditioned, humidity controlled, and purposely denied electrical outlets, all for fear of accidental detonation.)
He unsoldered the connectors from either end of the cable, resoldered them on the opposite ends, and completed the assembly of the bomb.
He knew that if he slipped and somehow set off the several tons of high explosive, he wouldn’t feel a thing.