|How do you keep the nuclear bomb stable until you're ready to use it? (For example, on the way to Hiroshima)
The nuclear material will only explode if it is assembled to the point of critical mass. If that assembly is done slowly, the material will overheat and melt, perhaps causing a minor explosion but creating more of a radiation hazard than a nuclear detonation. Only if the assembly is done rapidly to well over the critical mass will the bomb explode. To keep a nuclear weapon "safe", the bomb makers ensure that the assembly cannot occur prematurely. They probably remove the triggers for the high explosives or block the paths through which the nuclear material must move. In most cases, even an accidental triggering of the high explosives use in assembly wouldn't cause the bomb to explode because all of the high explosives must be triggered at the same time for the assembly to work properly. If only part of the explosives ignited, the bomb would fizzle (very loudly).