Correct, the missile has to be armed before it would explode and they aren't stored "armed".
A month ago I finished Last Stand of the Tin Can Sailors. Destroyers had a similar situation with depth charges. Depth charges had to be armed. So before battle, they would be armed. But if the ship was hit, sailors had to run around disarming them. If the ship sank with armed depth charges, charges would explode when they reached their programmed depth. The author said the concussion wave would result in a fatal enema for anyone in the water.
So nukes have similar safeguards. There is a very specific explosion to set off a nuke, a normal explosion would not. The most basic example is that the explosion has to come from the middle of the material and start the reaction uniformly. An explosion from one end would not do this.
But for this to work, the explosive surrounding the plutonium must be a perfect sphere, and the entire sphere must be detonated simultaneously. If the detonation merely starts at a single point on the sphere, the nuclear explosion will fizzle. To achieve uniformity of detonation, scores or hundreds of electric detonators must be embedded all over the outer surface of the sphere, and all must be fired at the same instant.
If the sphere of explosive were set off by an accident, it would not explode uniformly and would not compress the plutonium enough to cause a nuclear detonation. But it might set off a fast chain reaction short of a full-scale explosion - a reaction that would generate deadly radiation and spread radioactive debris.
www.nytimes.com