It was a "component" of a nuclear weapon (the other component being the plutonium core). A cannon doesn't stop being a cannon just because you take out the gun

It was a "component" of a nuclear weapon (the other component being the plutonium core). A cannon doesn't stop being a cannon just because you take out the gunpowder and fill it with sand. It may not be at risk of going off accidentally, but it's still a disaster to lose track of one if cannon design techniques are a closely-guarded state secret.