Re: What does happen when an ICBM launch fails?
The firing mechanism of modern nuclear bombs consists of an array of shaped explosive charges arranged around a hollow fissionable core and depends upon all of those shaped charges being detonated at precisely the right time, to within extremely tight limits, for the bomb to 'work'; premature detonation of one or more of the shaped charges, or failure of any one of the shaped charges to detonate at precisely the right time will result in the fissile core, along with the rest of the bomb, being blown apart. The biggest bang you'll get in these cases will be from the shaped charges detonating and the biggest problem will be the local radioactive pollution from the destroyed fissile core.
Whilst there are other safeguards preventing the premature arming and ignition of the firing mechanism, there's not really a need to be able to disarm a nuclear bomb in the event of a launch failure.
Edited to add: pretty much what Nigel 11 says above - started typing before he posted but got called away, so ended up posting after him.