I think most democratic nations put a rather large priority in ensuring that such a catastrophe does not wipe out government in its entirety. In the US, for instance, there is a long
chain of succession set up to replace the P, VP should anything happen to them, and all the members of this line can never be found within some specified radius of each other, should some catastrophe strike (if for no other reason than to give someone the responsibility to call for a nuclear strike, should the threat arise). If a giant chandelier fell on the President's podium during a State of the Union address, incapacitating Bush, Cheney and Pelosi, the person with his finger on the nuclear button would be a 91 year old man. They take the continuance of government pretty seriously out here.