I doubt if any governments are stocking up for such a disaster, other than a few paranoid "survivalists". Governments don't really think that far ahead. They would expect to duck and cover and then pop back out to a world that would continue as it was before.

As far as how to set up such a refuge. I would expect that you would need a few hundred people. For reference there have been some estimates that the number of people who first migrated out of Africa on the last try about 60,000 to 70,000 years ago may have consisted of as few as 70 people, if I recall correctly. They of course were emigrating into a well stocked world in which they were well prepared to live. Modern people would need quite a bit of support while we were recovering.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.