It took only 4 bombs to end civilization if nuclear war break out

[NGAD IS THE BEST]

Well, Chernobyl has radiation fallout out did all the Russians die? In times of crisis large urban populations will be moved out of the cities into underground That winter freeze is caused by dust eventually the dust will settled out

And how much your underground cities can hold how many people? How much long you survive In your underground cities? Debris will settled down few months to few years, how much your underground cities can hold food/water/clothing and combustible how long it last?

And after Debris will settled down and your people move to the surface everything will be contaminated with fallout/radiation no food no water, no nothing

And chernobyl was emptied by force immediately after the incident that's why no one die there, and chernobyl is still high in radiation producing mutated animals and plants

And Hiroshima and nagasaki is still suffering from birth defects issues in children

And chernobyl and Hiroshima and nagasaki bombing is minor as compare to current thermonuclear weapons which yields several hundred kiloton to megaton yields, so chernobyl nuclear bombing of Japan in WW2 is childish weapons/incident compare to current thermonuclear weapons

 

[Developereo]

if you are talking about a bunker for 5 to 10 people, maybe, anything more than 1000 it will be way over your yield.

Another issue with surviving in a bunker is you don't have unlimited space for waste management, bunker was never going to be a long term survival solution.

Waste can be recycled as much as possible or ejected to the exterior (think something built on the side of the Grand Canyon or a large river).

If it's built and managed by a government, they could scale it up to a decent size, maybe a few hundred but not in the thousands per shelter.

Countries like US and China would be able to build hundreds of such shelters and save a total in the tens of thousands.

And after Debris will settled down and your people move to the surface everything will be contaminated with fallout/radiation no food no water, no nothing

I am a big believer in technology. As long as something isn't forbidden by the laws of physics, we will find a solution.