Some valuable information here. Provided you survive the initial explosion, you then have to survive in deep shelter for two weeks before it is safe to walk away. Thus any type of shelter needs only two weeks of sustainability. If you did not survive you were too close. Also recall that a giant flash means you run to get underground and protected. There is a nasty shock wave on the way.
Cellars and underground parking is basically good enough if you prevent circulating air and that should be no problem in that case Somewhat like the response to a tsunami or general area flooding.
Recall that survival is primitive and essentially unpleasant but no more. What is clear is that initial survival is doable. It is after that you will need to access whatever organization exists...
What technology do modern day nuclear bunkers have? How many years could you survive in one?
Allen E Hall, Owner at Aerospace Eng & Mfg Consultant (2014-present)
The most you would need is 4–6 weeks no matter how many bombs went off. Radiation from nuclear weapons decays at a rate known as the 7/10 rule of thumb.
The 7:10 Rule of Thumb states that for every 7-fold increase in time after detonation, there is a 10-fold decrease in the exposure rate. In other words, when the amount of time is multiplied by 7, the exposure rate is divided by 10.
Detonation 1000 rad fallout (per hour)
7 hours later 100 rads
49 hours 10 rad
14 days 1 rad
100 days .1 rad
You would leave after 14 days (safe to exit the shelter and relocate to an area without any excess radiation)
What you see on TV is sensational nonsense. This is the actual reality.
There is quite a bit of interest in this answer so I am expanding it a bit.
The amount of radiation you can receive before any detectable changes occur is about 25 Rads. There is a 50/50 chance you will die with a 400 Rad exposure. Not everyone reacts the same to radiation, some people have survived much larger exposures with no ill side effects but there is no guarantee for anyone.
A key point to understand risk is the rating of your shelter. In the scenario above surviving would require that you are in a shelter rated greater than 2000. That is a rating that means at 1000 Rad external radiation level the shelter would see .5 Rad per hour. This is the minimal safe level at this point.
1000 Rads an hour means that you are close to ground zero of a very big bomb that was a surface burst. Chances are you have a lot of other problems besides radiation. Most areas farther out wont see levels higher than in the 10–100’s of Rads which makes survival easier.
Most shelters are rated no more than 1000. Here is a visual guide to help you understand radiation shielding in general. A shelter with a rating of 200 means that if the radiation outside is 10 Rads, inside it would be .05 Rads (10/200).
and this is the 7/10 chart curve.
Generally you need 8 feet of well packed dirt between you and the radiation to protect you