The type and severity of damage from a nuclear disaster depends on how close you are to it and how big the blast is. Other factors matter as well, such as weather, wind direction and type of bomb. Whether the bomb explodes at or near the ground will also greatly affect radiation exposure. The only way to know how much radiation you have received is to have a device that will measure the amount of radiation you have been dosed with. I have included a links to a site where you can download a book that tells you how to make your own simple device to let you know how much radiation you have gotten.
This information will allow you to make good choices to be able to survive a nuclear disaster.
FEMA's site says that People will get very sick at 90 Rs. They also say that you will receive that dose when you are 250 miles away from a nuclear strike. (That is assuming a strike by a 1 megaton bomb.)
Other US nuclear preparedness sites say that if you are in even a very basic shelter, you can survive a much closer nuclear strike.
Below is information about the book that includes how to make your own device (a KLM,) to measure the dose of radiation that you have received.
"Untrained families, guided only by these written instructions and using only low cost materials and tools found in most homes, have been able to make a KFM by working 3 or 4 hours. By studying the operating sections of these instructions for about IV2 hours, average untrained families have been able to success fully use this fallout meter to measure dose rates and to calculate radiation doses received, permissible times of exposure, etc.
The KFM (Kearny Fallout Meter) was developed at Oak Ridge National Laboratory. It is understandable, easily repairable, and as accurate as most civil defense fallout meters. In the United States in 1986 the least expensive commercially available dose-rate meter that is accurate and dependable and that measures high enough dose rates for wartime use is a British instrument that retails for $375. Comparable American instruments retail for over $1000.
A KFM is a simple electroscope-ionization chamber fallout meter with which fallout radiation can be measured accurately.
The long term penalty which would result from a dose of 100 R received within a few weeks is much less than many Americans fear. If 100 average persons received an external dose of 100R during and shortly after a nuclear attack, the studies of the Japanese A-bomb survivors indicate that no more than one of them is likely to die during the following 30 years as a result of this 100R radiation dose. These delayed radiation deaths would be due to leukemia and other cancers. In the desperate crisis period following a major nuclear attack, such a relatively small shortening of life expectancy during the following 30 years should not keep people from starting recovery work to save themselves and their fellow citizens from death due to lack of food and other essentials."
Here is the link to the whole book on preparing for and dealing with nuclear disasters. It includes instructions for making your own radiation meter. It is in PDF and another format. You can download the whole book on condition that you always keep the copyright information on it.
There are 317 pages in this book. I think everyone in the world who is capable of it should read it. We have too many trigger-happy world leaders, and terrorists, with too many bombs to ignore this information safely.
The book has how to build your own shelter along with blast doors and air filters that you can make. If you read this book and use it, you will have a good chance of surviving a nuclear disaster in good health.