After having explained all this, now I must tell you that there are some isotopes that unfortunately do not fall into either the short range of initial radiation (which we do not need to worry about because it does not extend out of the blast area), nor the medium range (that you will be protected from by a fallout shelter), nor the very long range (that decays over so many hundreds of years that their energy is too weak to concern us here).
These remaining isotopes are real meanies. There may be solutions to the problems they present but there are no simple solutions. There will not be enough of them around that they will make walking around dangerous for most people but the problem is that they get into the food chain and that they have relatively short half-lives, between five and 30 years.
That means that during the next couple of hundred years they are going to be giving off most of their energy. Fortunately, some of them are rather rare, and given that they are going to be widely dissipated in worldwide fallout we can largely ignore their effects.
Others may be concentrated in certain areas, certain types of soil and certain foods where we can avoid them also.
So they will not be that serious a problem.
Some others, however, particularly Cesium 137 and Strontium 90, present mayor problems in keeping them out of the food chain. Even here, there are available defense techniques. For example lime, gypsum, fertilizer, or organic matter (in practical amounts) may be applied to low calcium soil, or naturally high calcium soil may be used for growing certain crops which have an uptake preference for calcium over strontium.
There are known refining and purification techniques for some foods and milk, and there are some new techniques which I have discussed with some of the researchers at some of the leading nuclear laboratories, but which the world isn't ready to hear about as yet.
These methods along with others such as land denial, deep plowing, surface scraping, and selective utilization, are harsh realities that are going to have to be faced by the long-range survivors.