A good question and one that may not have an exact answer.

My understanding is that radioactive fallout, such as from a nuke explosion is relatively large and most of it can be filtered out without too much trouble.

I suspect the same is true of this kind of radioactive debris.

I suspect there is some that is going to get through no matter how good your filter is. Whether that is enough to harm you in any significant way is the real question.

Even an RO won't remove radioactive dissolved gasses.

The actual solution to this problem, if it really is a problem, and not just something someone dreamed up to worry themselves about, is to check the filtered water with a portable radiation detector before drinking it.

Keep in mind there is likely to be a fair amount of radioactive particles in the air in such an area, and that is probably more of a health hazard than the radioactive particles in the water.