Underground aquifer would be safest and not even need any treatment. If you've got that then there's little reason to work on this angle of prep unless you have to travel.

Other water sources, if given time for particles to settle, would likely be only lightly contaminated. Collecting water from the upper layers of the water body and letting it settle further in containers before boiling or filtering would help.

I suspect there is lots of useful advice in the old Civil Defense texts about surviving radioactive fallout from bombs. The idea there was to stay underground for 2 weeks while the radioactivity decays. I haven't read much further than that.

But it needs to be said that bomb fallout is probably a different distribution of radionuclides than that from a damaged reactor core.