For the mushrooms, as a side order, even every day would be harmless. If you follow some stone age diet, eating only meat, then MAYBE eating those boars every day might cause a measurable increase in cancer risk. Probably still less than simply breathing the air in a city, though.

> and the contamination was much higher in the first months/years after Chernobyl

In the first days and months after Chernobyl, there were real reasons to be somewhat careful. That was before the short-lived isotopes had decayed. Once the decays of those isotopes took over, and Cs-137 became the main source, the levels were pretty safe, even if they were double compared to today (the half-life of Cs137 is 30 years).

> The overall point is that the impact was not restricted to a 50 mile radius as suggested by the GP.

And this is where my main objection is. The "impact" as counted in measurable increase in cancer risk, is nil.

By employing the ultra-conservative "linear no-threshold model" (LNT) of radiatino risk, then theoretically, over a very large population, you could calculate some number of expected cancer cases. But there is (last time I checked) no consensus that the LNT hypothesis matches data better than a null-hypothesis that doses under about 100mSv are harmless.

In any case, at that level, the risk for any given individual is so small, that it disappears next to any other risk you can imagine from pollutants or similar, so there is no reason to take such hypothetical risks into account for your personal behavior.