You could start with Wikipedia
https://en.wikipedia.org/wiki/Radiation
The fact is though we all live with radiation every day, especially pilots and CC. For example my yearly allowed dose in my work environment is 50uSv, which is less one trans-Atlantic flight at 80uSv.
https://www.gov.uk/government/public...se-comparisons
Most people are unlikely to get any Gamma dose (except in hospital under controlled conditions) but the nasty stuff are the beta and alpha emitters, these would be produced in nuclear tests, accidents such as Chernobyl and the one we are discussing. Very small fragments of active material can be ejected and travel a very long way as aerosols.
Have a read about Downwinders
https://en.wikipedia.org/wiki/Downwinders
But there are a lot of factors:
- What elements/half-life - If short not much risk, its decayed before reaching anyone
- How far will it spread? wind, weather, remote location..
The health risk, the risk of a cancer depends on: how much is inhaled/eaten, the level of activation, how much it has already decayed and how long it stays in the body.
My point was its not impossible that one could encounter some of this material via flying through it. But the fact is it had already passed over Europe albeit in small quantities before we ever found out. And ref above we have many sources of radiation in our everyday lives, you just cant avoid it, so at least in my opinion its not worth worrying about.
(The exception to that is Radon, always worth finding out if you live in a susceptible area and taking precautions, because you can do something about it)