I suppose I got lucky with the search terms on YouTube, I remember watching a documentary showing the first sight of the reactor filmed from a helicopter, and the narrator saying "The flashes of light on the footage are due to radiation". I was hoping to find this heli video on YouTube, but all the "chernobyl heli" footage are from the TV series, or very very low res. The video above isn't what I was looking for, but it shows the effects of radiation very well.

Sheesh, but DuckDuckGo and Google are quite useless. I just tried to see if The Internet has a record of the heli that flew to Chernobyl to shoot that first footage of the blown reactor (IIRC from Moscow, although that does not seems a plausible flying distance), and the search engines think I want to instead find out about the heli that crashed after hitting a crane...