What caused the flashes in the video footage of Chernobyl?
The white flashes seen in the video are the result of radioactive contamination of the original film. Airborne radioactive particles were deposited onto the film and caused a local overexposure of the negative, quite similar to the historic experiment of Antoine Henri Becquerel. That's why the flashes only appear in individual spots in individual frames of the film. (This is also explained in the documentary The Battle of Chernobyl, which also contains these film excerpts.)



There was no corresponding visual phenomenon in the air that could be directly observed by people in Pripyat.

The ambient dose rates measured in 26 locations in Pripyat on 26 April 1986 at noon (i.e. about the time when the film was recorded) were about 0.038–1.9 R/h (i.e. about 0.38–19 mGy/h; with a geometric mean of 2.4 mGy/h). Such dose rates cannot cause any directly visible effects in air.

The flashes on the film are not related to any visual phenomenon that was caused by nuclear reactions, radiation, fire or other chemical reactions at the open reactor during the night of the accident.