You wrote "Initially there was a flat out denial that anything happened at all, but once the scale of the catastrophe became clear (within about a day)". That's not what happened, you can look it up in any of the numerous timelines and accounts of the accident and the reporting and Soviet disclosures. The evacuation of Pripyat started before the first public Soviet announcement as described here:
https://en.wikipedia.org/wiki/Chernobyl_disaster#Evacuation
And the section immediately following regarding the announcement.