> What is the scenario that could cause a catastrophic failure in the VVER design or implementation that would result in a disaster that would create a situation to evacuate cities 50kms away?

There is Neris river that is both near Astravets NPP and running through Vilnius - so it is not only 50 kms away and adds other risks.

I hope for the best but stuff like this does not give much confidence https://en.wikipedia.org/wiki/Astravets_Nuclear_Power_Plant#...