The high numbers are quite remarkably bad science, tough. Ive read what I can find in languages I understand, and.. eh, they ring every bell that gets set off when I read climate change denialist tracts, and they ring them hard. I have to classify some of them as deliberate attempts at deception, because nobody is that stupid and that clever at once.

 Good epistomology on it is however honestly very difficult, because chernobyl was located in the industrial heartland of the SU, which means it is horrifically polluted, chemically. The citizenry in those parts can be expected to suffer elevated rates of just about anything, with or without radiation, and we have no hard data on how radiation interacts with chemical toxicity in general. - We know that it is a very bad idea for radiation workers to smoke, but beyond that- nada.

The fatality numbers for fukushima are much more reasonably predictable, tough. And And while not zero, the main cause of death is not radiation, it is relocation stress.  - a lot of people got moved, and some of them were not in the best of health.