As far as I know, neither Nobel laureate IPPNW nor GfS is mainstream (by default as organisations being highly critical of the nuclear industry) but AFAIK both consist of radiation experts, and that particular study is a meta-study with focus on Russian-language studies not published before in English (or German). Their report is the main one I used five years ago as source for Chernobyl's Downplayed Victims. However, in the Russian-language studies meta-study category, Chernobyl: Consequences of the Catastrophe for People and the Environment (which I tracked down inspired by some link on ET recently, by das monde I think) is newer, deeper, more comprehensive and more detailed. It contains the 212,000 deaths estimate, but explains that even that is partial, and they add several more sub-sums, including statistical deaths in large populations across the world with low exposure, to arrive at a total of one million(!).

_{*Lunatic*, n.
One whose delusions are out of fashion.}