Which former Soviet republic has the greatest percentage of affected land and people due the Chernobyl accident (in relation to this republic's total national territory and population) ?

Belarus

Explanation:

The Chernobyl nuclear power station is situated in the north of Ukraine, close to that country's present boundary with Belarus, the closest of the former Soviet states, and where the remnants of the Communist regime survive today.

Radiation impacted 23% of the land of Belarus. During the incident, 70% of the radioactive substances released by the facility ended in the land of Belarus.

As a consequence of the catastrophe, 135,000 Belarusians have been evacuated from their homes and moved elsewhere, with radioactivity affecting many.