AST, I thought that too, but the creator of the show has a podcast called Chernobyl Podcast (Craig Mazin) where he explains why they don’t use Russian accents and it makes sense. He basically says they tried using Russian accents at first, but it always sounded comical and distracting so they quickly decided just to use non Russian accents and it worked out. Good podcast if you enjoy the show, a lot is explained after each episode.