The writer of Chernobyl Craig Mazin, said they tried using Russian accents initially, but it changed how the actors delivered their lines, he felt they were acting to the accents, rather than just acting out the scene, so he scrapped and let them use their natural accents.
The writer is seriously obsessive about attention to detail, the clothes, the hairstyles, the colour of the walls, the positioning of the mics in court, were all spot on, so it was surprising he didn't go for Russian actors or accents, but it took me about a minute to get over the British accents.