I read (can't remember where) that one of the brothers had a friend who had been evacuated from a town near Chelyabinsk 65 which was the site of a very serious

I read (can't remember where) that one of the brothers had a friend who had been evacuated from a town near Chelyabinsk 65 which was the site of a very serious accident in 1957, actually worse in some ways than Chernobyl. He had described the contaminated zone around there in graphic detail and the Strugatskys had used the idea for an alien visitation.