Letters from a Dead Man (or Dead Man's Letters) is a 1986 Russian post-apocalyptic science fiction film directed by Konstantin Lopushansky, who also directed Ugly Swans. This reminds me a bit of Threads. I learned about this movie from the list I mentioned yesterday.
Youtube has it online in 8 pieces. part 1:
part 2, part 3, part 4, part 5, part 6, part 7, part 8
Moria says, "Despite the bleakness of the subject, Letters from a Dead Man manages to be an extraordinarily optimistic film."