Considered by many to be one of the most realistic (and hence scariest) depictions of nuclear war.
https://en.wikipedia.org/wiki/Threads_(1984_film)
Other free sources that work in MX, maybe elsewhere too:
https://fawesome.tv/movies/10647273/threads
https://tubitv.com/es-mx/movies/531445/threads
https://flixhouse.com/video/2599/threads
That’s the one, thanks!