The most frightening and realistic depiction of a nuclear holocaust that I've seen is a 1984 BBC film called Threads.[1][2]

You can watch it for free on youtube.[3]

[1] - https://en.wikipedia.org/wiki/Threads

[2] - http://www.imdb.com/title/tt0090163/

[3] - http://www.youtube.com/watch?v=_MCbTvoNrAg