Threads
Check this movies out:
http://www.ibp-intl.demon.co.uk/nuke/threads.html
It goes into graphic detail about the aftermath of a nuclear war. I wouldn't say its a horror film but it will definately appeal to horror fans. Its VERY difficult to get hold of and usually ends up going for about £80 on E-Bay, I managed to see it in the 80s when It wasn't so rare.