It's not actually a horror movie at all, but i think that "Trinity and Beyond, The Atomic Bomb Movie" has some pretty horrifying scenes in it....
no, not *just* the mushroom clouds, yes those are scary, yet errily beautiful, the scary scenes are scenes with Navy sailors setting up animal test subjects on ships used in Project Crossroads, it's old, grainy, black and white film, but you can *see* the fear in the animals eyes as they're strapped into frames and cages......
the Bombs detonate, with the requisite jaw-dropping mushroom clouds, shot from multiple angles
then you cut to Navy sailors at ground zero, inspecting the ships, removing the broken, bloated animal corpses, while standing on the decks of the badly irradiated ships, with *no* protective gear, sucking up *major* doses of radiation......
later on in the movie, you see the detonation of the first thermonuclear bomb on Bikini Atoll, and the mushroom cloud just keeps growing....and growing....and growing
later, that explosion is *dwarfed* by a 15 Megaton explosion that literally *consumes* a small chain of islands (small barrier islands)
yet the most unnerving test to me, was Shot Grable, a "small" 15 Kiloton nuclear *artillery shell* fired from a standard artillery cannon
(most of the stock nuke footage you see on TV is from that test, the stand of trees whipping forward and back, the vehicles getting flattened and thrown around, building walls *collapsing* under the shockwave....)
then....you finally remember that these devices *still exist* and have only gotten *more* powerful since that footage was shot....