This is based on Nyrsth's USAF Orion Mod (it's in what I've uploaded, by permission, under my mods). Changes; I shrunk it. It's half scale, like a lot of KSP replica rocketry, so instead of a 10m pusher plate, it's 5m. BUT I've also included the 86foot edition as well (again at half scale, so 43foot or ~13.1m). And I've taken the liberty of half scaling the propulsion devices as well, which isn't technically possible, but it's a game, and has many more objectionable impossibles in it. I've split the engine. You now need the main engine piece, a floor or five of bomb conveyor (each floor piece has one type of bomb loaded) and on top of that an Engine-Payload-Interface, which is a fancy way of saying top. Some code changes as well.
There's a reasonably accurate portrayal of the effects of atmosphere. Orion's have the opposite issue as rocket engines, they work really well in an atmosphere, less so in a vacuum. By a bit over an order of magnitude. So start small. :-) Also, I scaled heating with atmosphere and velocity. Basically, a nuke in an atmosphere creates a fireball. If the code estimates you might spend some time in said fireball, it applies some extra heat. Using an Orion to decelerate in an atmosphere is risky business.