I think a lot of that is inaccurate. Without an atmosphere there is no fireball. The fireball in a nuclear explosion is radiation attenuating in air, heating it up. With no attenuation the only thing that happens is the surface with line of sight to it heats up. That's it. It would all happen pretty instantly.
The kinetics of the actual material of a nuclear bomb on its own are pretty wimpy. Pretty much all the kick is heated gas from radiation. Most of the material would evaporate. But because the volume of evaporated material isn't very much it's going to be pretty anti-climatic, all of the gaseous particles are going to launch radially, and anything in their way is going to feel a slight kick. Cody's lab did an interesting experiment of explosions in vacuums. I'm not sure if he used high explosive or low explosive but it shouldn't be that different. It's pretty wimpy.
Just because there's no atmosphere doesn't mean you can't achieve a fire effect, look at the sun, it's billions of nuclear explosions
The Sun is essentially Hydrogen and Helium. So it's like its own body is its own atmosphere.
In the case of a nuke detonating in space, you're still going to get a giant ball of heat and light, that's not going to be an invisible phenomenon
Now you aren't going to get a mushroom cloud nor a blast wave for obvious reasons, that's a given
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