Did a quick search of an explosion in a vacuum and got most of what I was looking for, but on CBS Sunday morning they had a segment on restoring and reevaluating all of the immense footage of America's atmospheric nuclear testing program. They showed some amazing footage of the expansion of the fireball and then the commensurate shock wave. It is clear, that a terrestrial blast would involve both the immense nearly instantaneous EM radiation expansion, but most of the damage occurs from the 700 mph shock wave. In a vacuum I believe there would be no shock wave, but there would still be the expanding ball of radiation that would do damage, but would it still be as devastating? What kind of damage would it be? How would it dissipate?