system
Given the following post, respond as if you're commenting on Reddit:
human
What is the fireball of a nuclear explosion composed of?

I assume it isn't the bomb itself due to the sheer volume of it.
gpt
The 'fireball' from the explosion is from the surrounding matter which has been vaporised by the energy released from the fission.

The nuclear fission releases ~~most of it's~~ energy in gammas and x-rays (and kinetic fission fragments and neutrinos and neutrons).  This energy get absorbed by the surrounding matter (rock, air, bomb casing, fission fragments etc) and they become superheated into plasmas ~~(a candle flame is a plasma)~~ which then rapidly expand (shockwave) and also re-radiate their energy all over the EM spectrum (heat, light).