Witnesses of nuclear explosions have described their hands becoming transparent, and that they could see the bones. For example, see here. How does that happen?
Have you never seen the bones of your hand when covering a flash light at night? Imo it was just a very bright light over a large area and trying to shield the eyes the bones were seen.
Skin and flesh are of different ability to stop light. Extremely bright light can be detected through a thin layer of skin.
Also, a nuclear weapon releases electromagnetic energy all up and down the spectrum. Different wavelengths have different ability to penetrate. Here is a guy showing interesting effects with infrared.
His example is a lot of fun because the wavelengths he is using penetrate but don't do any harm. The radiation released by a nuclear weapon includes wavelengths that are very harmful. But they can penetrate and scatter. When they scatter there is some tendency for them to scatter to lower wavelengths which are then visible.