Originally posted by gaweidert
You may be able to photograph one of these.

But I would use a very long lens.
Actually, the image you show is more a display of shutter speed than ND filters. Images such as this had a effective shutter speed around three millionths of a second using specialized cameras where the image sensor (film) was stationary and shot a single frame. The cameras were known as Rapatronic cameras.
Rapatronic camera - Wikipedia
"For a film-like sequence of high-speed photographs, as used in the photography of nuclear and thermonuclear tests, arrays of up to 12 cameras were deployed, with each camera carefully timed to record sequentially. Each camera was capable of recording only one exposure on a single sheet of film. Therefore, in order to create time-lapse sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time used was three microseconds."
ND filters were not used during the tests.
Remember the exposure triangle - from that you can guestimate the ASA (ISO) and aperture of the camera. By the way, nuclear explosion's produce a lot of light.