[muthas]

To put it another way - from an "immediate risk" point of view, the largest danger from critical-capable amount of fissile material is the prompt radiation emitted during a criticality: the pulse of neutrons, gamma and x-rays thrown off during that period the unit is sustaining a chain reaction.

For relatively brief power excursions like this one (which Slotin himself stopped by removing the reflector), the prompt burst was well over a lethal level but the radiation dose-rate from fission product decay even a tiny time later would not be immediately hazardous.

For those looking to learn more, Los Alamos has a great writeup of every publicly-known incident (in the US, USSR, and elsewhere) that details just how many times similar incidents happened: https://www.orau.org/ptp/Library/accidents/la-13638.pdf