The Chlorine paper mentioned in the video.
There is a nice explanation of fission chain reactions in the video. My only minor quibble is that neutron production per fission is 2.5 not 2.0. His point about neutrons being hard to detect is true for standard issue body dosimeters. They are not hard to detect with the right instruments. After all we can detect neutrons from a lightning strike caused by fusion.
These neutrons already possess enormous energy - 2.45 MeV. However, in the atmosphere of our planet they are capable of living at most for 0.2 seconds, during which they will inevitably meet with nitrogen atoms and be absorbed by them. This time period is sufficient for neutrons to fly a distance of one or two kilometers.The energy of these fusion neutrons are on the order of the energy of neutrons from fission. About 2 MeV.
Some of my previous posts on the criticality problem:
Worst Case Scenario
Surrealistic Cement Shoes
Core On The Floor
Is A Recriticality Accident Possible?
A Random Source Of Neutrons
Cross Posted at Classical Values