JakeJHecla wrote: ... accidental solution reactors have happened a number of times previously. In such a criticality accident, when the solution hits k=1, there's a large release of heat and radiation, but rarely an explosion. ... system will go critical yet again (often repeating many, many times)...
The Japanese built a facility called TRACY for doing that on purpose, with real fissile materials in solution, and plenty of variables. Some early results were useful for terminating the unplanned criticality at Tokai-mura in 1999. snipped from http://jolisfukyu.tokai-sc.jaea.go.jp/f ... 5/2_4.html
Today I couldn't re-find an online video clip that viewed a TRACY critical solution from above. Not only was the camera shielded, it was at the farthest corner of the enclosure, with telescope and mirrors in the optical path. In spite of all that, the image gets sparkly (and momentarily white) from irradiation of the camera sensor. Like the videos Andrew Seltzman has shown us from camera on conveyor cart in e-beam irradiation facility.
When the criticality at Tokaimura was over, I believe the total fission count (which power of ten) was determined by measuring the activation of coins found at the plant office and in neighbors' houses. They must have been looking at activation products with half-lives measured in hours or days. Maybe that's given in the reference given above by Jake. A friend of mine was in Japan on business at the time, and the accident interrupted his rail travel.