Criticality was reached upon the technicians adding a seventh bucket of uranium solution to the tank.[6] The nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation. The technicians, one of whom had his body draped over the tank, observed a blue flash of ionized air[7] and gamma-radiation alarms sounded.[8][9] The two technicians closest to the tank immediately experienced pain, nausea, difficulty breathing, and other symptoms. The technician closest to the tank lost consciousness in the decontamination room a few minutes later and began to vomit.[10] There was no explosion, but fission products (fission fragments of U‑235 with atomic masses typically around 95 and 137, such as yttrium‑94 and barium‑140) were progressively released inside the building.
NB in the description what happens when the water was drained off in the surrounding jacket: they dropped out of criticality. Many people talking about “criticality” obviously have no idea of the physics involved in creating and sustaining a critical threshold in a reactor.
BTW - we had an incident similar in result (3 dead) here in the US in the early 60’s. Unlike the incident you’re mentioning, it was a boiling water reactor, not a breeder reactor, the men were killed by mechanical effects of a nuclear mishap, and it resulted in a release of radioactivity.
The men were buried in lead-lined coffins.
http://www.inl.gov/proving-the-principle/
Jump down to “The SL-1 Reactor” if you’re impatient. That’s an example of “prompt criticality.”
BTW - we’re still here. That area of Idaho grows trainloads of wheat .