The film depiction had Merriman holding part of the sphere above the other half of the sphere with a screwdriver through an assembly of silvery metallic bricks, then raising & lowering the upper hemisphere to induce criticality of the mass. In the actual incident, Louis Slotin, a physicist working at Los Alamos, did pretty much what was depicted in the film but with no silvery bricks. I've not seen anything that explains what caused Slotin to lose control of the screwdriver, but this method was inherently precarious to say the least. The film suggests that Merriman was distracted by a falling coffee cup, but that may be speculation or artistic license by the filmmakers. In the end, the only answer to the question of the actual Slotin incident might have been "It slipped."
This immediately induced criticality with the resulting blue glow that was depicted in the film. The blue glow is the air around the nuclear mass ionizing into a plasma state. Considerable heat is generated as well, but the real damage was done by the tremendous burst of invisible radiation produced by the fission reaction taking place right in front of his eyes a couple of feet away! Slotin received full body radiation exposure, absorbing enormous quantities of radiation in those few seconds and he was a rendered a walking dead man. The film hints at the medical problems he experienced, but it was truly horrible what happened to Slotin (and to Daghlian earlier). Slotin lived for 9 days after the incident, Daghlian nearly a month.
After the incident there was considerable criticism of the whole procedure that Slotin used. The thing with the screwdriver and the absence of spacers in his assembly might be considered unconscionable, and in fact Slotin's laxness on safety was a major contributor to the incident, led to his death and quite likely the later death of others who were in the room at the time of the accident.
The Slotin incident took place AFTER the end of the war, in 1946, though it was depicted in the film as having occurred in August 1945, before Fat Man & Little Boy, actually using the dates associated with the fatal criticality experiment that killed Harry Daghlian, a physicist who had also been conducting criticality experiments before & after Trinity. In fact, Daghlian's accident occurred with the same plutonium core sphere that Slotin was working with in 1946. It became known as the 'Demon core' and later was used to fuel a nuclear bomb detonated at Bikini Atoll in the South Pacific on July 1, 1946. Harry Daghlian was the first to die from radiation exposure associated with the Manhattan Project.
By the way, these were called 'criticality experiments', with the term criticality referring to a nuclear mass going 'critical', or having nuclear fission begin to take place.
This was not the only such incident; a few other criticality experiments had similar accidents though not always involving human injuries, and there was another well known accident with radioactive fluids that caused a couple of horrible deaths.
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