... nobody forgot that the work they were doing at Los Alamos was both vital to national defense, and highly dangerous due to the radioactive substances involved. Wilson recalled his own brush with death while assisting a physicist in the Critical Assemblies Group with another experiment to determine when criticality was reached as one stacked a series of enriched uranium hydride cubes. He was surprised, and a bit dismayed, to find that the group didn't rely on the usual elaborate safety devices commonly used at cyclotron facilities at the time. Instead, the physicist arrived with a simple set-up involving a wooden table, a single neutron counter to monitor criticality, and a whole bunch of cubes of enriched uranium hydride.More on Louis Slotin from Wikipedia:
Wilson watched, rapt, as the physicist started stacking uranium cubes, and then noticed with alarm that the neutron counter wasn't, well, counting. Upon inspection, he discovered that the voltage supply was burnt out. When the counter was turned back on, it lit up immediately, to Wilson's horror. "A few more cubes and the stack would have exceeded criticality and could well have become lethal," he recalled. Furious, Wilson chewed out the physicist, his division leader, and even raged about it to Oppenheimer himself, but he had to leave for Trinity the very next day, so he let the incident drop. Had he stayed and pursued the matter, Wilson believed, "I might have saved the lives of two people. To this day, the incident is on my conscience."
Those two people were Harry K. Daghlian, Jr. and Louis Slotin, both of whom died of radiation sickness after accidents that occurred while conducting critical experiments with a plutonium core -- known as "tickling the dragon."
On May 21, 1946, with seven other colleagues watching, Slotin performed an experiment that involved the creation of one of the first steps of a fission reaction by placing two half-spheres of beryllium (a neutron reflector) around a plutonium core. The experiment used the same 6.2-kilogram (13.7 lb) plutonium core that had irradiated Harry K. Daghlian, Jr., later called the "Demon core" for its role in the two accidents. Slotin grasped the upper beryllium hemisphere with his left hand through a thumb hole at the top while he maintained the separation of the half-spheres using the blade of a screwdriver with his right hand, having removed the shims normally used. Using a screwdriver was not a normal part of the experimental protocol.One of the jobs I had was software safety for the detrous manipulator being built for the Space Station. It was hard to get the engineers to think hard about "what could go wrong". We are all optimists by nature. We love to extrapolate the upside. It is really hard, however, to think about failure modes, the "what if" when things go wrong. It helps if you've had previous experience to guide you. But the problem with new technology is that it is "new". We all love to see the wonderful new possibilities of the new, but few like to muck around with the grim downside. Worse, when people do consider what can go wrong, they tend to overshoot and come up with real gloom-and-doom scenarios (e.g. gray goo or the singularity) rather than the more mundane cascade of of unlikely faults that have hidden connections that lead you into a major disaster.
At 3:20 p.m., the screwdriver slipped and the upper beryllium hemisphere fell, causing a "prompt critical" reaction and a burst of hard radiation. At the time, the scientists in the room observed the "blue glow" of air ionization and felt a "heat wave". In addition, Slotin experienced a sour taste in his mouth and an intense burning sensation in his left hand. Slotin instinctively jerked his left hand upward, lifting the upper beryllium hemisphere and dropping it to the floor, ending the reaction. However, he had already been exposed to a lethal dose (around 2100 rems, or 21 Sv) of neutron and gamma radiation. Slotin's radiation dose was about four times the lethal dose, equivalent to the amount that he would have been exposed to by being 1500 m (4800 ft) away from the detonation of an atomic bomb.
As soon as Slotin left the building, he vomited, a common reaction from exposure to extremely intense ionizing radiation. Slotin's colleagues rushed him to the hospital, but irreversible damage had already been done. His parents were informed of their son's inevitable death and a number of volunteers donated blood for transfusions, but the efforts proved futile. Louis Slotin died nine days later on May 30, in the presence of his parents. He was buried in Winnipeg on June 2, 1946.
At first, the incident was classified and not made known even within the laboratory; Robert Oppenheimer and other colleagues later reported severe emotional distress at having to carry on with normal work and social activities while they secretly knew that their colleague lay dying.
The core involved was subject to a number of experiments shortly after the end of the war and was used in the ABLE detonation, during the Crossroads series of nuclear weapon testing. Slotin's experiment was set to be the last conducted before the core's detonation and was intended to be the final demonstration of its ability to go critical.
The accident ended all hands-on critical assembly work at Los Alamos. Future criticality testing of fissile cores was done with special remotely controlled machines, such as the "Godiva" series, with the operator located a safe distance away in case of accidents.