https://www.military.com/history/tic...gons-tail.html
Nearly a year had passed since the bombs exploded over Hiroshima and Nagasaki. The world knew the power of atomic weapons; the Cold War was dawning.
Yet, in Los Alamos, scientists were still "pushing uranium around with screwdrivers."
Critical assembly involves bringing two or more pieces of fissile material together to form a critical mass, the mass at which atomic chain reaction occurs.
The technique, called "tickling the dragon's tail," allowed Slotin to calculate critical mass, which would be necessary to detonate in an atomic weapon.
At that time, the screwdriver apparently slipped and the upper shell fell into position around the fissionable material. A "blue glow" appeared; a heat wave moved through the room. "Slotin lunged forward and grabbed the two hemispheres with his bare hands, ripped them apart and took the full brunt of a nuclear detonation right in his stomach," says Dr. Michio Kaku, a famous atomic scientist.