I suspect you're referring to this: https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_accident https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_acci

I suspect you're referring to this: https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_accident https://en.wikipedia.org/wiki/Louis_Slotin#Criticality_accid... "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 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 of neutron radiation."