You are probably thinking of Louis Slotin.
On 21 May 1946 at Los Alamos Laboratory, Slotin was demonstrating a criticality experiment using a plutonium core later nicknamed the “Demon Core.” He was holding two beryllium hemispheres around the plutonium, keeping them slightly separated with a screwdriver. When the screwdriver slipped, the assembly became briefly supercritical, producing an intense burst of neutron and gamma radiation. Slotin immediately knocked the hemispheres apart, likely preventing a worse accident, but he received a lethal radiation dose and died nine days later from acute radiation syndrome.
There was also an earlier fatal accident involving the same plutonium core. On 21 August 1945, Harry Daghlian accidentally dropped a tungsten carbide brick onto a neutron-reflector assembly around the core, causing a criticality excursion. He received a fatal radiation dose and died about 25 days later.
The two accidents are among the most famous nuclear criticality accidents in history, and the plutonium sphere became known as the Demon Core because it killed both Daghlian and Slotin.