"The core was placed within a stack of neutron-reflective tungsten carbide bricks and the addition of each brick moved the assembly closer to criticality. While

"The core was placed within a stack of neutron-reflective tungsten carbide bricks and the addition of each brick moved the assembly closer to criticality. While attempting to stack another brick around the assembly, Daghlian accidentally dropped it onto the core and thereby caused the core to go well into supercriticality, a self-sustaining critical chain reaction. He quickly moved the brick off the assembly, but received a fatal dose of radiation. He died 25 days later from acute radiation poisoning." https://en.m.wikipedia.org/wiki/Demon_core https://en.m.wikipedia.org/wiki/Demon_core The Wikipedia page describes the 2nd accident as well, involving a screwdriver and metal hemispheres.