Demon Core

When the Americans were developing the atomic bomb in the '40s under the
auspices of the Manhattan Project they had to find out how fast
different masses of fissile material would go critical. They did this by
getting some plutonium and steadily surrounding it with a 'neutron reflector' - a substance that reflects the neutrons that the plutonium is pumping out back on itself, which makes the plutonium throw out more neutrons, on and on, till you get Hiroshima.
But because this was the '40s and they didn't have health and safety
they just put the block of plutonium on a stand and stacked up bricks of
tungsten carbide around it till their Geiger counters started going nuts. The
guy doing this, by hand, one day accidentally dropped a tungsten carbide brick
onto the block of plutonium, so the plutonium went 'prompt critical' and
threw out enough radiation to kill the dude.
Later, they'd refined their technique a bit and had the same lump of plutonioum
inside two hemispheres of beryllium which, amazingly, they had a guy
hold open with a screwdriver. Of course the screwdriver eventually
slipped and the 2 hemispheres closed to form a sphere, the plutonium went
prompt critical again, and the guy holding the screwdriver, Louis
Slotin, felt a burning pain in his hand, a sour taste in his mouth, a
wash of heat and saw the air glow blue around him. He managed to yank
the hemispheres apart again, luckily stopping everybody in the vicinity
from dying horribly, but he had received a dose of radiation equivalent
to standing about a kilometre and a half away from an exploding atomic
bomb, so he died a few days later.
The lump of plutonium, really the hero in the tale, became known as the
'demon core.' Luckily we showed it what for, blowing it up in the 'Able' nuclear test at Bikini Atoll in 1946 (although this and other tests have made Bikini Atoll uninhabitable).