The center is radioactive, but the domes around the outside are reflective material that makes the radioactive core turn critical. So, the core being exposed is much safer than the outside being closed. Cause that's the thing that kills you
It's also memeable due to Louis Slotins blasé approach to safety, as the demon core was kept from going critical merely by shoving a screwdriver between the two half-sphere neutron-reflectors that it was kept inside.
If, or rather when (because it occurred twice), allowed to completely enclose it, they would cause it to go critical, emitting lethal amounts of radiation, instantly.
Louis and one other researcher died due to incidents involving accidental supercriticality. Many more were severely irradiated.
It was really stupid of him to be so cocky about it... eventually causing his own demise. But it is funny that the Wikipedia article says how he had done the experiment many times before, "often in jeans and cowboy boots" lmao.
