Tickling the Dragon and the Blue Flash: The first is a metaphor that refers to experimenting by careful trial & error how many & what configuration of Tungsten Carbide "tamper" bricks will reflect back enough neutrons to make a fissile sphere of bomb core Plutonium-239 go barely critical as in a self-sustaining nuclear reactor. The second term above is what happens when you unexpectedly, accidentally & grossly overshoot with the core going "prompt critical" -the instantaneous extreme high intensity radiation generated actually ionizes the electrons off the Nitrogen molecules in Air surrounding the Plutonium Core & brick pile with the characteristic color of the Blue Flash: if you're standing in close proximity right next to this catastrophic event, you will die an excruciatingly painful slow death by radiation exposure & poisoning within a few weeks.
"Tickling the Dragon", by the original Physicists of the WWII Atomic Bomb Manhattan Project, was akin to making an "eight-story House-of-Cards": you had to be focused, alert and stone sober. (Today this is never done manually up close on a workbench, but by remote control some quarter of a mile distance away). Scientist Harry Krikor Daghlian, "by the end of the war had tickled the dragon so many times that he was at that dangerous point where experience and confidence were so extreme, there was no need to be careful . . .as an Experimental Physicist of the Critical Assembly Group, he did not have the ignorance to be terrified of his tasks.
On August 21 1945, just six days after Japan surrendered to end WWII, Daghlian went back to the lab around 930pm knowing it was against regulations to perform a criticality experiment without an assistant and certainly forbidden to do so after hours. Thirty minutes later, after building a five stack house of Tungsten Carbide bricks around the core, while holding & slowly lowering the top brick in his left hand to cap the structure he was suddenly startled by the increased sound of the neutron radiation counters through the loudspeakers, and jerked the brick away from the pile, but lost his grip: the Security Guard sitting twelve feet away with his back to the assembly heard the increased neutron counters chatter through the loudspeaker then suddenly go quiet (overloaded & off-scale), along with the clunk of the brick landing atop the pile; and saw the Blue Flash light up the wall in front of him. He amazingly survived.
"Daghlian had caused a problem, and every instinct told him to erase the problem. With his right hand, he knocked off the brick on top of the assembly, glowing a pretty blue, and noticed the tingling sensation of direct sensory neuron excitation. . . He stood there, arms limp by his sides, coming to grips with what had just happened. He decided to dismantle the pile of bricks and calmly told the Security Guard what had occurred. . ."
"Daghlian's right hand endured a high dose of x-rays, gamma rays and high speed neutrons, somewhere on the order of 20,000 to 40,000 rem -it essentially turned necrotic and died two days later; his left hand around 5,000 to 15,000 rem as the brick hit the pile. His body absorbed 590 rem: A fatal dose inducing radiation sickness is usually around 1,000 rem. He slipped into a coma and expired 25 days later, the first victim of acute radiation poisoning to die in history's first mini-disaster of nuclear fission out-of-control." [abridged & taken from Atomic Accidents: A History of Nuclear Meltdowns & Disasters: From the Ozark Mountains to Fukushima, by Jim Mahaffey].
