What I don't understand; and please excuse my naivety, is when the accident on May 21st, 1946 occurred and Slotin's screwdriver slipped causing the reflector to close around the core and the core becoming supercritical:
On the day of the accident, Slotin's screwdriver slipped outward a fraction of an inch while lowering the top reflector, allowing the reflector to fall into place around the core. Instantly there was a flash of blue light and a wave of heat across Slotin's skin; the core had become supercritical, releasing a massive burst of neutron radiation estimated to have lasted about a half second. He quickly flipped the top shell to the floor. The heating of the core and shells stopped the criticality within seconds of its initiation, but Slotin's reaction prevented a recurrence and ended the accident. Slotin's body's positioning over the apparatus also shielded the others from much of the neutron radiation. He received a lethal dose of 1000 rads neutron/114 rads gamma in under a second and died nine days later from acute radiation poisoning.
Why did the core not explode/react? My understanding is that the core 'is' the bomb and once the chain reaction begins and occurs in trillionth's of a second could not possibly be stopped in time by the reactions of Slotin alone?
I have tried to source information to fill my knowledge gap, but am perhaps using the wrong terminology to ask the question. Can anyone assist?
Finally, and once again, please excuse the naivety of my question.