Kalothar

Using his own unapproved protocol, Slotin did not place any shims. The top half of the reflector was resting directly on the bottom half at one point, while 180 degrees from this point a gap was maintained by the blade of a flat-tipped screwdriver in Slotin’s hand.

often in his trademark blue jeans and cowboy boots

Enrico Fermi reportedly told Slotin and others they would be “dead within a year” if they continued performing the test in that manner.

On the day of the accident, Slotin’s screwdriver slipped outward a fraction of an inch while he was lowering the top reflector…there was a flash of light; the core had become supercritical.

Slotin reportedly turned to his colleagues in the hushed room and famously uttered the remark, “Well, that does it.”

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The video is kinda amazing and also from the article

The sunfish were already dead in both events, so the ramming is thought to be a processing technique rather than a means of delivering a killing blow.