Yeah, for being brilliant physicists, they weren't particularly smart. From the second incident:

On May 21, 1946, one of Daghlian's colleagues, physicist Louis Slotin, was demonstrating a similar criticality experiment, lowering a beryllium dome over the core.

Like the tungsten carbide bricks before it, the beryllium dome reflected neutrons back at the core, pushing it toward criticality. Slotin was careful to ensure the dome – called a tamper – never completely covered the core, using a screwdriver to maintain a small gap, acting as a crucial valve to enable enough of the neutrons to escape.

The method worked, until it didn't.

The screwdriver slipped and the dome dropped, for an instant fully covering the demon core in a beryllium bubble bouncing too many neutrons back at it.

After an initial bout of nausea and vomiting, he at first seemed to recover in hospital, but within days was losing weight, experiencing abdominal pain, and began showing signs of mental confusion.

A press release issued by Los Alamos at the time described his condition as "three-dimensional sunburn".

https://www.sciencealert.com/the-chilling-story-of-the-demon-core-and-the-scientists-who-became-its-victims-plutonium-bomb-radiation-wwii