One day in Oppenheimer's Manhattan Project, a brief, casual moment of carelessness killed one scientist and severely injured another. In this specially illustrated story, the artist and writer Ben Platts-Mills recounts what happened to these atomic bomb-makers – and why their accident holds powerful lessons for today.

"...In the search for a harmonious attitude towards life, it must never be forgotten that we ourselves are both actors and spectators in the drama of existence." – Niels Bohr, physicist //

On 21 May 1946, the physicist Louis Slotin was in his final weeks of working for the Project. He was an expert in bomb assembly and had played a central role, hand-building the "Trinity" device for the first test in July 1945, just a month before the Fat Man and Little Boy atomic bombs were dropped on Japan. But, like Oppenheimer, in the months that followed, he came to object to the continuation of the nuclear weapons programme and had decided to go back to civilian life.

Slotin was giving a tour to Alvin Graves, the scientist who was due to replace him. A little before 15:00, in the middle of one of the laboratory buildings, Graves spotted something he recognised: the "critical assembly", which was Slotin's specialism. Like an experimental nuclear bomb, it was used to safely test the reactivity of a plutonium core.

Graves commented that he had never seen the assembly demonstrated. Slotin offered to run through it for him.

From the other side of the room, Raemer Schreiber, Slotin's colleague, agreed. However, he encouraged him to proceed slowly and with caution: //

There are conflicting reports about what went wrong. An onlooker said Slotin's approach on this occasion was "improvised". Others said what he did was perfectly normal. In Schreiber's official report, he said Slotin acted "too rapidly and without adequate consideration", but that the others in the room "by their silence, agreed to the procedure".

"I turned because of some noise or sudden movement," wrote Schreiber. "I saw a blue flash... and felt a heat wave simultaneously." It seems the screwdriver had slipped and the plutonium had gone "prompt critical" as the reflector dropped down over it. It happened, as Schreiber wrote, in "a few tenths of a second." Slotin flipped the upper reflector to the floor, but his reaction was already too late. In the moments after the accident, the room was silent.

Then Slotin said quietly: "Well, that does it." //

Slotin died nine days later from organ failure. "A pure and simple case of death from radiation," as a colleague would later describe it. //

In fact his boss, Enrico Fermi, had explicitly warned Slotin only a few months earlier about his approach to critical assemblies. "You'll be dead within the year, if you keep doing that," he had said.

But it seems Fermi's was a lone voice in an institution that tended to downplay the dangers of its work. //

Relatively unscathed by the accident, Schreiber went on to help re-design the way procedures like the one that killed Slotin were conducted, with a greater emphasis on safety.

Standing at Slotin's shoulder, Graves received a high dosage of radiation and became critically ill. //

Floy Agnes Lee, the haematologist treating Graves after the accident described in a 2017 interview how severe his condition was. "His white blood cells were so low that they didn’t understand why he was still living," she said. "I don’t remember how long it took before his hair started growing back again."