At the heart of every nuclear weapon lies the core or pit, a sphere of plutonium the size of a baseball and weighing around 6 kilograms. When a nuclear weapon is triggered, a shell of conventional high-explosive “lenses” wrapped around the core detonates, creating a spherical shock wave that squeezes the core to criticality and sets off the nuclear chain reaction. Outside of a bomb, however, a nuclear core is relatively harmless. Though plutonium is one of the most toxic substances known to man, in this solid, monolithic form it cannot be absorbed into the human body. Even the radiation it emits can easily be blocked by a plastic sandwich bag. The only indication of the tremendous destructive power contained within the core is the mild, constant heat given off by its radioactive decay. But under very specific circumstances, a plutonium core can become very unstable and inflict a truly horrifying death, as two scientists would tragically discover in the immediate aftermath of the Second World War.

Haroutune Krikor Daghlian, known professionally as “Harry,” was born to Armenian immigrant parents in Waterbury, Connecticut, on May 4, 1921. A gifted student, he enrolled at MIT in 1938 at the age of only 17. After two years studying mathematics, he switched to particle physics and transferred to Purdue University, obtaining his Bachelor of Science in 1942. He then began his doctoral studies under physicist Marshall Holloway, working on the design and construction of a cyclotron particle accelerator. J. Robert Oppenheimer, scientific head of the Manhattan Project, invited Holloway and his students to join physicist Otto Frisch’s Critical Assembly Group in Los Alamos, New Mexico. Daghlian remained at Purdue for another year to finish his dissertation before joining the project in 1944.

At Los Alamos, Daghlian became involved in experiments to determine the criticality of atomic bomb cores. Criticality is the state in which a mass of nuclear material is capable of sustaining a chain reaction, and is dependent on both the total amount of material present – known as the critical mass – as well as the shape of that mass. For example, a mass that is critical when formed into a sphere may be rendered sub-critical by forming it into a ring or hollow cylinder, since these shapes allow more neutrons to escape into the outside environment and prevent a chain reaction from starting. Determining the criticality of a plutonium core is essential to efficient bomb design, as too little plutonium and the bomb will not work, while too much plutonium and the extra material will simply be vaporized and scattered without contributing to the chain reaction.

The procedure for measuring the criticality of a core involved stacking bricks of tungsten carbide around the plutonium sphere, forming a reflector that bounced neutrons back into the core. Bricks were progressively added until a neutron detector indicated that the core was near-critical. This procedure, considered highly dangerous, soon earned the nickname “tickling the dragon’s tail.”

Around 9:55 PM on August 21, 1945 – barely a week after the end of the Second World War – Daghlian was performing a criticality experiment at the Omega Site Laboratory at Los Alamos. He was alone, the only other person in the building being a security guard, Private Robert Hemmerly, seated at a desk four metres away. This was strictly against regulations, as dangerous experiments were not supposed to be performed alone or after hours. The core Daghlian was working on, nicknamed “Rufus,” was the third ever manufactured. The first had been used in “The Gadget,” the world’s first atomic bomb detonated on July 16, 1945 near Alamogordo, New Mexico; while the second had been used in “Fat Man,” the bomb dropped on Nagasaki on August 9. Daghlian’s core had been prepared for a potential third bombing scheduled for August 24, but the Japanese surrendered before it could be used and the core was retained at Los Alamos for experiments.

Daghlian had stacked the tungsten carbide bricks four layers high and was starting on the fifth when his neutron detector indicated that the core was near-critical. But when Daghlian pulled the brick away, it slipped from his hand and fell into the middle of the assembly, causing the core to go prompt critical and blasting Daghlian with neutron and gamma radiation. Thinking fast, Daghlian pulled out the brick and disassembled the reflector with his bare hands to halt the reaction. But it was too late: Daghlian had received a fatal radiation dose of around 510 rem. He was immediately hospitalized, but despite intensive treatment including multiple blood transfusions his condition quickly deteriorated. His hands, which had received the highest dose, swelled up, blistered and turned black, while the rest of his body displayed what doctors likened to a “three-dimensional sunburn”. As days passed, the symptoms of acute radiation poisoning piled up, including uncontrollable vomiting and diarrhea, hair loss, intestinal paralysis, widespread burns and blisters, and delirium. His mother and his sister were flown in at Army expense to tend to him, while Army doctors performed countless tests in order to better understand radiation exposure. After 25 days of agony Harry Daghlian slipped into a coma and died on September 15, 1945 at the age of 24 – the first recorded victim of a nuclear criticality accident. Due to the secrecy surrounding the atomic bomb project, his cause of death was officially given as “chemical burns.” The only other witness to the accident, Private Hemmerly, received only a 5 rem dose and died of leukaemia in 1978 at the age of 62.

Daghlian’s accident should have alerted the scientists at Los Alamos to the dangers of performing criticality experiments, but such was the freewheeling approach to nuclear physics in those days that the incident had little impact on official policy, and many scientists were more than willing to flaunt the rules in the name of expedience. So it was that exactly eight months later the same core that killed Daghlian would claim another victim, this time a brash young Canadian named Louis Slotin (“sloh-tin”).

Louis Slotin was born on December 1, 1910 in Winnipeg, Manitoba, the son of Russian Jewish immigrant parents. Showing an early interest in chemistry, he obtained his Masters in Geology at the University of Manitoba in 1933 before attending King’s College in England, where he obtained his doctorate in physical chemistry in 1936. Something of a braggart and prone to telling tall tales, he claimed to have fought in the Spanish Civil War on the Republican side as an aircraft tail gunner, though there is no evidence that this is true. After unsuccessfully applying to work at the Canadian National Research Council, Slotin accepted a research associate position at the University of Chicago and began working on cyclotron designs. On December 2, 1942, he was present at the first demonstration of Chicago Pile 1, the world’s first nuclear reactor built in a disused squash court beneath the University. Shortly thereafter he joined the Manhattan Project proper, working first on Plutonium production at Oak Ridge, Tennessee. Here he displayed his trademark bravado and disregard for safety by crawling under a running nuclear reactor to repair a broken instrument rather than waiting an extra day for the reactor to power down, receiving an estimated dose of 87 rad.

Slotin then transferred to Los Alamos, where he quickly became a leading expert in the design and assembly of Plutonium bomb cores, earning the unofficial title “chief armorer of the United States.” Indeed, it was he who assembled and inserted the core into the “Gadget,” the world’s first atomic bomb, prior to its historic test on July 16, 1945.

After the war, Slotin turned his attention to criticality testing of bomb cores. Since Harry Daghlian’s accident, the experimental procedure had changed significantly. The core was now placed in a depression in a large hemispherical Beryllium reflector while another hollow Beryllium sphere was lowered over it. The official procedure called for the two hemispheres to be held apart by a stack of shims, which would gradually be removed to bring the halves closer together and nudge the core towards criticality. But Slotin, with characteristic bravado, discarded this method in favour of  holding the top hemisphere with his left hand while keeping the two hemispheres separated with the blade of a flat-headed screwdriver. His colleagues were aghast at his recklessness, with famous physicist Enrico Fermi warning him that he would be “dead in a year” if he continued performing the experiment in this manner. But Slotin carried on regardless, and by May 1946 had “tickled the dragon’s tail” nearly 40 times – often while wearing his trademark blue jeans and cowboy boots.

But by this time Slotin had become weary of his involvement in the Manhattan Project, bemoaning the fact that he was:

“…one of the few people left here who are experienced bomb putter-togetherers.”

Thus, on May 21, 1946, Slotin decided to demonstrate his criticality technique to Alvin C. Graves, who was slated to replace him once he left Los Alamos. Also in the room were fellow scientists Samuel Kline, Marion Cieslicki, Swight Young, Raemer Schreiber, and Theodore Perlman – all working on their own experiments – as well as security guard Private Patrick Cleary. While Slotin had performed this experiment dozens of times, on this day his luck finally ran out. At around 3:20 PM, the screwdriver blade slipped and the two hemispheres slammed together. The neutron detector jumped off the scale as the core went critical, and observers reported seeing a flash of blue light and feeling a surge of heat wash over them. By instinct Slotin flipped his wrist, flinging the upper hemisphere to the ground and halting the reaction.

Slotin, ashen-faced, turned to Graves and said:

“I’m sorry I got you into this. I’m afraid I have less than a 50 percent chance of living. I hope you have

better than that.”

He then walked out of the room and promptly vomited.

Meanwhile, all the others in the room began feeling a strange sour taste in their mouths – a classic symptom of acute radiation exposure. All were taken to the laboratory hospital and closely monitored. There, it was determined that Slotin had absorbed nearly 5000 rem of radiation – nearly eight times the lethal dose and the equivalent of being exposed to an atomic bomb blast at a distance of 1500 metres. Alvin Graves, who was standing next to Slotin, received 830 rem, while the other scientists in the room received between 45 and 255 rem. Slotin’s body had blocked most of the radiation, while his quick reflexes had saved the others from a more severe exposure. But this was likely small comfort for Slotin, whose condition, like Daghlian’s, rapidly deteriorated as the massive radiation dose caused his body to shut down and disintegrate. Louis Slotin died at 11 A.M. on May 30, 1946 at the age of 36 with his family at his bedside. His body was taken back to Winnipeg and buried on June 2.

At first Slotin was hailed as a hero for saving his fellow scientists, with one report stating:

“Dr. Slotin’s quick reaction at the immediate risk of his own life prevented a more serious development of the experiment which would certainly have resulted in the death of the seven men working with him, as well as serious injury to others in the general vicinity.”

However, many such as General Leslie Groves, military director of the Manhattan Project, saw Slotin as unnecessarily reckless and the incident as symptomatic of Los Alamos’ lax attitude towards safety. Following the accident, hands-on criticality experiments were banned and all further tests were conducted using remote-control machines operated from bunkers a quarter-mile away. The “demon core,” as the deadly plutonium sphere became known, as slated to be used in the “Charlie” shot of Operation Crossroads, the first post-war nuclear test conducted at Bikini Atoll in the south Pacific. However, after the July 24 “Baker” detonation produced more radioactive fallout than anticipated, the “Charlie” test was cancelled at the demon core was quietly melted down for reuse in other weapons.

Meanwhile, four days after the accident, Alvin Graves’s beard stopped growing on the left side of his face and his hair began to fall out. However, he eventually recovered and suffered few lasting effects except for a mild radiation-induced cataract in his left eye. The other scientists in the room also suffered mild symptoms and died decades later, though their deaths were likely accelerated by their radiation exposure, with two of the seven dying from leukaemia. Security guard Patrick Cleary died in Korea in September 1950.

Harry Daghlian and Louis Slotin were the first peacetime victims of the atomic age, and they would not be the last. In the intervening 80 years dozens of workers have been injured and killed building the weapons that were meant to keep America – and the rest of the world – safe. These casualties serve as a constant reminder of the tremendous power of nuclear energy – a power that must never be taken for granted, for it can turn on the unwary any second.

But Daghlian and Slotin’s deaths also speak to the heroic, freewheeling spirit of the pioneering days of nuclear technology, a spirit perhaps best captured by a poem written by Thomas Ashlock, associate editor of the Los Alamos Times, two weeks after Louis Slotin’s death:

May God receive you, great-souled scientist!

While you were with us, even strangers knew

The breadth and lofty stature of your mind

Twas only in the crucible of death

We saw at last your noble heart revealed.

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