Demon Core: How The Third Nuclear Bomb Destined For Japan Killed a Bunch of American Scientists

President Harry S. Truman knew that one bomb would not be enough to force Japan to surrender, so he ordered two. What many don’t know is that there was a third bomb in reserve, just in case.

This third bomb had not been assembled yet, but its plutonium core—the heart of the bomb—was ready, and kept at the Los Alamos National Laboratory. When it became clear a third bomb would not be necessary, nuclear scientists at Los Alamos were delirious with excitement. Here there was in their hands, the rarest of the rare material—a 6.2 kg core of pure plutonium. They probed and prod the shinny metallic sphere and subjected it to countless experiments, until two sloppy scientists nearly blew up the laboratory and ruined it for every one. Both of them were dead within days, and the core acquired the nickname “demon core”.

A replica of the demon core in a mock setup used by one of the physicists during a fatal experiment. 

Nuclear bombs are different from conventional bombs. There are no fuses or detonators. What we have instead is a mass of nearly critical radioactive materials. At the time of detonation, this mass is made supercritical by bringing more radioactive materials close together so that flying neutrons knock out more neutrons free from other atoms creating a self-sustaining chain reaction. At that time, there was no easy method to determine how much uranium or plutonium would be required to achieve critical mass. So scientists figured it out the hard way.

At the head of the Critical Assemblies group was Otto Robert Frisch, an Austrian physicist. Frisch’s method was simple and dangerous. Frisch cut up the fissile materials into small bars 3 centimeters long, and stacked them up while keeping an eye on his radiation meter until criticality was achieved. To help slow down the reaction, he used uranium hydride instead of pure uranium. One day Frisch almost caused a runaway reaction when he casually leaned on the stack of uranium bars causing his body to reflect neutrons back into the stack. Out of the corner of his eye he caught the red lamps, that flickered intermittently when neutrons were around, glow continuously. Realizing what was happening, Frisch quickly scattered the bars with his hand. Frisch received a generation amount of radiation in the mishap, but not large enough to kill him. The same, however, couldn’t be said for his fellow scientists who were to repeat his experiments later.

The first fatality was 24-year old physicist Harry K. Daghlian, Jr. Daghlian was working alone late on the night of August 21, 1945, building a neutron reflector. He was placing bricks of tungsten-carbide around the sphere of plutonium to see how many bricks it would take to reflect enough neutrons back into the core for it to go critical. While working on the experiment, the neutron counter he used to measure the radiation coming off the plutonium core indicated that placing the last brick would cause the assembly to go supercritical. So Daghlian stopped and cautiously retracted his hand which held the brick he was about to lay. But then he dropped it, right over the core.

A re-creation of the experiment Harry Daghlian was involved in.

The moment the brick hit the assembly, the core went super-critical. Daghlian reported seeing a blast of blue light and a wave of heat. He instinctively knocked the dropped brick to the floor with his hand, but it was too late. In those few moments, Daghlian received a fatal does of radiation. He died 25 days later from acute radiation poisoning. A security guard, who was sitting at his desk twelve feet away when Daghlian dropped the brick, developed radiation-caused leukemia and died 33 years later.

Harry Daghlian's blistered and burnt hand after he received his fatal radiation dose.

You might think that a mishap as such would prompt scientists to become more cautious when conducting further critical mass experiments, which renowned physicist Richard Feynman likened to “tickling the tail of a sleeping dragon.” Yet, exactly nine months later, a second accident occurred with the exact same core.

This time, senior physicist Louis Slotin, who succeeded Otto Robert Frisch, was conducting a different type of criticality experiment with the plutonium core. Slotin’s method involved lowering half a shell of beryllium over the core until the core was just about completely covered. The beryllium shell reflected the neutrons radiated by the core back into the core until the core achieved critical state. The idea was to stop just before this stage. Slotin used the blade of a flat head screwdriver which he wedged between the two components to keep them apart. The screwdriver was the only thing that kept Slotin and his fellow scientists safe from certain doom.

Slotin conducted this experiment so many times in the past that he became brash. That afternoon of May 21, 1946, Slotin arrived in his trademark blue jeans and cowboy boots, and began ‘tickling the dragon’s tail’ in front of seven colleagues. But this time his screwdriver slipped. The beryllium shell fell completely covering the core and in an instant the room was covered with a flash of bright blue light as the core went super-critical. Everybody started yelling at once. There was total pandemonium. The security guard in the room understood very little what was happening, but when he saw the blue flash and others started yelling, he ran out of the room and continued running towards the hills. He was later called back as the scientists tried to figure out where everybody was standing and estimate how much radiation each received.

A recreation of Slotin's experiment made by the public historian Richard G. Hewlett.

Slotin, who was closest to the core, took the bulk of the blow. He was exposed to over 1,000 rads of radiation, a terribly high dose anyone has ever taken. For comparison, when the bomb went off in Hiroshima, the radiation at a distance of one kilometer from ground zero was 400 rads. According to physicist Raemer Schreiber, who was present in the room during the fatal slip of the screwdriver, Slotin’s first words immediately after the incident were “Well, that does it.” He died after an agonizing nine days in the hospital.

Nuclear weapon historian Alex Wellerstein provides a description of Slotin’s deteriorating health from the day of the incident to his death:

Slotin vomited once prior to being examined, and several times more in the next few hours, but stopped by the next morning. His general health seemed acceptable. But his left hand, initially numb and tingling, became increasingly painful. This was the hand that had been closest to the core, and scientists later estimated that it had received more than fifteen thousand rem of low-energy X rays. Slotin’s whole-body dose was around twenty-one hundred rem of neutrons, gamma rays, and X rays. (Five hundred rem is usually fatal for humans.) The hand eventually took on a waxy blue appearance and developed large blisters. Slotin’s physicians kept it packed in ice, to limit the swelling and the pain. His right hand, which had been holding the screwdriver, suffered lesser versions of these symptoms.

On the fifth day, Slotin’s white-blood-cell count dropped dramatically. His temperature and pulse began to fluctuate. “From this day on, the patient failed rapidly,” the medical report noted. Slotin suffered nausea and abdominal pain and began losing weight. He had internal radiation burns—what one medical expert called a “three-dimensional sunburn.” By the seventh day, he was experiencing periods of “mental confusion.” His lips turned blue and he was put in an oxygen tent. Eventually, he sank into a coma. He died nine days after the accident, at the age of thirty-five.

A recreation of Slotin's experiment made by the public historian Richard G. Hewlett.

Standing near Slotin and watching over his shoulder was Graves. Slotin’s body partially shielded him, so he received a high but non-lethal dose of radiation. Graves was hospitalized for several weeks with severe radiation poisoning and developed chronic neurological and vision problems as a result of the exposure. He died 20 years later, at age 55, of a heart attack which may or may not have been caused by radiation exposure.

Another physicist, Marion Edward Cieslicki, died of acute myelocytic leukemia, 19 years after the accident.

It took two deaths for Los Alamos to finally bring an end to hands-on criticality experiments. They were always known to be dangerous—Enrico Fermi himself had warned Slotin that he would be "dead within a year" if he continued performing the test in that manner. Further experiments were carried out using remote-control machines and TV cameras while all personnel stood at a quarter-mile distance.

The plutonium core that killed Daghlian and Slotin was originally nicknamed Rufus, but after the accidents it came to be called the demon core. After the accident, the core was still highly radioactive and needed time to cool off. It was slated for test at Crossroads, but when the test was cancelled, the core was melted and the material was recycled to make new cores.