Demon Core: The 3.5-Inch Nuclear Sphere That Caused the Deaths of Two Physicists

After the conclusion of World War II and the catastrophic consequences of the Hiroshima and Nagasaki nuclear attacks, the Cold War was imminent. The devastating impact and formidable capabilities of atomic bombs compelled global superpowers to engage in a fervent pursuit of nuclear research. The United States, for instance, was willing to deploy a third atomic bomb on Japan if deemed necessary, while other nations developed their own stockpiles as a means of deterrence or protection.

Access the “demon core”. Weighing a substantial 6.2 kilograms (13.7 pounds) and measuring 8.9 centimeters (3.5 inches) in diameter, this spherical object made of radioactive plutonium (referred to as “Rhufus” at the time) was specifically created for nuclear research as a fissile core for early versions of the atomic bomb. From 1945 to 1946, the demon core underwent experimentation and could have potentially been used in an atomic bomb targeted at Japan if they had not surrendered.

True to its foreboding name, the demon core proved to be unforgiving to the nuclear physicists involved. Created with the purpose of serving as a bomb core, it possessed a very small margin before reaching a point where its radioactivity would intensify and become supercritical (a state in which the fission reaction accelerates). Hence, it is imperative to closely monitor any external factors that may enhance reactivity, such as the compression of the core, which is the mechanism behind the detonation of a fission bomb, in the vicinity of the demon core.

Provoking the dragon’s anger
Notwithstanding the peril, scientists utilized the core as a test subject for supercriticality, employing neutron reflectors to maximize its potential. Neutron reflectors are employed to encompass the core, and during the nuclear fission process, they redirect neutrons towards the nuclear material in order to augment the extent of fission occurring.

In 1945, physicist Harry Daghlian, working in isolation in his laboratory, conducted an experiment involving a neutron reflector on the demon core. However, due to an error, he accidentally dropped a block of reflective tungsten carbide onto the core, causing it to reach a state of supercriticality and resulting in the emission of a highly lethal burst of neutron radiation. Following a three-week struggle with acute radiation sickness, Daghlian ultimately succumbed to his injuries, prompting the implementation of more stringent regulations regarding nuclear research in the Manhattan Project, although these measures were still not considered sufficiently strict.

Undeterred by the horrific demise of his predecessor and colleague, Louis Slotin proceeded with Daghlian’s research to determine the proximity of the core to supercriticality. In order to accomplish this, the method involved gradually descending a neutron reflector onto the core while monitoring the level of activity within. As previously mentioned, any contact between the reflector and the demon core would result in a catastrophic outcome. Therefore, spacers were employed to maintain a safe distance between the two.

Louis Slotin, a skilled physicist and audacious nuclear enthusiast, was not granted access to the established procedure. Instead, Slotin developed his own technique that involved less time and effort but significantly increased risk. Slotin eliminated the spacers that prevented the core from touching the reflectors. He would then use a flathead screwdriver to ensure that there was a gap between them, and he would control the screwdriver with one hand during the entire experiment. He rapidly gained recognition for his experiment, which was named “tickling the dragon’s tail” by renowned physicist Richard Feynman, who compared the audacious act to provoking a dragon. Slotin continued to repeat the experiment despite warnings from well-known experts.

Regrettably, Slotin’s display of boldness would ultimately lead to his downfall. On May 21, 1946, Slotin was showcasing his experiment to a group of individuals at the Los Alamos Scientific Laboratory. During the demonstration, his screwdriver accidentally slipped, causing the neutron reflector to enclose the core slightly and trigger a state of supercriticality. The nucleus suddenly released a burst of blue light and a powerful wave of heat that engulfed Slotin and several of his coworkers. Slotin expeditiously removed the reflectors from the core, promptly halting the reaction. However, the detrimental effects had already occurred.

Slotin would succumb to the lethal effects of the radiation burst within a span of nine days after being exposed. Alvin Graves, who was present next to him during the accident, also received a substantial amount of radiation but managed to survive the incident and live for an additional 20 years before passing away. Thanks to Slotin’s prompt decision-making and strategic body positioning, which effectively absorbed the majority of the radiation, the remaining spectators were protected from the explosion and lived to recount the incident.

After the accidents, the core was ultimately named the demon core before being repurposed into other fissile cores.