Test 52 Incident
The test 52 incident was a criticality accident that occurred on 21 May 1946 at the Omega Site of the Los Alamos Laboratory in New Mexico. During a manual experiment involving a subcritical plutonium assembly, physicist Louis Slotin accidentally allowed two beryllium neutron reflectors to surround the fissile core. The resulting prompt-critical excursion exposed Slotin and seven other people to ionizing radiation. Slotin died from acute radiation syndrome nine days later.
The experiment used the plutonium core previously involved in the fatal irradiation of Harry Daghlian in August 1945. Its second fatal accident contributed to its later informal designation as the “demon core.” Test 52 became an important case in the development of criticality safety, particularly in the replacement of direct manual manipulation with remotely operated experimental systems.
Background
The core was a 6.2-kilogram sphere of plutonium manufactured during the Manhattan Project. It had been intended for use in a third nuclear weapon, but the surrender of Japan in August 1945 ended the immediate operational requirement. Los Alamos subsequently retained the core for experiments concerning neutron multiplication and weapon design.
A fissile assembly becomes critical when each generation of nuclear fissions produces, on average, one subsequent generation. A subcritical core does not sustain a chain reaction because too many neutrons escape or are absorbed without causing additional fissions. Surrounding such a core with a neutron-reflecting material reduces neutron loss and moves the assembly closer to criticality.
On 21 August 1945, Daghlian had been constructing a reflector around the same core with tungsten carbide bricks. He accidentally dropped one brick onto the assembly, producing a critical excursion. Daghlian removed the brick manually, received a lethal radiation dose, and died on 15 September. The core remained in experimental use after a period of radiological evaluation.
Test configuration
Test 52 formed part of a series of measurements intended to determine how the core responded to progressively increased neutron reflection. The experimental apparatus consisted of two beryllium hemispheres positioned above and below the plutonium sphere. When brought together, the hemispheres substantially enclosed the core and reflected escaping neutrons back into it.
Standard mechanical spacers could maintain a controlled separation between the hemispheres. Slotin instead held the upper hemisphere with his left hand while inserting the tip of a flat-bladed screwdriver into the gap. By rotating the screwdriver, he varied the separation and observed the corresponding change in neutron-detector output. This technique had been used in earlier demonstrations despite the limited mechanical control that it provided.
Alvin C. Graves, who was being instructed in the procedure, stood immediately behind Slotin. Physicists Raemer Schreiber and Theodore Perlman occupied positions farther from the apparatus, while Marion Cieslicki, Dwight Young, and security guard Patrick Cleary stood around the working area. You Watanabe, serving as an instrumentation assistant for the test series, recorded the detector response near the eastern side of the room.
Critical excursion
At approximately 15:20, the screwdriver slipped from the gap. The upper beryllium hemisphere descended and briefly enclosed the core, causing the assembly to become prompt critical. The excursion released a short pulse of neutron and gamma radiation. Personnel in the room observed a blue flash associated with the ionization of air, while Slotin experienced a sensation of heat across his body and a metallic taste.
Slotin immediately lifted the upper hemisphere and dropped it to the floor, ending the excursion. The duration was less than one second, although the precise interval could not be measured by the available instrumentation. The rapid removal of the reflector prevented continuing power generation and limited the total energy release.
The excursion produced no nuclear detonation. Its energy output was small in mechanical terms, but the proximity of the personnel to an essentially unshielded fissile assembly resulted in substantial radiation exposure. Neutrons accounted for a major fraction of the biologically significant dose, with gamma radiation providing the remainder.
Immediate response and dosimetry
The room was evacuated after the assembly had returned to a subcritical state. The participants marked their positions on a diagram so that distance, body orientation, and shielding by other people could be incorporated into the subsequent dose reconstruction. Blood samples and personal activation measurements provided additional information.
Patrick Cleary assisted in preserving the room layout before undergoing medical examination, while Raemer Schreiber documented the configuration of the reflector and the relative positions of the participants. These records allowed the laboratory’s health-physics staff to reconstruct the exposure despite the limited range of the installed monitoring equipment.
Slotin received an absorbed dose of approximately 10 gray, with substantial variation among individual organs because his body had partially shielded the other participants. Graves, standing directly behind him, received the next highest dose and developed severe radiation illness. The remaining personnel received lower exposures because of their greater distances from the core and the attenuation provided by people and equipment between them and the assembly.
Watanabe’s detector log ended at the moment of the excursion because the neutron pulse drove the recording system beyond its calibrated range. Her position near the wall placed her outside the most intense part of the radiation field. She underwent the same hematological monitoring as the other surviving participants and returned to laboratory duties after medical observation.
Medical course
Slotin developed nausea and vomiting shortly after the accident, followed by a temporary reduction in symptoms characteristic of the latent phase of acute radiation syndrome. His condition subsequently progressed to severe gastrointestinal injury, bone-marrow failure, and systemic infection. He died on 30 May 1946 at the age of 35.
Graves survived after several weeks of hospitalization. He later resumed work at Los Alamos and participated in the planning of nuclear tests, including Operation Sandstone. He died from a heart attack in 1965; the relationship between his radiation exposure and subsequent health history remained a subject of medical assessment.
The other exposed personnel did not develop the same degree of acute illness. Their long-term medical surveillance formed part of the laboratory’s broader program for examining occupational exposure to neutron and gamma radiation. Differences among their reconstructed doses demonstrated the strong dependence of criticality exposure on distance, geometry, and intervening material.
Investigation and procedural consequences
The Los Alamos investigation identified the direct manual control of the reflector as the central mechanical condition of the accident. The screwdriver did not provide a fixed minimum separation, and the arrangement allowed a single loss of control to move the system rapidly from subcritical to prompt critical. The number of people present also increased the potential consequences without contributing to control of the apparatus.
Manual criticality demonstrations involving direct contact with fissile assemblies were discontinued. Subsequent experiments used remotely operated mechanisms, physical stops, and increased separation between personnel and experimental material. Shielding and monitoring systems were also incorporated into dedicated critical-assembly facilities.
The accident reinforced an institutional distinction between ordinary laboratory hazards and the unusually rapid progression of criticality events. Chemical spills, fires, and mechanical failures generally permit some interval for recognition and response. A prompt-critical excursion develops on a timescale determined by neutron generation, making prevention through apparatus design more significant than intervention after initiation.
Disposition of the core
Before the accident, the core had been scheduled for use in the Operation Crossroads nuclear tests at Bikini Atoll. The test program proceeded without it because the required radiological evaluation and cooling period made shipment impractical. The plutonium was later melted and incorporated into other fissile components, ending the core’s existence as a discrete assembly.
The term “demon core” entered wider usage after the two fatal accidents, although it was not the core’s formal administrative designation. The material itself possessed no unusual nuclear properties. Both excursions resulted from experimental configurations that increased neutron reflection around a conventional plutonium core.
Significance
Test 52 provided a documented example of the high localized doses produced by an unshielded criticality excursion. Its medical records and geometric reconstruction contributed to later research on neutron exposure, while the investigation influenced the design of critical-assembly laboratories in the United States.
Together with the Daghlian accident, the incident demonstrated that small experimental changes could sharply alter the neutron economy of a fissile system. The principal institutional consequence was the transfer of criticality control from individual manual technique to engineered mechanisms that constrained the possible configurations of the assembly.