Criticality Accident - Incidents

Incidents

Since 1945 there have been at least 60 criticality accidents. These have caused at least 21 deaths: seven in the United States, ten in the Soviet Union, two in Japan, one in Argentina, and one in Yugoslavia. Nine have been due to process accidents, with the remaining from research reactor accidents.

Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors.

• On 4 June 1945, Los Alamos scientist John Bistline was conducting an experiment to determine the effect of surrounding a sub-critical mass of enriched uranium with a water reflector. The experiment unexpectedly became critical when water leaked into the polyethylene box holding the metal. When that happened, the water began to function as a highly effective moderator rather than just a neutron reflector. Three people received non-fatal doses of radiation.

• On 21 August 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.

• On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident (called the "Paharito accident" at the time) using the very same sphere of plutonium responsible for the Daghlian accident. Slotin surrounded the plutonium sphere with two 9-inch diameter hemispherical cups of neutron-reflecting material (beryllium); one above and one below. He was using a screwdriver to keep the cups slightly apart, which kept the assembly subcritical. When the screwdriver accidentally slipped, the cups closed completely around the plutonium, sending the assembly supercritical. Immediately realizing what had happened, he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin succumbed to radiation poisoning nine days later.

• On 16 June 1958, the first recorded uranium-processing–related criticality occurred at the Y-12 Plant in Oak Ridge, Tennessee. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55-gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for forty-four days. All eight workers eventually returned to work.

• On 15 October 1958, a criticality excursion in the heavy water RB reactor at the Vinca Nuclear Institute in Vinča, Yugoslavia, killed one and injured five. The initial survivors received the first ever bone marrow transplant in Europe.

• On 30 December 1958, the Cecil Kelley criticality accident took place at the Los Alamos National Laboratory. Cecil Kelley, a chemical operator working on plutonium purification, switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 rads according to later estimates. The other operators reported seeing a flash of light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later.

• On 23 July 1964, a criticality accident occurred at the Wood River Junction facility in Charlestown, Rhode Island. The plant was designed to recover uranium from scrap material left over from fuel element production. An operator, intending to add trichloroethane to a tank containing uranium-235 and sodium carbonate to remove organics, erroneously added uranium solution instead, producing a criticality excursion. The operator was exposed to a fatal radiation dose of 10,000 rad (100 Gy). Ninety minutes later a second excursion happened when a plant manager returned to the building and turned off the agitator, exposing himself and another administrator to doses of up to 100 rad (1 Gy) without ill effect. The operator involved in the initial exposure died 49 hours after the incident.

• On 10 December 1968, Mayak, a nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution"; in other words, the operators were decanting plutonium solutions into the wrong type of container. After most of the solution had been poured out, there was a flash of light and heat. "Startled, the operator dropped the bottle, ran down the stairs, and from the room." After the complex had been evacuated, the shift supervisor and radiation control supervisor re-entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident and possibly attempted to pour the solution down a floor drain, causing a large nuclear reaction that irradiated the shift supervisor with a fatal dose of radiation.

• On 23 September 1983, an operator at the RA-2 research reactor in Centro Atomico Constituyentes, Buenos Aires, Argentina received a fatal radiation dose of 3700 rads (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured.

• On 30 September 1999, at a Japanese uranium reprocessing facility in Tokai, Ibaraki, workers put a mixture of uranyl nitrate solution into a precipitation tank which was not designed to dissolve this type of solution and caused an eventual critical mass to be formed, and resulted in the death of two workers from radiation poisoning.

• Based on incomplete information about the 2011 Fukushima I nuclear accidents, Dr. Ferenc Dalnoki-Veress speculates that transient criticalities may have occurred there. Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) “emphasized that the nuclear reactors won’t explode.” By March 23, 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring. Additionally, on April 15, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radiation release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality", most unlikely.