September 30, 1999, at 10:35 a.m., three workers at the Japan Nuclear Fuel Conversion Company (JCO) uranium-processing facility in Tokaimura, Japan, received severe radiation doses in one of the worst criticality accidents in decades.


Nuclear criticality is basically a self-sustaining fission reaction or a self-sustaining chain reaction. Fission occurs when a sufficient amount (critical mass) of fissionable material, such as Uranium 235 or Plutonium 239, absorbs a thermal (low-energy, slow-moving) neutron. This absorption results in an unstable nucleus that decays by splitting into two almost equal portions, releasing energy and more neutrons. In the case of Uranium 235, about 208 MeV (million electron volts) of energy are released-83 percent in the form of kinetic energy (heat) and six percent in the form of radiation, gamma rays, x-rays, neutrons, and electrons. The remaining energy is emitted as radiation from decay products produced as a result of the fissioning process.

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The minimum critical masses for Uranium 235 and Plutonium 239 are listed above.

For a mass of fissionable material to achieve criticality, the number of neutrons in one generation must equal or exceed the number of neutrons in the next succeeding generation, or when Keff is equal to or greater than one. In other words, neutron production equals or exceeds neutron losses from the system. Several factors can affect the multiplication of neutrons in a critical assembly. They include materials in and near the fissile material, low Z (molecular weight) materials such as hydrogen-containing substances, carbon, beryllium, and concrete. These materials, if close, will increase neutron flex (multiplication).

The shape of the fissile material can affect the multiplication of neutrons in a critical assembly. The worst shape (geometry) a critical assembly can be in is the sphere, which has the lowest surface area per unit volume, which will increase neutron flux. The flat sheet is the best shape, since it has the largest surface area per unit volume.

The physical arrangement of the parts that form a critical assembly (critical mass) can also affect the multiplication of neutrons in the critical assembly.

From the above, it can be seen that controlling criticality involving fissionable materials involves preventing the accumulation of a critical mass of fissionable material from developing, controlling the shape (geometry) of a subcritical mass of fissionable material, and controlling the surrounding area with respect to materials that can increase neutron flux or reflect neutrons back into the subcritical mass.

Criticality accidents result from the uncontrolled release of energy caused by nuclear fission. A characteristic phenomenon produced during criticality is Chernkov Radiation, a blue light observed in highly radioactive solutions or in the water around nuclear reactors during operations. It results from beta particles traveling faster than the speed of light through water. In air systems, it results from the ionization of the air. It requires approximately 72107 R/sec to produce Chernkov radiation in air.


Between 1944 and 1998, there have been nine criticality accidents, resulting in significant radiation exposure. Following are some examples of those incidents.

  • Los Alamos, New Mexico, June 4, 1945. This experiment was designed to measure the critical mass of enriched uranium surrounded by a hydrogenous material. Enriched uranium blocks were staked in a pseudo-spherical arrangement in a polyethylene box in a steel tank. Water was added to the tank. Within a few seconds, the fission count rate began to increase, and a blue glow surrounded the tank. The water supply line was closed, and the drain valve was quickly opened. Two men received a radiation dose of 66 rem1; a third man received a dose of 7.4 rem.
  • Los Alamos, August 21, 1945. During the process of making critical mass studies, Harry Daghlin, working in a laboratory at night, alone except for a guard who was seated 12 feet away, was staking blocks of tamper (neutron-reflecting) material around a mass of fissionable material. As the assembly reached critical configuration, Daghlin was lifting one of the last pieces of tamper material into place. As he moved his hand to set the block at a distance from the assembly, he dropped the piece of tamper material on top of the setup. He observed a blue glow. Daghlin disassembled the entire assembly. He died 28 days later. It is estimated he received a radiation dose of 1,350 rads.2
  • Los Alamos, May 21, 1946. Louis Slotin was demonstrating the technique of critical assembly and associated study measurements to another scientist. The technique involved bringing a hollow hemisphere of beryllium around a mass of fissionable material contained in another hemisphere of beryllium. The system was maintained subcritical by keeping the two hemispheres separated by spacers. At the time of the accident, Slotin was using a screwdriver in place of one of the spacers to hold the two hemispheres apart. The screwdriver slipped, and the upper hemisphere fell into position around the fissionable material. A blue glow was observed, and heat was felt. Slotin immediately removed the upper hemisphere. He died nine days later.

  • It was estimated he received a radiation dose of 3,000 to 000 rads. The student received a sufficient dose of radiation to cause serious injury and disability.
  • Woods River Junction, Rhode Island, June 24, 1954. Mr. Peabody, a technician at Unified Nuclear Corporation Fuel Recycling Plant, poured 11 liters of concentrated urinayl nitrate solution into a tank containing 0.54 molar sodium carbonate. He thought he was adding TCE to the solution. There was the typical blue flash and physical ejection of material from the tank. Peabody died 49 hours post-exposure. He is estimated to have received a radiation dose of between 14,000 and 46,000 rads.
  • Los Alamos, December 30, 1958. A chemical operator introduced a highly concentrated plutonium solution, believed to be a dilute solution, into a tank already containing a concentrated solution of plutonium emulsion. A criticality excursion occurred immediately after the operator started the motor to a propeller-type stirrer. A blue flash and a loud noise occurred. The operator received an estimated dose of 12,000 rem. He died 35 hours later.


Reprocessing of spent fuel recovers the uranium and plutonium. The uranium is enriched and converted to the oxide form for reuse in reactors. Japanese reactors use a mixture of plutonium oxide and enriched uranium, called “mixed oxide” or “MOX.” The official line is that MOX kills two birds with one stone. First, using MOX in reactors cuts consumption of uranium by 25 percent. Second, using MOX deals with the problem of what to do about the surplus of plutonium accumulated over the years. Burning it as MOX keeps it out of the hands of those who might want to convert it into bombs.

Japan’s nuclear program has another odd feature. The country has clung to its fast-breeder reactors. Unlike conventional reactors, which use fuel whose uranium content has been enriched to about three percent, fast-breeder reactors use fuel containing plutonium and highly enriched uranium enriched to about 20 percent in their cores.

The accident at Tokaimura was caused in part because the plant was handling uranium that had been highly enriched so it could be used in a fast-breeder reactor.


The Tokaimura facility is part of JCO Co.’s Tokai Works. The site is one of 15 nuclear sites in Tokaimura, a city of about 34,000 people. The entire facility is unshielded because process material is unirradiated. The plant had no criticality accident response plans or criticality accident alarm system because it was assumed “critical fission chain reactions could not occur.” More probably, according to the Report on the Preliminary Fact Finding Mission following the accident at Tokaimura, “Emergency planning was not required because management and authorities assumed a criticality accident was not credible if appropriate criticality safety programs were implemented.”

This plant usually converts UF6 to UO2 at a five percent maximum enrichment (commercial reactor fuel). However, it occasionally converts U3O8 to uranyl nitrate at about 20 percent enrichment, which is blended with plutonium oxide to form MOX, a breeder reactor fuel.


The first thing the three technicians pouring uranium solution into a sedimentation tank at Tikaimura on September 30, 1999, noticed was a blue flash. Then, they began to experience nausea and some difficulty in breathing. One worker became unconscious. What they did not realize was that they had accidentally dumped more than six times as much fissile materials than they had meant to and had triggered a runaway chain reaction. Unwittingly, they had built a makeshift nuclear reactor.


The accident at Japan Tokaimura occurred when three men poured 16 Kg of 19 percent enriched uranium in solution into a sedimentation tank, creating a critical mass and a critical geometry. The company that operates the uranium-processing plant at Tokaimura had quietly and illegally compiled a manual that encouraged workers to cut corners to reduce costs. Workers were allowed to bypass the time-consuming, automatically controlled mixing process designed to prevent such a criticality event. Instead of pumping carefully metered quantities of solution into the reaction vessel, the operators were instructed to mix the solution by hand in 10-liter stainless-steel buckets. The operators then carried these buckets to the sedimentation tank and tipped them in. The three workers making the fuel were woefully inexperienced; two of them never made fuel before. They did not understand, nor did the company explain to them, the factors that can lead to a criticality accident and steps to be taken to prevent such occurrences.

The sedimentation tank is a vertical cylinder with a flat top, a dished bottom, and a water-cooling jacket. It has a 50-cm inner diameter and a 70-cm inner height. The top has few penetrations, most notably a centrally located vertical pipe 25 cm in diameter, an angled pipe 10 cm in diameter, and four vertical diameter pipes. There are also a drain line in the tank bottom and two horizontal lines on the side, a few centimeters above the bottom. The 55-cm outer diameter water jacket surrounds the bottom and lower third of the tank. The cooling water in the water jacket acted as a neutron reflector.


A flash of blue light signaled the start of the fissioning process and the initial burst of radiation, gamma rays, and neutrons. There was no explosion, although there was a release of gaseous fission products (krypton, xenon, and iodine). The room became radioactive because the system oscillated (pulsed) between super and subcritical states for more than 20 hours, following the initial pulse and the reaction-produced fission products in the container-including radioactive cesium, which emits gamma radiation. Available reports do not indicate the number of pulses, their magnitude, or frequencies, but measured radiation dose rate values at the nearest site boundary seemed fairly steady for hours, indicating the pulse frequency was probably fairly rapid.

Tokyo Electric Power Company rushed 880 pounds of borated material to the JCO plant; however, responders could not use it right away because they had no way of remotely adding it to the reaction tank. Working in shifts of a few minutes at a time, workers attempted to drain the water from the water jacket surrounding the reaction vessel. Finally, workers cut a drainpipe outside, at the valve’s upstream side, and injected argon gas, which successfully drained the water from the cooling jacket. The tank was finally subcritical. The chain reaction was arrested around 6 a.m. on October 1, 1999, nearly 20 hours after the blue flash. Responders added borated solution to the tank at 8 a.m. to further ensure system stability. These responders found in the opening the funnel through which workers had added the materials to the tank.


It seems that emergency responders were notified and activated separately from authorities. Plant personnel completed notifications of JCO officials within 10 minutes. Apparently, none of these officials instructed plant personnel to notify or establish communications with city or regulatory authorities. The first notification to outside officials was made at 11:15 a.m., but city authorities indicate they were not notified until around 11:30 a.m. The accident occurred at approximately 10:30 a.m.; residents were not notified until two and a half hours after the first pulse.

City officials activated their emergency operations center (EOC) and their communications network but were left to determine response actions for residents on their own. Firefighters, police, and army personnel had radios, but communications between other responders in different locations were almost exclusively by telephone and, later, by fax.

All roads within 0.6 miles of the plant were closed; they were open only to emergency vehicles sometime between 11:55 a.m. and 12:15 p.m. About 160 people within a radius of 380 yards of the plant were advised to remain indoors at about 12:30 p.m. and were evacuated from the area at about 3 p.m., September 30, about four and a half hours after the first pulse of a cycling accident. They were not permitted to return to their homes until 6:30 p.m. on October 2.

Evacuees were gathered at a shop about 758 yards from the plant and were then bused to a village evacuation center 1,500 yards from the plant. However, after a night in temporary shelters, some evacuated residents reportedly returned home to care for pets or retrieve fresh clothing.

At approximately 10:30 p.m., authorities advised people within six miles to shelter in place (stay inside with doors and windows closed). This order remained in effect until 6:35 p.m. on October 1. About 310,000 of the population were affected. At the same time, national authorities decided to close all 135 schools and 50 post offices. The Japan railways suspended train service within the area. The Ibaraki Traffics (the dominant bus company in the region) suspended all service, and the Japan Highway Services closed the Mito/Hitachiminami-Ohta section of the Joban Highway, which connects Tokyo and the Tokai regions. Local businesses in the area also shut down.

Authorities warned people not to eat produce or drink milk from the affected area until testing had been completed. The ban was lifted on October 4.


The worker standing on the floor holding the funnel over the entry port received the highest dose of radiation, somewhere in the range of 10 to 20 Gy (1,000 to 2,000 rads). This is considered a lethal dose. His coworker, who was standing on a platform leaning over the vessel and pouring the solution through the funnel, received an estimated dose of 6 to 10 Gy (600 to 1,000 rads), an often fatal dose as well. A third employee, who was in an office about 16 feet away, received an estimated radiation dose of about 1.2 to 5.5 Gy (120 to 550 rads). Besides the three JCO workers noted above, three firefighters, 56 plant workers at the site, and seven members of the public who were near the site boundary at the time were exposed to radiation. At the time this was written, the two workers who received the largest radiation exposures have died. The third worker is recovering slowly.

Twenty-seven emergency workers at the Tokaimura nuclear facility received planned exposure as they sought to contain the problem. The highest dose received by workers involved in controlling the accident was about 10 rem.

In all, more than 7,000 people were checked for radiation exposure. Some of these individuals were slightly radioactive because of the formation of radioactive sodium-24 in their bodies, resulting from neutron activation of the sodium in their body. This was initially misinterpreted as radioactive contamination when, in fact, there was no radioactive contamination.

Three firefighters received radiation exposure in the range of 0.5 to 3.9 mGy (950 to 300 mrads) during the almost one-hour time period they spent in the accident room treating the three victims. The most severely exposed worker was having seizures; responders assumed he was an epileptic. The responders did not realize the seizures were the result of the high radiation dose he had received. The firefighters entered the area without appropriate personal protective equipment. They were not advised of conditions or of the type of accident before entry. Respirators and anticontamination clothing would help protect wearers from some fission products and splashed solution. Other protective measures such as limiting the time in the radiation area and using better-shielded routes were also not used to reduce radiation exposure.

There was some exposure to area residents. The highest measured gamma and neutron dose rates reported were 0.84 mSv/hr (84 mrem/hr) and 4.5 mSv/hr (450 mrem/hr) at the eastern site boundary (location 4) between 11:30 and 11:50 a.m. on September 30, 1999. The gamma dose rate declined slightly over time until midnight October 1, 1999, when it began to drop more rapidly. Near normal levels of gamma radiation were measured by 6:30 a.m. on October 1, 1999. Neutron levels were nondetectable at this time.

Evidence that neutrons reached the residential neighborhoods around the nuclear facility was found when a gold necklace in a house 650 feet away became radioactive because of a reaction in which neutrons were captured by a stable material, creating a small amount of a radioactive isotope, gold-198 (Au-198).


  • Never assume something can’t happen. Anything that can happen will. The Japanese assumed that a criticality accident could not occur. It did.
  • As part of your community’s hazard assessment, identify facilities that store, manufacture, use, process, or transport radioactive materials. Learn what radioactive materials are on-site and in what quantities, what the occupancies do, and what types of accidents could occur.
  • Review and update your emergency response plan and standard operating guidelines as they relate to radioactive materials emergencies. Base your planning on a thorough hazard assessment. The Japanese did not have a plan for dealing with this type of emergency. Their hazard assessment indicated this type of accident was not credible. They did have plans for dealing with a nuclear power plant accident. They quickly found out these plans could not be applied to this type of incident.
  • In any planning that involves possible evacuation, consider and plan for dealing with pets and livestock.
  • Consider and plan for dealing with the psychological impacts of the incident.
  • Get training for yourself, your staff, and members of your response community on the fundamentals of radioactive materials, radiation protection principles, radiation, instrumentation, and response to radiation incidents.
  • Acquire radiation-monitoring equipment, dosimetry, and personal protective equipment. Make arrangements for acquiring specialized detection, monitoring, and sampling equipment and laboratory support should you need them.
  • Each community should have on its EOC staff an individual who can provide advice and guidance to elected officials and the public about matters dealing with radiation and radioactivity within those first crucial minutes and hours post-accident.
  • Get to know the people who regulate radioactive materials in your community. Build them into your planning, training, test, and exercise. Use them as resources.

  • Test and exercise the radiological annex to your emergency response plan. Nte the Nuclear Power Plant Annex, if you have one. You do that every two years anyway. Take your exercise through the recovery phase, even if it’s just tabletop. Learn from your exercise, revise your plan, and exercise it again.
  • Remember those who fail to plan, plan to fail.


Tokaimura residents’ attitudes range from calm to angry over nuclear safety issues. However, most do not seem to be worried about long-term health effects from this accident. The Japanese government probably will not revise its nuclear policy. However, legislators plan to write a nuclear emergency law and to revise an existing nuclear safety law. The United States, the United Kingdom, and several other countries are reviewing criticality safety programs and safety at all nonreactor and possible reactor nuclear facilities in light of the accident.


  1. Dosage of ionizing radiation that will cause the same biological effect as one roentgen of X-ray or gamma-ray exposure.
  2. A unit of absorbed dose of ionizing radiation equal to an energy of 100 ergs per gram of irradiated material.


  • Baker, Michael C., “Criticality Accident Summary Significantly Updated,” Radsafe, Oct. 7, 1999, and Oct, 14, 1999.
  • Choppin, C.R., and J. Rydberg, Nuclear Chemistry Theory and Application, Pergamon Press, 1980.
  • Anonymous, Major Radiation Accidents: Human Experience 1944-Dec. 1998, DOE/REACTS/TS Radiation Accident Registry.
  • Anonymous, “Japanese Nuclear Games,” The Economist, Oct. 9, 1999, 101-102.
  • International Atomic Energy Agency, “Report on the Preliminary Fact Finding Mission Following the Accident at the Nuclear Fuel Processing Facility in Tokaimura, Japan,” 1999. Interviews with the Director General, Department of Civil Affairs and Environment, Ibaraki Prefecture, Oct. 14, 1999; the mayor of Tokaimura, Oct. 14, 1999; and the staff of JCO Company LTD, Oct. 15, 1999.
  • Munger, Frank, “Oak Ridger returns from Japan with details of radiation tragedy,” The Knoxville News-Sentinel, Nov. 15, 1999.
  • NRC, Operational Accidents and Radiation Exposure Experience Within the United States Atomic Energy Commission, 1943-1970, U.S. Government Printing Office, Washington, D.C.

ANTHONY M. GAGLIERD, RO, EMT, is the radiological officer for Allegheny County (PA) Emergency Services, an emergency medical technician, and a team leader for the East Boroughs Hazardous Materials Red Team. He also is an adjunct instructor at the Federal Emergency Management Institute, the Pennsylvania State Fire Academy, the Allegheny County Fire Academy, and Point Park College. He has a bachelor’s degree in behavioral science and chemistry from Point Park.

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