Tokaimura nuclear accident
Encyclopedia
The Tokaimura nuclear accident (), which occurred on 30 September 1999, resulted in two deaths. At that time, it was Japan
's worst civilian nuclear radiation
accident. The criticality accident
occurred in a uranium
reprocessing facility operated by JCO (formerly Japan Nuclear Fuel Conversion Co.), a subsidiary of Sumitomo Metal Mining Co. in the village of Tōkai
, Naka District
, Ibaraki
.
The accident occurred as three workers were preparing a small batch of fuel for the Jōyō
experimental fast breeder reactor, using uranium enriched to 18.8% with the fissile
radionuclide
(radioisotope) known as U235
(with the remainder being the non-fissile U238
). It was JCO's first batch of fuel for that reactor in three years, and no proper qualification and training requirements appear to have been established to prepare those workers for the job. At around 10:35 a.m., a precipitation tank reached critical mass
when its fill level, containing about 16 kilograms of uranium, reached about 40 liters (about 35 pounds and 11 gallons respectively).
Criticality was reached upon the technicians adding a seventh bucket of an aqueous uranium solution known as uranyl nitrate
to the tank. The nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation. The technicians, one of whom had his body draped over the tank, observed a blue flash of Cherenkov radiation
and gamma-radiation alarms sounded. The two technicians closest to the tank immediately experienced pain, nausea, difficulty breathing, and other symptoms. The technician closest to the tank lost consciousness in the decontamination room a few minutes later and began to vomit. There was no explosion, but fission products (fission fragments of U235 with atomic masses typically around 95 and 137, such as yttrium94
and barium140
) were progressively released inside the building.
Being a wet process with an intended liquid result, the water promoted the chain reaction by serving as a neutron moderator
, whereby neutrons emitted from fissioned nuclei are slowed so they are more readily absorbed by neighboring nuclei, inducing them to fission in turn.
The criticality continued intermittently for about 20 hours. As the solution boiled vigorously, steam bubbles attenuated the liquid water's action as a neutron moderator (see Void coefficient
) and the solution lost criticality. However, the reaction resumed as the solution cooled and the voids disappeared. The following morning, workers permanently stopped the reaction by draining water from a cooling jacket surrounding the precipitation tank since that water was serving as a neutron reflector
. A boric acid
solution (boron
being a good neutron absorber) was then added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to radioactivity.
The direct cause of the criticality accident was workers putting uranyl nitrate solution containing about 16 kg of uranium, which exceeded the critical mass for the precipitation tank. The tank was not designed to hold this type of solution and was not configured to prevent criticality.
A dose of 50 millisieverts
(mSv) is the maximum allowable annual dose for Japanese nuclear workers. For context, 8000 mSv (800 rem) is normally a fatal dose and more than 10,000 mSv is almost invariably fatal. Normal background radiation amounts to an annual exposure of about 3 mSv. There were 56 plant workers whose exposures ranged up to 23 mSv and a further 21 workers received elevated doses when draining the precipitation tank. Seven workers immediately outside the plant received doses estimated at 6–15 mSv (combined neutron and gamma effects).
The three operators' doses were far above permissible limits at 3,000, 10,000, and 17,000 mSv; the two receiving the higher doses died several months later. The most severely exposed worker had his body draped over the tank when it went critical. He suffered serious burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count.
The cause of the accident was said to be "human error and serious breaches of safety principles", according to the International Atomic Energy Agency
.
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
's worst civilian nuclear radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
accident. The criticality accident
Criticality accident
A criticality accident, sometimes referred to as an excursion or a power excursion, is an accidental increase of nuclear chain reactions in a fissile material, such as enriched uranium or plutonium...
occurred in a uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
reprocessing facility operated by JCO (formerly Japan Nuclear Fuel Conversion Co.), a subsidiary of Sumitomo Metal Mining Co. in the village of Tōkai
Tokai, Ibaraki
is a village located in Naka District, Ibaraki, Japan. It is approximately 120 km north of Tokyo, Japan on the Pacific coast.As of 1 January 2005, the village has an estimated population of 35,467 and a population density of 946.29 persons per km²...
, Naka District
Naka District, Ibaraki
Naka is a district of Ibaraki, Japan.Following the January 21, 2005 formation of the city of Naka, the district is coextensive with the village of Tōkai. As of January 1, 2005 population data, the district has an estimated population of 35,467 and a density of 946.29 persons per km²...
, Ibaraki
Ibaraki Prefecture
is a prefecture of Japan, located in the Kantō region on the main island of Honshu. The capital is Mito.-History:Ibaraki Prefecture was previously known as Hitachi Province...
.
The accident occurred as three workers were preparing a small batch of fuel for the Jōyō
Joyo (nuclear reactor)
is a test sodium-cooled fast reactor located in Ōarai, Ibaraki, Japan, operated by the Japan Atomic Energy Agency. The name comes from the previous country name of the area around Ibaraki....
experimental fast breeder reactor, using uranium enriched to 18.8% with the fissile
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
radionuclide
Radionuclide
A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
(radioisotope) known as U235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
(with the remainder being the non-fissile U238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
). It was JCO's first batch of fuel for that reactor in three years, and no proper qualification and training requirements appear to have been established to prepare those workers for the job. At around 10:35 a.m., a precipitation tank reached critical mass
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
when its fill level, containing about 16 kilograms of uranium, reached about 40 liters (about 35 pounds and 11 gallons respectively).
Details of the accident
Uranyl nitrate
Uranyl nitrate is a water soluble yellow uranium salt. The yellow-green crystals of uranium nitrate hexahydrate are triboluminescent.Uranyl nitrate can be prepared by reaction of uranium salts with nitric acid...
to the tank. The nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation. The technicians, one of whom had his body draped over the tank, observed a blue flash of Cherenkov radiation
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...
and gamma-radiation alarms sounded. The two technicians closest to the tank immediately experienced pain, nausea, difficulty breathing, and other symptoms. The technician closest to the tank lost consciousness in the decontamination room a few minutes later and began to vomit. There was no explosion, but fission products (fission fragments of U235 with atomic masses typically around 95 and 137, such as yttrium94
Isotopes of yttrium
Natural yttrium is composed of only one isotope 89Y. The most stable radioisotopes are 88Y which has a half-life of 106.65 days and 91Y with a half-life of 58.51 days. All the other isotopes have half-lives of less than a day, except 87Y, which has a half-life of 79.8 hours, and 90Y, with 64 hours...
and barium140
Isotopes of barium
Naturally occurring barium is a mix of six stable isotopes and one very long-lived radioactive primordial isotope, barium-130, recently identified as being unstable by geochemical means...
) were progressively released inside the building.
Being a wet process with an intended liquid result, the water promoted the chain reaction by serving as a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
, whereby neutrons emitted from fissioned nuclei are slowed so they are more readily absorbed by neighboring nuclei, inducing them to fission in turn.
The criticality continued intermittently for about 20 hours. As the solution boiled vigorously, steam bubbles attenuated the liquid water's action as a neutron moderator (see Void coefficient
Void coefficient
In nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant...
) and the solution lost criticality. However, the reaction resumed as the solution cooled and the voids disappeared. The following morning, workers permanently stopped the reaction by draining water from a cooling jacket surrounding the precipitation tank since that water was serving as a neutron reflector
Neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, and tungsten carbide, or other materials...
. A boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
solution (boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
being a good neutron absorber) was then added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to radioactivity.
The direct cause of the criticality accident was workers putting uranyl nitrate solution containing about 16 kg of uranium, which exceeded the critical mass for the precipitation tank. The tank was not designed to hold this type of solution and was not configured to prevent criticality.
Evacuation
Five hours after the start of the criticality, evacuation commenced of some 161 people from 39 households within a 350 meter radius from the conversion building. Residents were allowed home two days later after sandbags and other shielding ensured no hazard from residual gamma radiation. Twelve hours after the start of the incident residents within 10 km were asked to stay indoors as a precautionary measure, and this restriction was lifted the following afternoon.Aftermath
Dozens of emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours. At least 667 workers, emergency responders, and nearby residents were exposed to excess radiation as a result of the accident.A dose of 50 millisieverts
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...
(mSv) is the maximum allowable annual dose for Japanese nuclear workers. For context, 8000 mSv (800 rem) is normally a fatal dose and more than 10,000 mSv is almost invariably fatal. Normal background radiation amounts to an annual exposure of about 3 mSv. There were 56 plant workers whose exposures ranged up to 23 mSv and a further 21 workers received elevated doses when draining the precipitation tank. Seven workers immediately outside the plant received doses estimated at 6–15 mSv (combined neutron and gamma effects).
The three operators' doses were far above permissible limits at 3,000, 10,000, and 17,000 mSv; the two receiving the higher doses died several months later. The most severely exposed worker had his body draped over the tank when it went critical. He suffered serious burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count.
The cause of the accident was said to be "human error and serious breaches of safety principles", according to the International Atomic Energy Agency
International Atomic Energy Agency
The International Atomic Energy Agency is an international organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons. The IAEA was established as an autonomous organization on 29 July 1957...
.
See also
- Fukushima I nuclear accidents
- Nuclear power in JapanNuclear power in JapanNuclear energy was a national strategic priority in Japan, but there has been concern about the ability of Japan's nuclear plants to withstand seismic activity...
- Criticality accidentCriticality accidentA criticality accident, sometimes referred to as an excursion or a power excursion, is an accidental increase of nuclear chain reactions in a fissile material, such as enriched uranium or plutonium...
- 5 yen coin
External links
- What Happened at Tokaimura?
- Tokaimura Criticality Accident - What happened in Japan
- International Atomic Energy Agency: “Report on the preliminary fact finding mission following the accident at the nuclear fuel processing facility in Tokaimura, Japan”, 1999 (9.5 MB PDF, here )
- Criticality accident at Tokai nuclear fuel plant (Japan) Wise Uranium project