Tokaimura nuclear accident

Coordinates: 36°28′47.00″N 140°33′13.24″E / 36.4797222°N 140.5536778°E / 36.4797222; 140.5536778

Tōkai-mura

Location of the Tokaimura nuclear accident

There have been two Tokaimura nuclear accidents at the nuclear facility at Tōkai, Ibaraki.

1. On 11 March 1997, a small explosion in a Dōnen plant.
2. On 30 September 1999, a serious criticality accident in a JCO plant.

When time is not specified, in most cases the 1999 incident is the one being referred to.^[1]

In 1997

The first Tokaimura nuclear accident was the accident which occurred on 11 March 1997, in a nuclear reprocessing plant of the Dōnen (Power Reactor and Nuclear Fuel Development Corporation). Another name is the Dōnen accident (動燃事故 (Dōnen jiko)).

On the night of Tuesday 11 March 1997, a small explosion occurred in a nuclear reprocessing plant of the Dōnen. Windows were smashed and smoke escaped to the atmosphere.^[2] On Thursday, workers repaired thirty broken windows and three doors with duct tape. They had been damaged during the blast. At least 37 workers were exposed to elevated levels of radiation during the incident.^[3]

A week after the event, meteorological officials detected unusually high levels of Cesium 40 kilometers (25 miles) south-west of the plant.^[4]

In 1999

The second and more serious Tokaimura nuclear accident (Japanese: 東海村JCO臨界事故Tōkai-mura JCO-rinkai-jiko) indicates the nuclear disaster which occurred on 30 September 1999,^[5][6][7] resulting in two deaths.^[8] It was the worst civilian nuclear radiation accident in Japan prior to the Fukushima Daiichi nuclear disaster of 2011.

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 Prefecture.^[9]

The accident occurred as three workers, Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa,^[10] 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 U‑235 (with the remainder being the fissionable-only U‑238). 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 precipitation tank reached critical mass when its fill level, containing about 16 kilograms (35 pounds) of uranium, reached about 40 liters (11 U.S. gallons).^[9]

Details

Criticality was reached upon the technicians adding a seventh bucket of an aqueous uranyl nitrate solution to the tank. The nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation. At the time of the criticality event, Ouchi had his body draped over the tank while Shinohara stood on a platform to pour the solution into it; Yokokawa was sitting at a desk four meters away. All three technicians observed a blue flash (possibly Cherenkov radiation) and gamma-radiation alarms sounded.^[8][11]

Technicians Ouchi and Shinohara immediately experienced pain, nausea, difficulty breathing, and other symptoms. Ouchi then began to vomit in the decontamination room a few minutes later and lost consciousness shortly after.^[12] There was no explosion, but fission products (fission fragments of U‑235 with atomic masses typically around 95 and 137, such as yttrium‑94 and barium‑140) 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.^[9]

Cause

The cause of the accident was the workers adding a uranyl nitrate solution which contained about 16 kg of uranium into the precipitation tank. This greatly exceeded the tank's uranium limit of 2.4 kg and caused an instantaneous and uncontrolled nuclear fission. Under correct procedures, the uranyl nitrate would have been stored inside a buffer tank and then pumped from there into the precipitation tank at intervals of the correct volume level not exceeding 2.4 kg.

In this case, the workers bypassed using any buffer tanks entirely and instead poured the uranyl nitrate directly into the precipitation tank with a stainless steel bucket rather than using a pump. The buffer tank would have actually held this solution safely, as it had a tall and narrow geometry and was designed to prevent criticality. The precipitation tank however had not been designed to hold this type of solution and was not configured to prevent criticality.^[10]

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 with sandbags and other shielding to protect 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.^[9]

Aftermath

Dozens of emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours; 39 of the workers were exposed to the radiation.^[13] At least 667 workers, emergency responders, and nearby residents were exposed to excess radiation as a result of the accident.^[8]

By measuring the concentration of sodium-24, created by a neutron activation whereby sodium-23 nuclei were rendered radioactive by absorbing neutrons from the accident, it was possible to deduce the dose received by the technicians. According to the STA, Hisashi Ouchi was exposed to 17 sieverts (Sv) of radiation, Masato Shinohara received 10 Sv, and Yutaka Yokokawa 3 Sv.^[8][10] By comparison, a dose of 50 mSv is the maximum allowable annual dose for Japanese nuclear workers.^[9] A dose of 8 Sv (800 rem) is normally fatal and more than 10 Sv almost invariably so.^[10] Normal background radiation amounts to an annual exposure of about 3 mSv.^[8] 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).^[14]

The two technicians who received the higher doses, Ouchi and Shinohara, died several months later. Ouchi suffered serious burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count.^[8]

The cause of the accident was said to be "human error and serious breaches of safety principles", according to the International Atomic Energy Agency.^[9]

See also

• Nuclear power in Japan
• 2011 Japanese nuclear accidents – On 11 March 2011, narrowly escaped explosion in the Tōkai Nuclear Power Plant.
• Fukushima Daiichi nuclear disaster
• 5 yen coin#Use in nuclear accident investigation

References

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