Abstract
A nuclear accident occurred in Japan that is rated 4 on the international scale of 0 to 7 developed by the International Atomic Energy Agency and the Organisation for Economic Cooperation and Development. On 30 September at 10:35 am a critical accident occurred at JCO Co. Ltd, a nuclear fuel processing company in Tokai, Japan. Supercriticality was reached at 10:35 am and the criticality then remained until the morning on 1 October. Three workers at the JCO were heavily exposed and transferred to Mito Hospital (a state-run hospital in Mito, Ibaraki Prefecture) by an ambulance. The symptoms of those three patients were severe and consequently they were transferred to the National Institute of Radiological Sciences (NIRS) in Chiba Prefecture by helicopter since NIRS is assigned to be the Third Stage Hospital in the case of a radiological emergency.
Scope of the accident
Tokai is about 130 km northeast of Tokyo, and 13 nuclear facilities are located there including the Japan Atomic Energy Institute (JAERI) and the Japan Nuclear Cycle Development Institute (JNC). JCO Co. Ltd (previously called Japan Nuclear Fuel Conversion Co.) operated the uranium processing plant. It has been found since the accident that the company used a hidden procedure for handling uranium for seven or eight years, and on that day three workers deviated even further from the company's unauthorised work manual, bypassing a part of the procedure and using stainless steel buckets to put uranium nitrate into a precipitation tank instead of a solution tank. They were working on the refining of uranium fuel for the Joyo experimental fast breeder reactor in Ibaraki Prefecture by hand, and they mixed 16 kg of 18.8% enriched uranium, which is nearly seven times the allowable amount of 2.4 kg of uranium, into the tank. The two workers beside the tank (Mr O, age 35, and Mr S, age 39) saw a blue flash with a sound like `bashi' and fled from the site. The third person (Mr Y, age 54) stayed outside of the room and also saw a reflected blue light on the wall. The three men escaped from the control area, and at the decontamination room one of them (Mr O) fainted while Mr Y tried to use the telephone to report the accident. Mr O, who received the most radiation, suffered vomiting and convulsions. Other workers came into the area and took them outside after hearing the alarm from an area monitor at the site. Mr O vomited several times as they waited for an ambulance a few tens of metres away from the accident site. During the trip to Mito Hospital Mr O vomited and suffered diarrhoea. Mr S also vomited but had no diarrhoea.
Reporting
A gamma dose rate of 0.84 mSv h-1 was measured at the boundary of the company on Thursday morning (30 September), where the normal value is around 0.2 µSv h-1. The Science and Technology Agency received the first report on the accident from JCO at 11:15 am. Ibaraki Prefecture was then informed of the accident at 11:33 am. A neutron monitor at the Naka branch of JAERI located 1.7 km away from the JCO showed a sharp peak at 10:35 am on 30 September. The gamma monitor at the same site showed a relatively small increase. Measurements of neutron doses at several points around the facility were started several hours after the accident. The highest dose of 4.5 mSv h-1 due to neutrons was observed.
Response of local government
Police immediately sealed off an area of 200 m radius around the facility at 12:41 am. The Tokai Municipal Board of Education instructed kindergartens, primary and junior high schools in the village to close all windows in their buildings and not to let pupils outdoors on the afternoon of Thursday, 30 September. Tokai village authorities issued an evacuation advisory at 3:18 pm to 175 people in 50 families living within a 350 m radius of the plant and relocation at a public facility about 1 km away. The Ibaraki governor issued an advisory to the 313 000 residents living within a 10 km radius of the plant to stay indoors at 10:30 pm. Traffic was banned in a 3 km radius around the accident. On Friday morning (1 October) the local government ordered that no water be drawn from the nearby Kuji River until a water-quality analysis was performed. They also recommended that people avoid using well water for the time being. They said tap water in the area was safe because it was being drawn from the Naka River. In addition they instructed farmers to refrain from harvesting crops in the area until they were checked for radiation contamination. Dairy product shipments were also ordered to be suspended. Meanwhile, East Japan Railway Co. said that services on the Joban Line between Hitachi and Mito stations and on the Suigun Line between Mito and Hitachi-Daigo stations several kilometres west of Tokai had been suspended for most of Friday (1 October). Services resumed gradually from around 4 pm after it was announced safe for residents in the 10 km radius to leave their homes. National Route 6, a major highway running through Tokai, and other prefectural roads that were close at some points were reopened to traffic in the late afternoon. As a precaution, 224 Ibaraki prefectural schools were closed for the day (1 October).
Remediation
A pair of workers was sent to the plant nine times to halt the chain reaction. At 6 am Friday, 1 October, the nuclear chain reaction was stopped by draining the cooling water around a precipitation tank by blowing the water away from the cooling water jacket with argon gas. The chain reaction lasted for roughly 16 hours. Within two hours of the release of the cooling water the neutron monitor at the plant read zero. This was confirmed by the neutron monitor at the plant at 6:15. The workers who risked exposure to high levels of radiation received doses in the range of 50 to 100 mSv. JCO employees injected a boron solution into the precipitation tank to ensure that the criticality stopped.
Subsiding
The Chief Cabinet Secretary announced at 3:00 pm on the afternoon on 1 October that the government would allow residents living within a 10 km radius of the accident site to leave their homes now and resume their normal lives because the area was deemed safe. The government had been urging these people to stay indoors since Thursday night (30 September). However, the government left the evacuation order for the 350 m zone near the plant in effect. It was at 6:30 pm on Saturday (2 October) that the government announced that its order cordoning off the area was lifted after intensive examination of radiation levels both inside and outside homes in the area. The authorities also conducted inspections of agricultural products both in the field and on supermarket shelves.
Radiation levels near the plant had almost returned to normal as of Friday morning but the plant was still emitting gamma rays as of Saturday morning. The firm piled up concrete blocks around the plant on Saturday morning as part of the effort to prevent radiation leakage.
Victims
So far, 69 people have been reported as having been exposed to the radiation, including the three workers at the plant who were hit with the initial radiation burst. Two of them received especially high doses inside the room where they had manipulated uranium nitrate. The third man, who stayed outside the room at the time of the accident, was exposed to radiation through the door. These victims were airlifted to NIRS from Mito Hospital in the afternoon on the day of the accident. Two of them were carried into the institute on stretchers by medical staff wearing masks and white antiradiation suits before being rushed into a radiological emergency ward and then into a sterile isolation room.
Radiation safety staff in NIRS surveyed the radiation level of the body as well as the inside of the ambulance. No contamination was found in the ambulance or on the attendants to the patients. However, the radiation levels around the patients were very high, especially for the two highly exposed workers (Mr O and Mr S). Those two men were in so serious a condition that medical treatment was urgently required. They were moved to sterile isolation rooms. The third man (Mr Y) walked into the hospital and was treated in a normal ward.
The white blood cell counts for Mr O and Mr S were sharply up and their immune systems weak. Mr O showed very strong early radiation symptoms including nausea and diarrhoea. Mr S also showed nausea. They were not fully conscious and their blood pressure was low although they responded to doctors' questions.
Dose estimation
The dose estimation team was established and measured the patients' blood as well as other items. Gamma spectrometry of clothes and vomited material and smear tests of their noses showed no trace of uranium-235 nor fission products except the daughter products (91Sr, 139Ba) of the noble gases 91Kr and 139Xe. In contrast, a large amount of Na-24 was observed in addition to 80Br, 82Br and 42K, which were derived from the neutron activation of the body elements. Urine samples were also measured and showed a large amount of Na-24 as well. Gamma measurements revealed that the victims were not contaminated with uranium-235 or fission products except the daughter products of the noble gases Kr and Xe externally as well as internally. The main source of their radioactivity was Na-24, which was produced by the neutron capture reaction of Na-23 in the body. Since there was no internal contamination it was not necessary to use some chelate material for decontamination. The measurements of Na-24 in their blood samples showed that their initial concentrations of Na-24 at the time of exposure were around 200, 100 and 30 Bq ml-1, respectively. Using a dose conversion factor shown in the draft of IAEA about the criticality accident at Sarov, the doses to the three patients were estimated to be 10, 6 and 1.4 Gy. Gamma equivalent doses are derived to be 17, 10 and 3 GyEq, respectively, using a RBE value of 1.7, which was obtained in our institute for acute GI syndrome. Whole body counting of Mr Y could also be carried out since Mr Y was in a relatively good condition. The body burden of Na-24 was estimated from the measurement and then converted to dose. Some other measurement techniques using coins and hair were applied to estimate neutron flux. Stable Na, P and S were also measured to improve the dose estimation.
The dose estimation based on the number of lymphocytes in their blood was treated as the first dose estimation since the three men did not wear any dosemeters during their work. Using the counts of lymphocytes in their blood for the first few days the doses were estimated to be more than 8 Sv for the two highly exposed victims (Mr O and Mr S) and 3 to 5 Sv for Mr Y.
On Monday, 4 October, another temporary dose estimation was completed by the chromosome aberration counting method and showed almost the same doses as derived from Na-24 activity measurements in blood.
Tentative dose ranges for three men which were estimated by the four methods mentioned above (lymphocyte count, Na-24 in blood, Na-24 WBC, chromosome aberration) were 10 to 20, 6 to 10 and 0.7 to 5.5 GyEq for Mr O, Mr S and Mr Y, respectively.
Further medical treatment
NIRS has organised the Medical Network for Radiation Emergency since last year as a permanent supporting system for medical emergencies. NIRS held a meeting of the Medical Network after the initial treatment for the three patients. Medical doctors attending the meeting discussed their future treatment and decided on Saturday afternoon (1 October) to transfer Mr O to the Hospital of the University of Tokyo for peripheral blood stem cell transplantation as well as for more intensive care. On 6 and 7 October, peripheral blood stem cell transplantations were carried out using blood from his sister. Mr O remained in a critical condition, with fluid accumulating in his lungs as a result of dehydration, one of the early effects of radiation. The number of lymphocytes in his blood reached zero, causing his immune system to deteriorate dramatically. If the transplantation is successful, Mr O's white blood cells should start increasing. If the treatment is ineffective, they will consider a bone marrow transplant.
The white blood cell and lymphocyte count for Mr S also dropped significantly. No proper matching of peripheral stem cells was found for Mr S. The Network Group meeting then decided to transfer Mr S to the Research Hospital of the Institute of Medical Science at the University of Tokyo to conduct umbilical cord blood stem cell transplantation. Mr S received a transplantation of umbilical cord blood stem cells on Saturday morning (9 October) to activate his blood-producing functions.
Mr Y remains in a relatively stable condition and takes a simple diet in NIRS. The percentage of lymphocytes is lower than normal. However, the level has remained constant and the blood-producing function could be expected to recover by itself. Therefore, no transplantation is planned now.
Japanese authorities are now conducting a full investigation of the worst nuclear accident in Japan technically as well as legally.
Kenzo Fujimoto
In the UK the efforts to assist in the recovery of the JCO incident were hampered by the news footage shown. Throughout Thursday 30 September the BBC news programmes repeatedly led with the JCO story highlighting the `hole in the roof'. With the knowledge of the JCO process and facilities together with on-hand criticality expertise, how the explosion occurred was incomprehensible. On Friday 1 October, after the JCO accident was under control, it was suspected that the BBC had used `archive footage' of the previous fire at Tokai. This turned out to be the case. An apology was issued to a private individual by the BBC on Friday 8 October - a week later. The apology stated that the early versions of the story were wrongly reported. The BBC have now corrected their archives.
K E Bhanot