[Newsletter] Dec 9,1999

Japan's Nuclear Criticality Accident at a Fuel Conversion Facility

Dec. 9, 1999
Japan Atomic Industrial Forum, Inc.

Criticality Accident in Nuclear Heartland of Japan

On September 30, around 10:35 a.m., the Japan's first nuclear criticality accident occurred at a fuel conversion facility in Tokai-mura, Ibaraki Prefecture. The accident occurred in the Conversion Test Facility of JCO Co., Ltd. Tokai Works when uranyl nitrate solution made by dissolving triuranium octaoxide (U3O8) in nitric acid was being poured into a precipitation tank.

Usually, JCO Co., Ltd. Tokai Works was doing a job to convert enriched uranium hexafluoride (UF6) into uranium dioxide (UO2) powder and deliver it to fuel fabrication companies, as a part of fuel manufacturing processes for nuclear power plants of boiling water reactors (BWRs).

The Conversion Test Facility where the accident occurred is not a fuel conversion facility for ordinary nuclear power plants, and was handling uranium of higher enrichment (up to 20% permitted) as compared with nuclear fuel for power plants (3 to 5% enrichment). The highly enriched fuel being processed was for experimental fast reactor Joyo of Japan Nuclear Cycle Development Institute, and its enrichment was 18.8% (See Fig. 1).

Fig.1: Facility Layout in JCO Tokai-Works
Fig.1 Boundary of Site
Boundary of Peripheral
Monitoring Area

Conversion Facilities

In the Conversion Test Facility, like the First and Second Conversion Facilities for power plant fuel, devices were designed and arranged under geometry control so as to avoid nuclear criticality accidents, each device being of narrow structure and arranged at sufficient distances. Restriction was also imposed on the amount of uranium to be processed. This means that the operation in the Conversion Test Facility should have taken processes of dissolving uranium oxide (U3O8) in nitric acid in the solution column and homogenizing the solution on a one-batch basis in the geometrically controlled narrow storage.

In the operation on September 30, however, three workers dissolved sequentially 2.4 kg uranium powder in a ten-liter stainless steel bucket with nitric acid and pure water, and in order to homogenize thus obtained uranyl nitrate solution, poured the solution into a five-liter beaker, and fed it from the beaker directly into the thick and large (45 cm in inner diameter and 61 cm high) precipitation tank complete with a stirrer, using a funnel inserted into the sampling hole (See Fig. 2). The last step was done by two workers: one holding the beaker, and the other supporting the funnel. The remaining one, the operation team chief, was in the neighboring room.

Fig.2: Regular Procedure of Conversion (upper) and Illegal Procedure Taken at the Accident

In the normal process, it is supposed that the amount of uranium fed to the precipitation tank is limited to about 2.4 kg per batch. Actually, however, about 16.6 kg of uranium, equivalent to six or seven batches, was fed into the precipitation tank, which caused criticality. It has been three years since the last converting operation of fuel for Joyo. This time the operation just started on September 22.

To Finish the Job Earlier

The operation procedure that caused the accident is totally different from the one approved by the government. It is reported that the procedure of feeding the solution from a bucket to the precipitation tank was proposed by the three workers, and carried out for the first time. It is also reported that they did not obtain approval of their supervisor, foreman, about use of the precipitation tank.

In the initial stage of criticality, pulse-like nuclear fission of about 0.1-second duration occurred, and intense neutrons and gamma rays were radiated. However, the reaction was not explosive, and rise in the solution temperature is estimated to be tens of degrees. The temperature rise would have decelerated the reaction, and then as the temperature lowered, the reaction would have been accelerated again, gradually reducing the reaction output with evaporation of the solution. The reaction continued for about 20 hours until the cooling water surrounding the precipitation tank was drained. The estimated total nuclear fission number is 2.5 x 1018 (See Fig. 3).

Fig.3: Progress in Critical State

Most of the radioactive materials generated by fission remained in the solution, but volatile xenon and iodine were emitted to the atmosphere through the precipitation tank sampling hole. These materials a little contaminated the environment, but the contamination level was low enough, causing no public health problems.

Criticality Finally Comes to an End

Late at night, September 30, the Science and Technology Agency decided to start an operation to drain water from the cooling jacket surrounding the precipitation tank in order to suppress criticality. The precipitation tank is almost cylindrical, and its lower half is surrounded by the cooling jacket, assuming a double-bottomed structure. There was a large possibility that criticality was continuing because water in the cooling jacket, intended for cooling, was functioning as a neutron reflector.

The draining operation started around at 2:30 a.m., on October 1. Water was drained by opening the valve of the piping from the cooling jacket, and this operation was voluntarily conducted by JCO's personnel. As a result, criticality ceased around at 6:30 a.m. The company's personnel have the best knowledge of the devices, not to mention recognition of the risks involved in the operation, so it must be the personnel who carry out the actual operation to terminate the accident.

Then, boric acid having a function to absorb neutrons was added to assure the end of criticality. Termination of criticality was confirmed at 8:50 a.m.

Evacuation of Residents - Drastic Measures Taken by Mayor of Tokai-mura

The village government of Tokai-mura, where JCO Co., Ltd. is located, set up an "Anti-disaster Headquarters" at 0:15 p.m., on September 30, the day of the accident, which was earlier than national government's setting-up of a task force. At 0:30 p.m., 15 minutes after setting-up of the Headquarters, issuance of recommendation to the residents to stay indoors was started, and the prefecture road running beside the facilities was closed. At 1:45 p.m., the village government temporarily stopped taking water for running water from Kuji River.

At 3:00 p.m. on the same day, Tatsuya Murakami, mayor of Tokai-mura, recommended that the residents living within 350-meter radius from the JCO facilities be evacuated to "Ishigami Community Center," a public facility of the village, located at 2 km from the accident site, and 150 residents were evacuated.

Evacuation of 150 residents continued even after confirmation of criticality termination at 8:30 a.m., on October 1. This was because radiation dose and the amount of radioactive materials around the facilities had to be checked by detailed radiation monitoring. After safety was confirmed, Chief Cabinet Secretary Nonaka announced at 7:15 p.m., on October 2, "there is no problem about lifting of the evacuation recommendation for residents within a 350-meter radius." Following this announcement of the government view, Mayor Murakami announced lifting of the evacuation recommendation for the residents concerned. More than two days and four hours had passed. Before returning people home, the village government confirmed safety by checking every house for any radioactivity in the presence of its resident.

Measures were taken not only for residents within a 350 meters radius but also for people living in a much broader range. At 10:20 p.m., on September 30, the vice-minister of STA advised the governor of Ibaraki Prefecture to keep indoors the residents living within a 10-km radius (about 310 thousand people). In response to this advice, the governor requested the relevant municipal governments to inform their residents. Thus "sheltering indoors" (i.e., staying indoors) was implemented for 310 thousand people living within a 10-km radius. The sheltering recommendation was lifted by the governor of Ibaraki Prefecture around at 4:30 p.m., on October 1, following the Chief Cabinet Secretary Hiromu Nonaka's announcement of government view at 3:00 p.m.: "there is no problem about lifting of the sheltering recommendation for residents within a 10-km radius." Residents were forced to stay home almost for a day.

Characteristics of the Criticality Accident

STA announced that the accident rated "level 4" on the International Nuclear Event Scale, the highest of the Japan's past accidents, though this rating was provisional. The fire and explosion accident at the bituminization facility in Tokai Reprocessing Plant (March, 1997) rated level 3, and the sodium leakage accident at prototype fast reactor Monju (December, 1995) rated level 1.

Unlike the Chernobyl accident in Soviet Union, which emitted a large amount of intense radioactive materials over a wide range, the nuclear criticality accident at the JCO Fuel Conversion Facility caused little radioactive contamination in the surroundings, and had no radioactive effects on agricultural and marine produce, not to mention on residents (See Fig. 4). This is because no explosive reaction occurred in the precipitation tank, and most of the fission products produced by criticality remain in the solution. Some fission products, volatile xenon and iodine gas were emitted to the environment, but their dose was about 0.1 mSv even in the neighborhood of the site, and radioactivity survey conducted by the central and prefecture governments on soil and river water detected little amount of radioactive materials.

The accident is notably characterized not by the effects of radioactive emission, as mentioned above, but by the three worker's exposure to a huge radiation dose due to neutrons and gamma rays emitted by fission chain reaction. Neutrons and gamma rays emitted by uranium criticality reaction were radiated in every direction from the precipitation tank. Intensity of radiation is reduced in inverse proportion to the square of distance from the radiation source. That is, the longer the distance from the radiation source, the lower is the radiation exposure.

Such a characteristic of the accident caused the three workers working around the precipitation tank to be exposed to a huge radiation dose, but there was no need for residents to worry about effects of radiation on their health.

Fig.4: Differences between the JCO Accident and Large Scale Accidents

JCO, What Kind of Company?

JCO Co., Ltd. is a wholly-owned subsidiary of Sumitomo Metal Mining Co., Ltd. It was established in October, 1979, as Japan Nuclear Fuel Conversion Co., and renamed to JCO Co., Ltd. in 1998. The stock capitalization is \1 billion.

The Tokai Works of the company has a production capability of 715 tons (in terms of uranium) of BWR fuel whose enrichment is 5% or less, and 3 tons (in terms of uranium) of research reactor fuel whose enrichment is less than 20%.

Financial status of the company shows that the annual sales decreased from \3.25 billion in FY1991 to \1.72 billion in FY1998, and the annual amount of uranium conversion dropped from 552 tons to 365 tons during the corresponding period. The number of employees also decreased from 162 to 110.

Standing in contrast to this, in FY1998, the annual sales of Mitsubishi Nuclear Fuel Co., Ltd. that has a uranium conversion facility and fuel fabrication facility for PWR was \16.9 billion, that of Japan Nuclear Fuel Co. fabricating fuel for BWR was \21 billion, and that of Nuclear Fuel Industries, Ltd. fabricating fuel for PWR and BWR was \28.8 billion. Unlike these companies engaged in highly value-added fuel fabrication consisting of such processes as uranium powder baking, molding, and assembling fuel rods into fuel assemblies as long as four meters with scrupulous care, JCO Co., Ltd. is engaged only in the conversion process. This implies that JCO has suffered troubled finances in the midst of severer international competition.

3 Employees Exposed - 2 Seriously

The three employees who had caused the criticality accident were taken to the National Mito Hospital in an ambulance, given first aid, and then transferred by an emergency helicopter of Ibaraki Prefecture to the National Institute of Radiological Sciences in Chiba Prefecture. In NIRS, doctors belonging to the medical network for exposure emergency preparedness, which had been already set up, participated in treatment.

Of the three employees, Hisashi Ouchi, 35, who was in the most "dangerous condition" requiring peripheral blood stem cell transplantation, was transferred to the University of Tokyo Hospital. He had been by the precipitation tank, pouring uranyl nitrate solution from a five-liter beaker into the precipitation tank. The radiation dose to which Ouch had been exposed is estimated to be 17 Sv. He initially had serious diarrhea, disturbance of consciousness, increase in the white blood corpuscle (25,000), and a fever.

Then, Ouchi assumed typical symptoms of acute radiation syndrome such as burn, respiratory disorder, and decrease in immunocompetence. The hospital has repeated such treatments as hematogenous function recovery treatment, blood transfusion, and large-scale skin transplantation, but November 27 he fell into a "dangerous condition." (At 11:21 p.m. on December 21, 1999, Hisashi Ouchi died of multiple-organ failure at the Tokyo University Hospital.)

Masato Shinohara, 40, who had been working with Ouchi by the precipitation tank, had been exposed to a radiation dose of 10 Sv. Initially he responded to callings though he had a fever and disturbance of consciousness. After treatment at NIRS, he was transferred to the Research Hospital of the Institute of Medical Science, the University of Tokyo, for umbilical cord blood transplantation, and is now in a stable condition as a result of treatments there.

Yutaka Yokokawa, the operation team chief, had been exposed to a radiation dose of 3 Sv, which is less than those for the other two employees since he had been in the next room when the accident had occurred. Since then, Yokokawa has been in a stable condition with clear consciousness.

No Other Persons Who Were Adversely Affected in Terms of Health

In addition to the three employees, 56 employees were exposed to radiation doses up to 3.9 to 24.0 mGy, three persons from fire station who took the three employees from the accident site to the hospital were exposed to 0.5 to 4.1 mGy dose, and seven persons including residents and workers who were in the vicinity of the JCO site were exposed to 0.5 to 9.4 mGy. The exposure doses for 18 employees who drained precipitation tank cooling water and six employees who injected boric acid were 0.03 to 120 mSv. All these values are under the exposure levels that would have some effect on human health.

"Probabilistic effects, exemplified by increase of cancer, are in general supposed to emerge only when the effective dose exceeds about 200 mSv. Therefore there would be no need to become immediately concerned about effects of the accident on health, such as increase of cancer," announced STA concerning future effect of exposure on health of those persons except the three employees.

Central Government's Response

As radiation doses around the facility did not decrease even in the afternoon, September 30, STA set up about at 2:30 p.m. an accident-management task force headed by Minister Arima of the Agency. The central government also set up a "Government Task Force" headed by Prime Minister Keizo Obuchi about at 3:00 p.m. "Recognize that the situation has become so serious that we must be concerned about effects of radiation, and take steps giving priority to residents' safety, " told the Prime Minister at the first government task force meeting at the Crisis Management Center in the annex of the Prime Minister's Official Residence. He directed that the Defense Agency, National Police Agency, Fire and Disaster Management Agency, and other government authorities join forces and take every step to assure safety of residents and prevent spread of harm by radiation.

"Very fundamental rules are not observed. I am concerned about weakened sense of mission and morality," said Arima, STA Minister on October 1. On the same day, Kaoru Yosano, Minister of the International Trade and Industry told, "I take it seriously and earnestly as though the accident had occurred in a facility under control of our ministry," expressing his will to do his best for settlement of the issue. This is because the government reorganization scheduled January 2001, will make MITI into the "Ministry of Industry and Economy," which will take charge of safety regulation of nuclear power and fuel cycle facilities.

Prime Minister Obuchi announced to postpone for a while a Cabinet reshuffle originally scheduled for October 1, and decided to handle issues on the accident with the unreformed Cabinet. The new Cabinet started finally on October 5, when the most urgent steps had been finished.

At the press interview after start of the second reformed Obuchi Cabinet, October 5, Mr. Obuchi said: "The accident was an unexpected one caused by human activity making considerably light of safety. In order to prevent such an accident occurring again, first its cause should be thoroughly investigated. And on that basis, we like sooner to establish and implement measures to prevent recurrence of such an accident. We have immediately started urgent check of other nuclear fuel facilities. As for the government's crisis management, we must reconsider what should be reconsidered in order to assure further thoroughness. Lastly, I, from the bottom of my heart, express my respect for devotion and courage of those who took a lot of trouble for settlement of situation at the accident site regardless of their own danger."

Criminal Suit against JCO to Be Filed

Based on the Law for Regulations of Nuclear Source Material, Nuclear Fuel Material and Reactors, STA started on-site inspection of JCO from October 3 when the local situation began to show stability. As mentioned by Prime Minister Obuchi, the Agency also conducted on-site inspection of other 20 companies, including fabrication companies and reprocessing companies, to confirm safety of their facilities.

In order to trace out the accident cause, "Investigation Committee for Criticality Accident at Uranium Processing Plant" set up on October 4 under the Nuclear Safety Commission started investigation. Ibaraki Prefecture Police Headquarters set up an investigation headquarters, which started investigation on October 3. The Ministry of Labor also started investigation for any problem against the Industrial Safety and Health Law.

The Investigation Committee under the NSC officially announced "Urgent Recommendations - Interim Report" on November 5. The report consists of accident situation and its consequences, response to the accident, cause of the accident, urgent recommendations, and items for the further investigation and examination. The full text is available on STA web site (http://www.sta.go.jp/genan/jco/jco-e.html for English).

For health consultation for residents around the accident site, a local consultation center of STA was set up in the Tokai-mura village office, where doctors from NIRS and other specialists held, October 18, a briefing session concerning effects of radiation on human health. In addition, doctors sent to Tokai-mura and those at National Mito Hospital conducted medical examination and health consultation of each resident in order to dissolve residents' anxiety.

Tokai-mura and Its Residents Most Trusted "Nuclear Energy People"

Tokai-mura is a village facing the Pacific Ocean, covering an area of 3,567 ha (7.5 km north to south and 6.6 km east to west) with an population of 32,727 or 10,864 households as of 1995.

In 1955, Muramatsu-mura and Ishigami-mura merged into Tokai-mura. In 1956 Japan Atomic Energy Research Institute was set up there, and in 1959 Nuclear Fuel Corporation, the forerunner of Japan Nuclear Cycle Development Institute, was established. Following it, in 1960, construction of Japan's first commercial nuclear power plant, the Japan Atomic Power Company's Tokai Power Plant, started. Now Tokai-mura is dotted with 15 nuclear sites (including JCO). Thus Japanese nuclear technology was born and grown in the bosom of Tokai-mura.

When two villages merged into Tokai-mura, the population was about 10 thousand. Then the population gradually increased, and now nuclear-engaged people including their family members count about 10 thousand, and people commuting to factories in Hitachi City and Hitachinaka City, north and south to Tokai-mura, respectively, count about 10 thousand.

Since the beginning of the peaceful uses of nuclear energy, residents of Tokai-mura have shown understanding to nuclear energy development, lived along with nuclear facilities, and expected much of nuclear research, development, and utilization in Japan. Many nuclear-related people visit the village from abroad, and Tokai-mura is a sister village of Idaho Falls, Idaho of the United States, where nuclear facilities are located like the village. Because the village is well known as "Tokai-mura" (mura means a village) in foreign countries, there is no need to change its name that the village of 30 thousand population does not change its name to "Tokai-machi" (machi means a town).

Thus they have lived with nuclear energy, they as the whole village have trusted people engaged in the nuclear energy development, and they have set a good example of local people of a region where a new nuclear facility is sited. This even more increases their shock about nation's first occurrence of nuclear criticality accident in Tokai-mura, and even more amplifies their distrust of nuclear safety.

Supports from Neighboring Nuclear Entities - Possible only in Tokai-mura

The accident at JCO facility occurred in Tokai-mura where as many as 15 nuclear entities are located. This may have helped rapid response of such organizations as Japan Atomic Energy Research Institute, Japan Nuclear Cycle Development Institute, and Japan Atomic Power Company. Specialists from them rushed to the neighborhood of the scene to take necessary steps including radiation monitoring and accident-terminating activities. Persons from all of the nine electric power utilities also joined the supporting group. This is appreciated by Tokai-mura people concerned.

Some people think that the fact of the JCO accident having occurred in Tokai-mura led to rapid holding-out of helping hands by nuclear researchers and engineers who live together in the same community. However, it is still a problem how to build up a disaster prevention system that urges specialists to the scene wherever the accident may occur.

Damage Spreads on Agricultural and Marine Products Due to Rumors

There have been found an economical damage on the local medium and small-sized businesses and the local agriculture, forestry and fisheries due to this accident.

With respect to the medium and small-sized businesses whose plant operation is affected directly and/or indirectly by this accident; for instance, their employees cannot work at plant or their working is delayed due to the accident, as of October 1, the Government (MITI) decided to apply the "Disaster Restoration Loan" provided by the governmental medium and small-sized financial institutions to the medium and small-sized businesses in Ibaraki prefecture, and the applications have already started.

In some areas out of big consumer cities such as Tokyo, there has been seen such a situation that agricultural, marine and livestock products produced in Tokai-mura, its surrounding municipalities and the entire Ibaraki prefecture disappeared from the retail outlets including supermarkets due to the accident. Such a damage due to rumors has spread and become noticeable. To cope with this problem, the abovementioned JAIF being a united organization for parties involved in the nuclear industry has earlier appealed to its member companies to willingly purchase the agricultural, forest and marine products of Ibaraki growth. Following that, electric power utilities and component manufacturers have also promoted the purchase of such products, which is leading to mitigation of the damage.

To cope with the claim for the direct and indirect damages such as the damage on agricultural products due to rumors and a temporary stop of plant operation, JCO itself set up the window to contact for such claims on October 4. On October 22, the "Nuclear Damage Compensation Dispute Review Panel" to settle and intermediate the disputes related to damage claim was set up within STA. In addition, the "Nuclear Damage Investigation Workshop" aiming to quicken the certification of damage was set up and has initiated its activities.

According to the Law on Compensation for Nuclear Damage, the total amount of compensation for damage from an insurance company is limited to 30 billion yens for a nuclear power plant accident (the law has been under amendment to raise the amount to 60 billion yens even prior to the JCO accident) while the total amount is limited to 1 billion yens in the case of the JCO accident. The actually claimed amount for damages, however, is reported to be already over 6 billion yens; therefore, a parent company of JCO will reportedly bear the legitimately claimed amount over 1 billion yens.

Pursuing Internal Reformation Thoroughly - Response by Nuclear Industry

On October 5, Tuesday in the week following the accident, a special meeting of the board was held by the Japan Atomic Industrial Forum, Inc. (JAIF, Chairman: Takashi Mukaibo, the ex-president of the University of Tokyo; number of its members: 760 companies), which is a "private organization aiming at the peaceful uses of nuclear energy from a standpoint of the nation as a whole." In the meeting, they concluded a statement "Towards Self-Reformation of Private Parties Involved in Nuclear Energy" (http://www.jaif.or.jp), delivered it to its members and agreed to force everyone concerned to understand the concept. In the statement, they said, any parties concerned handling such nuclear materials have to pursue their self-reformation and make every effort to recover trust of the local community and the nation as well, and they proposed the five items for actions to be taken including the item that securing the nuclear safety is the biggest subject among the top management and therefore they are required to set up a corporate policy placing its top priority on safety.

Taking advantage of the self-reformation statement as a good opportunity, the "International Network for Safety Assurance of Fuel Manufacturers (INSAF)", a voluntary safety check system by the uranium processing operators, is decided to start. INSAF, which is to be formed by appealing to the operators processing nuclear materials in the countries such as France, U.K. and U.S.A., aims to build a safety culture at a worldwide common level through the cooperation of the processing operators in the world towards safety enhancement and their efforts to review mutually. A preparatory general meeting for establishing INSAF is to be temporarily held on December 6 with participation by seven organizations of Japan.

It is also determined that the "Nuclear Safety Network" (NS Net) temporarily comprising thirty-five organizations such as electric power utilities, fuel processing operators, plant manufacturers and research institutes will be established so that the entire nuclear industry can have a safety culture in common and improve its level. This organization is expected to perform the activities such as enhancement of safety culture, mutual evaluation and check among members (peer review), exchange of safety information, education and support based on the past cases, and is assumed to be a Japanese version of the World Association of Nuclear Operators (WANO), an international organization consisting of electric utilities in the world. A general meeting for establishing the NS Net is to be held on December 9 with participation of all the thirty-five organizations.

Some Reflection on the Response to Accident

As described in the "Urgent Recommendations - Interim Report" issued November 5 by the Nuclear Safety Commission's Investigation Committee for Criticality Accident at Uranium Processing Plant, the Disaster Prevention Program based on the Disaster Measures Basic Law and the "Disaster Measures at Surrounding Area of the Nuclear Power Plant" prepared by the NSC have been developed assuming the accidents at the nuclear power plants, not the criticality accidents in the fuel processing facilities.

For this reason, a periodical inspection usually performed once a year at a nuclear power plant is not required at a fuel processing plant, where any inspection on the operating conditions and the conditions of systems has never been carried out once a permit for installation was issued through the inspection for facility design including criticality accident.

At the moment in Japan the prefectural governments are responsible for the disaster prevention system of a nuclear facility like the case of natural disaster. However, much knowledge and expertise related to nuclear energy are actually owned by the national government rather than the local governments as a review for construction permit of nuclear installations is conducted by the national government. In this context, the accident made it clear that it is very difficult for the local authority to respond to the emergency properly. It is also reported that the national government's instruction and advice were not provided properly and smoothly due to the poor communication of information and the delay of the national government's response during accident, which eventually caused the delay in issuance of residents' evacuation and other measures.

Accordingly the residents were not provided with proper information and could not but rely on the mass media for the information which circulated a lot of inaccurate information. As for the communication to the medical people concerned, there is a lot of things to be reconsidered.

To cope with these problems, the national government has decided to introduce a system to newly require an on-site inspection at fuel processing facilities, a periodical inspection on the processing operators, and an inspection on the management of plant operation and the educational program for employees, by partially amending the "Law for Regulations of Nuclear Source Material, Nuclear Fuel Material and Reactors." The government also has decided to newly enact the "Law for Nuclear Disaster Prevention Special Measures" and incorporate into the law the new requirements such as a proper understanding of information, a smooth distribution of information, sharing of countermeasures and organic cooperation among the national and local governments, reinforcement of the national emergency response system, role of the processing operators, and development of monitoring system. The amendment law and the new Law for Nuclear Disaster Prevention Special Measures were approved November 25 in the House of Representatives and circulated to the House of Councilors.

Chronology of the Accident

September 30, 1999
10:35Area alarm (criticality warning device) sounded when uranium solution with enrichment of 18.8% to be used for Japan Nuclear Cycle Development Institute (JNC) was fed into the precipitation tank in the Conversion test facility at Tokai Works of JCO Co. Ltd. (JCO). The first "criticality accident" occurred.
11:15The first report of the accident from JCO reached Science and Technology Agency (STA), suggesting "possibility of criticality accident."
Around 12:40Prime Minister Keizo Obuchi received the first report of the accident.
14:00STA reported the accident to Nuclear Safety Commission (NSC).
14:30STA established its Countermeasure Headquarters.
15:00The decision was made to establish the Government Accident Countermeasure Headquarters consisting of Mr. A. Arima, Minister for Science and Technology as the head and representatives from the ministries and agencies involved. At 16:50 the first meeting was held.
Around 15:30Three workers, who were exposed to radiation, were transferred to the National Institute of Radiological Sciences from National Mito Hospital by a helicopter.
Around 17:00A result of neutron measurement at the facility's site boundary was reported to be around 4 mSv/h. It was possible that the criticality condition was continuing.
18:00The first meeting of the Emergency Technical Advisory Body in NSC was started. In the meeting, it was decided that two commissioners of NSC would be dispatched to Tokai.
21:00The first meeting of Cabinet Task Force for the Accident consisting of the Prime Minister Obuchi as the chief and ministers involved.
22:20Permanent Vice Minister for Science and Technology advised the governor of Ibaraki Prefecture to let the residents living within a 10-kilometer range from the facility be stay indoors, as a precautionary measure.
October 1, 1999
Around 3:00An operation to drain cooling water from the precipitation tank was started to terminate criticality.
Around 4:00It was confirmed that the neutron dose rate at the site boundaries decreased, and around 6:30 the neutron dose rate fell below the detection limit.
Around 8:00Boric acid water was injected into the precipitation tank.
9:20Chairman of NSC announced that "the criticality had apparently ended."
8:20It was confirmed that a total of 49 persons (JCO employees including the three workers, three fire fighters engaged in the rescue operation, and local residents) had been exposed to radiation.
16:40The Governor of Ibaraki Prefecture announced the lifting of the recommendation for the residents living within a 10 km range to stay indoors. (The recommendation by the Tokai-mura Mayor for the residents living within a 350 m range to evacuate continued.)
October 2, 1999
15:45One of those three workers, who was in serious condition, was transferred to the hospital in the University of Tokyo from the National Institute of Radiological Sciences.
18:30Mayor of Tokai-mura announced the lifting of all the evacuation of the residents living within a 350 m range from the facility.

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