Tkai village nuclear campus in Ibaraki Prefecture, Japan has experienced two major accidents. The first incident happened on March 11, 1997, when a fire broke out in the PNC radioactive waste bituminization facility caused by an experimental batch of solidified nuclear waste.
Radiation was leaked to almost twenty people. The second occurred on September 30, 1999, when liquid uranium fuel was handled incorrectly and caused a criticality accident at a separate facility owned by Japan Nuclear Fuel Conversion Co.
The incident lasted for around 20 hours during which time 667 people were exposed to radiation and two workers lost their lives.
1997: The PNC Tokai Nuclear Accident
At PNC’s bituminization facility, the first major nuclear-related event in the village of Tokai happened on 11 March 1997. Dnen is an acronym of PNC’s full Japanese name, Dryokuro Kakunenryu Kaihatsu Jigydan, therefore the accident has also been known as the Dnen accident.
The facility was testing a novel asphalt-waste mix which required 20% less asphalt than usual, on that day. Previously, the facility had encased and solidified low-level liquid waste in molten asphalt (bitumen) for storage.
At around 10:00 a.m., a slow chemical reaction inside one new barrel ignited the already-hot contents, quickly spreading to several other neighboring barrels.
Employees were unable to put out the fire completely and the building had to be evacuated due to smoke and radiation alarms. Inflammable gases built up inside the building ignited and detonated about 8 p.m., just as people were getting ready to reenter the structure.
The explosion shattered windows and doors releasing smoke and radiation into the neighborhood. The Science and Technology Agency of the government proclaimed this to be the worst nuclear accident in the country to date, exposing 37 surrounding workers to low levels of radiation.
This event was given a rating of 3 on the International Nuclear Event Scale. High quantities of cesium were identified by meteorological officials 40 kilometers (25 miles) southwest of the plant a week after the occurrence.
Overhead photos of the nuclear processing plant after the fire and explosion revealed a severely damaged roof that allowed radiation to continue to leak out into the atmosphere. Management at PNC compelled two employees to provide a false account of the sequence of events that led to the evacuation of the building.
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1999: Second Tokai Nuclear Disaster, Unfortunate Catastrophe Near PNC Site
On September 30, 1999, around four miles from the PNC site, a second, more significant Tokai nuclear disaster happened at a fuel enrichment plant run by JCO, a subsidiary of Sumitomo Metal Mining Corporation. Before the 2011 Fukushima Daiichi nuclear disaster, this was Japan’s worst civilian nuclear radiation mishap.
After the uranium mixture reached criticality, the incident exposed the nearby populace to harmful nuclear radiation. A gasoline mixing accident took the lives of two of the three technicians.
The tragedy happened because of a combination of factors, including a lack of regulatory oversight, a lax safety culture, and poor training and education for technicians.
The JCO plant processed UF6 to produce UO2 for nuclear power. This was the initial process of creating fuel rods for use in nuclear reactors at power plants and research reactors in Japan. There is a high degree of difficulty and potential danger involved in enriching nuclear fuel.
Combining nuclear products can result in a fission reaction and subsequent radiation release if not done correctly. Uranium fuel enrichment calls for a distinct chemical purification process.
The process of making uranyl nitrate from nitric acid involved adding small amounts of a uranium oxide powder to a special dissolving tank.
Afterward, the solution is transferred to a buffer tank that was designed for the task. The combined components are stored in a buffer tank that is engineered to stop fission activity before it reaches criticality. Ammonia is added to a precipitation tank, where it becomes a solid.
Any lingering radioactive waste pollutants will be contained in this tank. Finally, uranium oxide is placed in dissolving tanks until refined using a wet-process technology developed in Japan that does not include enriching the isotopes.
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