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Tokaimura
In 1999, an accident at a Japanese nuclear fuel facility less than 100 miles from Tokyo led to three workers receiving high doses of radiation. Two of these workers sadly died due to the incident.
The accident was caused by too much nuclear material (uranium) being brought together which caused an uncontrolled nuclear chain reaction (also called a criticality) that lasted for around 20 hours.
The International Atomic Energy Agency (IAEA) says the cause of this accident was “human error and serious breaches of safety principles”.
- Location of Tokaimura on map of Japan
The Accident
The Tokaimura accident occurred in a small fuel preparation plant operated by a company who at the time were known as Japan Nuclear Fuel Conversion Co. This plant was not involved in producing electricity and instead existed for experimental research.
The company had a regulator approved process for dissolving uranium oxide powder in acid to ensure safety. However, this procedure was changed by the company without permission from the regulator. The company did this to speed up their processes without thinking about the safety implications.
On 30th September 1999, three workers were preparing fuel using this new unapproved process. No proper qualification or training requirements had been established to prepare these workers for the job, and they had no understanding of the potential for criticality with highly enriched levels of uranium. During the fuel preparation, the workers added too much uranium solution to a tank, which surpassed the critical mass, and a criticality incident began.
At criticality, the nuclear chain reaction began to emit intense gamma and neutron radiation which triggered alarms at the plant. To stop the chain reaction, additional workers had to remove the cooling water which surrounded the tank as this acted as a neutron reflector and kept the chain reaction going. The water was replaced with a boric acid solution as this absorbs the neutrons which keep the reaction alive. Once the additional workers had controlled the reaction, the next task was to install shielding to protect people outside the building from gamma radiation.
Buildings which house nuclear processing facilities are usually operated at a lower pressure than the atmosphere meaning any air leak is inwards and air filters can pick up contamination before it leaks out of the building. This was thankfully the case at Tokaimura.
The company did conduct an emergency evacuation drill every year, but there were no pre-established procedures or arrangements for a criticality event such as this one. It was considered to be an unrealistic scenario.
The Science
If you’re new to nuclear, heading back to the knowledge hub and reading about Fission and Criticality will help understand the science behind why the Tokaimura accident happened.
An uncontrolled, self-sustaining chain reaction between fissionable material and neutrons lead to a criticality event. To keep nuclear material in a non-critical state, there is a list of factors that need to be controlled. This list includes but is not limited to mass, geometry and enrichment.
During the Tokaimura accident, the procedure for fuel preparation was changed. The initial procedure involved transferring dissolved uranium oxide from a dissolution tank into a storage column for mixing, following by transfer to a precipitation tank. This procedure was originally approved by the regulator. The prevention for criticality was based upon limiting mass and volume in tanks which had a critically-safe geometry.
When the company changed this process without approval, the uranium oxide was dissolved in steel buckets and tipped manually into the precipitation tank. This bypassed criticality controls. No longer having the approved geometrically favorable tank, and an increased enrichment of uranium, critical mass was reached kickstarting the criticality.
Effects
The incident was classified by Japanese authorities as not having a significant offsite risk and did not release any radioactive materials.
A total of 119 people received a radiation dose, but only the three operators’ doses were above the allowable limit. These three workers were hospitalized, with two sadly passing away.
While the incident was occurring, people living within 350m from the building were evacuated from their homes and people living within 10km were asked to remain indoors. Both of these restrictions were lifted within two days.
Japan Nuclear Fuel Conversion Co. (now known as JCO) admitted that it violated both normal safety standards and legal requirements. They faced legal action. The plant’s operating license was revoked in 2000.
Lessons learned from the incident have been shared through nuclear professionals worldwide.
The International Atomic Energy Agency (IAEA) says the cause of this accident was “human error and serious breaches of safety principles”.
Lessons Learned
Nuclear accidents of any scale are undeniably catastrophic events with potentially far-reaching consequences. However, these tragic incidents have served as hard-learned lessons for the nuclear industry. An incident like that at Tokaimura spurs a collective commitment to enhance safety protocols, technological advancements, and regulatory frameworks. Stringent international standards and regulations, coupled with advancements in reactor design and emergency response procedures, have transformed the nuclear sector into one of the safest industries globally.
Examples of some of these lessons learned from the Tokaimura accident are highlighted below.
