Accident treatment plant Tokaimura nuclear fuel
The treatment plant is uranium fuel in Tokaimura (Japan), 120km northeast of Tokyo, in Ibaraki Prefecture. Property is currently owned by JCO Company.
The nuclear accident at the facility took place on September 30, 1999, in the building of the plant conversion.
The installation consists of three auxiliary buildings uranium conversion:
- One with an annual capacity of 220 tons of uranium per year for low-enriched (about 5%).
- Another with an annual capacity of 495 tons of uranium per year for low-enriched (less than 5%).
- Another, who had the accident, with an annual capacity of slightly more than 3 tons of uranium per year for high enrichment (not exceeding 20%).
In this third building occurs uranium oxide powder concentrate from the processing of uranium hexafluoride. It used to work continuously, employing only highly specific production orders immediately. Almost alone was running two months a year.
Causes of the accident
To understand what happened we must first briefly explain the process of uranium enrichment plant in Tokaimura.
The uranium enrichment process is performed previously converted into a compound of uranium, uranium hexafluoride, which is gaseous under normal conditions. The next step is the conversion of enriched uranium in the form of uranium hexafluoride to uranium oxide, which is achieved in a tank with an aqueous solution of uranyl nitrate.
The compound was converted by precipitacióny sedimentation, followed by calcination in ceramic fuel pellets, which constitute the fuel elements of some nuclear reactors.
According to the internal operating procedure established, the dissolution of uranium oxide (U3O8) in a tank should be provided for this purpose, transferring despuésa nitrate solution pure uranyl and homogenized with a nitrogen purge gas.
Subsequently, the mixture was discharged into the precipitation tank of water cooled to remove residual heat generated by the exothermic reaction occurs.
To prevent the occurrence of a criticality (a fission chain reaction self-supporting), the procedure set limits to the amount of uranium that would be transferred to the tank precipitations n, a maximum of 2.4 kg of uranium.
The working procedure was amended in November 1996, without authorization by the competent regulatory authorities, allowing treatment of the dissolution of uranium oxide in stainless steel buckets, which did not meet the appropriate action. This new method of work had been carried out several times before the accident occurred.
Thus, in preparing the JOYO reactor fuel in September 1999, workers U3O8 powder dissolved in nitric acid in stainless steel buckets and poured the solution directly into the precipitation tank .
The solution used 16 liter uranium oxide, enriched to 18.8% uranium-235, was divided into four stainless steel buckets to pour into the precipitation tank.
On the morning of September 30, when the volume reached 40 liters, equivalent to 16 kilograms of uranium, much higher than the amount initially limited, reached the critical mass necessary for the initiation of a reactionary , n of self-supporting nuclear fission chain, accompanied by the emission of neutrons and gamma radiation.
Consequences of the accident
The accident directly affected three workers who prepared the sample, which were hospitalized, two of them in critical condition, and one died at 12 weeks and another, after 7 months.
In addition, 56 plant workers were exposed to radiation, of which at least 21 people received significant doses and had to be under medical evaluation.
Within a radius of 200 meters around the facility, was restricted access, and additionally, the Japanese authorities established evacuation measures 161 people, from areas to a distance of 350 meters from the plant .
As a precaution, the 310,000 people living within 10 km were advised not to leave their homes, until the situation was under control, confinement lasting 18 hours.
Criticality Once completed, adding boric acid to the solution tank precipitation, and thanks to the site containment systems, always with respect to depression external radiation levels in the outer returned to normal.
According to the IAEA, radiation levels in the areas near the plant, in mid October 1999, had recovered natural background levels. The measurement of iodine-131 in soil and vegetation outside the facility, found that food had not been affected.
The incident was classified as level 4 as INES Scale ("crash without significant risk off-site"), since the amount of radiation released to the outside were very small, and inside the Li , set limits, but within the site, the damage to the equipment and biological barriers were significant, besides the fatal exposure of workers.
Since the accident, which all indications as human error, the fuel fabrication plant in Japan, were fully automated to ensure that a criticality accident not recur, equipping the systems with neutron monitoring equipment, and conversion using dry methods, inherently safer.