The gamma-irradiation facility (GIF), located in the town of Stamboliyski, was put into operation in 1980, charged with 49 sealed Co-60 sources with a total activity of 426 TBq. The GIF was owned by the Agricultural Academy and had been used for the irradiation of food and agricultural products, as well as for scientific experiments.
From 1997 till 2005 the GIF has not been used for its intended purpose and the sources were temporarily stored in the facility. In 2005, the sources were transferred for long-term storage to the Novi Han RAW Repository.
In 2005, "Gitava" Ltd. company became the owner of the facility. In 2005, Gitava Ltd. started with the reconstruction of the GIF, which was put into operable condition in 2007. GIF has been put into operation and licensed in 2008, as the facility maintenance is performed by the company personnel.
The GIF consists of a shielded container (buried in the ground) with six vertical cylindrical channels, providing a panoramic irradiation field. Three channels are loaded with cylindrical tubes containing the sources (called assemblies) and the remaining three channels are loaded with blank tubes filled with lead (called imitators).
On 14 June 2011, the GIF has been loaded with 12 Co-60 sources with a total activity of 421 TBq.
Planned operations for changing of the configuration (rearrangement) of assemblies and imitators in the GIF started at 9:30 on 14 June 2011, under the leadership of the company manager and with the participation of 4 other company employees. The purpose was to remove one imitator and to prepare the GIF for subsequent loading of an additional assembly with sources. All workers had valid individual licenses.
Activities for removal of one cylindrical tube started at 13:00 o'clock. According to the load diagram from 2008 this tube should have been an imitator (blank tube). During the withdrawal of the tube, the radiometer used to monitor this operation gave an alarm signal. The tube was put back into the channel. Following discussions on whether this was an imitator (as indicated on the load diagram) or a loaded assembly (as the radiometer alarm suggested), the work continued under conditions where the exact configuration of the tubes in the shielded container was unclear.
At 13:15 the tube was taken out by hand and was put upright against the wall, close to the entrance door of the hall. No alarm from the radiometer was heard during this manipulation. Next, workers tried to take out another tube using the crane and a refueling container. This was unsuccessful and the tube that had already been taken out was put back in the channel to reduce the gamma radiation penetrating through the opening of the channel.
When finally the staff managed to withdraw the stuck tube they realized that this was an imitator and respectively the tube that was taken out and put upright against the wall was a charged assembly, and not an imitator as they assumed. The staff realized that they had made a mistake and that they have been exposed to a high radiation field.
The time during which the staff has been exposed to radiation was estimated to 25-30 minutes.
As a result of the event, five workers received total doses above the authorized limits. overexposed during the event. There was no impact to public and the environment. The results from biological dosimetry are listed in the table atttached.
On 14 June 2011, workers started the planned operations in violation of the quality assurance program, since no work plan had been developed and respective responsible officers and executors were not designated. Activities on rearrangement of tubes (assemblies or imitators) started without the necessary preparation, according to the instructions.
The load diagram with the location of tubes inside the shielded container that had been used was created in 2008 during the initial charging of the GIF with Co-60 sources. The diagram appeared to be wrong and mislead the staff, who proceeded with the manipulation of tubes being convinced that they knew exactly the location of the blank and loaded tubes. This initial misjudgment lead to a series of errors and improper actions by the staff.
The employee who withdrew the assembly had eyesight problems (2.5 diopters) and worked without his glasses. Because of this fact, negligence or inattention he did not realize that this assembly was a loaded one. Assemblies with sources have a distinctive head of hexagonal shape, while the imitators have heads with round ends.
During the withdrawal of the tube by hand, the alarm signal of the portable radiometer was activated and the tube was immediately put back in the channel. This alarm signal indicates that the tube in the channel is a loaded assembly and not an imitator. Instead of performing a visual verification of the shape of the tube head (see above), the workers solved the issue through a discussion. An additional factor is the lack of labeling ("Co-60" for assemblies and "Pb" for imitators) on the upper surface of the shielded container. The presence of such labeling is a requirement of the approved procedures. The existence of such labeling could have prevented the event.
In the subsequent operations, no dose control has been carried out in the hall irrespective of the fact that such radiometric and dose control equipment has been available. Workers were wearing their individual film dosimeters, but no one was wearing the available on-site individual alarm dosimeters. On the day of the accident these dosimeters were not usable because of empty batteries. The Regulation on radiation protection during activities with sources of ionizing radiation requires the use of backup individual alarm dosimeters with direct reading during such high risk activities.
In addition, the licensee did not inform NRA about the planned radiation hazardous works and had not applied for a permit to modify the configuration of the tubes, as required by the license conditions.
INES Rating of the event
In the first days, when the available information about the event and its consequences was incomplete or inaccurate, a provisional rating "Level 3 - serious incident" was assigned to the event using the INES scale.
As a result of the event of 14.06.2011, one worker received a total dose of 5,63 Gy and four other workers received total doses in the range of 1,23 Gy to 3,44 Gy. As a result of the overexposure, deterministic effects occurred (acute radiation syndrome).
According to the criteria specified in paragraph 2.3.1 of the INES Manual if the event involves "likely occurrence of a lethal deterministic effect as a result of whole body exposure, leading to an absorbed dose of the order of a few Gy" the minimum rating is Level 4. Based on the fact that one individual has received a dose of 5,63 Gy (50% likelihood of a fatality - Appendix II), the event has been rated at "Level 4 - Accident with local consequences", which is the final INES rating of the event.
Event date: Tue, 14-06-2011