(From database EAN - www.eu-alara.net)
Description of the incident
A monthly personal dosemeter assigned to a truck driver working for an accredited radioactive source distributor recorded a dose of 11.3 mSv. Every Saturday morning, the driver collected six technetium-99 generators from an authorized depot at the airport of city A, and then delivered them to hospitals in city B (a 3-hour drive) and city C (a further 3-hour drive).
When the dose report was received, the distributor carried out an investigation and drew up an incident report. It was discovered that the driver did not use a cart or trolley to transport the generators but carried them himself (manually) from the parking lots to the points of delivery. The driver wore his dosimeter in his trouser pocket or on his belt during each work day.
The investigation determined that the driver could have carried the generators himself over a period of two to three hours. Thus, the distributer concluded that carrying the generators from the truck to the reception points were the cause of the high dose recorded.
Investigation by the Regulatory Authority
A further investigation was carried out by the Regulatory Authority. This included a series of measurements on similar technetium-99 generators, to reconstruct the dose received by the driver. Using dose rate meters and dosimeters, a series of measurements were made around the generators, and also at the driver’s seat (in the front of the vehicle). Other measurements were also carried out at approximately 5 cm from the load in order to simulate the generators being loaded directly behind the driver’s seat.
The dose rate at the surface of the generators was 500 - 600 μSv/hr. The dose rate at 5 cm from the load was close to 500 μSv/h. When the generators were loaded in the rear of the van, the dose rate at the driver’s seat varied between 5 and 10 μSv/hr.
On the basis of the information provided by the driver and the measurement results, the manual transportation of generators was estimated to give rise to a maximum dose of 1 - 1.5 mSv. The additional exposure during driving (assuming the generators were loaded into the rear of the vehicle) was 0.4 mSv, giving a total of approximately 2 mSv over the exposure period. This dose is not compatible with the dose of 11.3 mSv registered by the personal dosemeter.
During more detailed discussion with the driver, it transpired that he did place the generators directly behind his seat to facilitate handling. Under these circumstances, the driver was exposed to significant dose rates for several hours each Saturday. Thus, it was estimated that, over the period that the dosimeter was worn (between four and six weeks), a dose between 9 and 14 mSv could have been received, ie just from driving the vehicle.
The Regulatory Authority concluded that this incident resulted from a lack of appreciation by the driver of the nature of the radiation hazards associated with the generators. In turn, this indicated a lack of suitable training. Loading the generators at the front of the vehicle was not compliant with "Instructions to Drivers" drafted by the Regulatory Authorities, and was also a contravention of the working procedures established by the distributor.
The distributor was asked to update and amend his "Radiation Protection Procedures" and to reinforce the training of his staff in order to emphasize the following points:
- Technetium generators must be loaded in the rear of the transport vehicle in order to guarantee dose rates of less than 20 μSv/hr at the driver position.
- Once unloaded from the vehicle, the technetium generators must never be transported other than on a dedicated cart, provided by the employer. The drivers must not carry the generators themselves between the vehicle and the point of reception.
This incident also demonstrated:
- The value of the driver wearing a personal dosimeter. In this case, the problems may never have been identified if a dosemeter had not been provided. In addition, if the driver had been provided with a dose rate meter, this wiould have helped him to restrict his own radiation exposures.
- The value of the dose reconstruction methods employed in the investigation by the Regulatory Authority.