pdf Exposure during the transportation of sealed sources of iridium 192 (85 kB)
Description of the incident
Following the receipt of a package, an abnormally high dose rate was measured: 4 mSv/hr at 25 metres instead of 2 mSv/hr on contact. The type B package contained 366 TBq of iridium-192 in tablet form (intended for the manufacture of sources for industrial radiography). The tablets (3000) were packaged in three tubes (cases) in screw top metal capsules.
The sequence of events was as follows:
- 6:00 PM: Receipt of the package at the airport. The package was grouped with other packages in an aircraft container. It is then handled by a transport company who unload the package using a platform elevator and transfer it using a cart to the dangerous substances storage and sorting area located 100 metres from the aircraft parking area. Having arrived at the storage area, the individual packages were removed from the container and dispatched according to their final destination. The defective package was deposited in another aircraft container, on a mechanical palette in the area reserved for hazardous products. It was then taken to the parking area in order to reduce the loading time. The handling is undertaken with a forklift truck, and the package was not moved manually during its passage through the airport.
- 11:30 PM: The container with the package in question was transferred to the foot of an aircraft. The package remained in transit at the airport for about five and a half hours, and assuming 30 minutes transfer time to/from the storage area, the package was present within the transport company facilities for four and a half hours.
- An inspection ofthe package subsequently revealed that two of the three cases containing the tablets were open, probably because of poorly attached screw lids.
Radiological consequences
A subsequent investigation indicated that two agents of the transport company received doses of 100 and 15 mSv, respectively.
The original consignor of the package said that checks carried out upon departure did not highlight abnormal dose rates around the package and that the values were consistent with those expected for this type of package, i.e.:
- less than 2 mSv/hr. on contact, and.
- 25 µSv/hr. at 1 metre, and a Transport Index of 2.5 (Yellow Label III ).
The measurements taken in the aircraft (cockpit) before departure, and the dosemeters worn by the aircraft pilots, did not detect any radiological anomalies, although this could be due to the orientation of the package, the distance between the cockpit and the package (20 metres), and the shielding provided by other freight.
The destination company measured 4 mSv/hr at 25 meters from the upper side of the package, and 0.01 mSv/hr on the other sides. Thus, the radiation leak was in the form of a beam, rather than uniformly around the package.
Lessons to be learned from the incident
- A radiation detection system with a pre-programmed alarm threshold installed at the entrance to the dangerous substances storage area could have identified the problem sooner. In case of an alarm, additional measurements should be made by suitably trained persons.
- Employees of transportation companies, called upon to handle a large quantity of packages containing radioactive materials, can potentially receive significant radiation doses - high enough to require classificiation, even where no abnormal incidents occur. Personal dosimetry should be provided for such staff, as should appropriate measures to optimise their exposures.
- As well as training on the requirements of the regulations for the transport of radioactive materials, relevant staff should receive training on radiation protection requirements.
pdf Exposure during the transportation of sealed sources of iridium 192 (85 kB)