pdf Persons exposed due to failing to wind in NDT source between shots (95 kB)
Description of the incident
The incident occurred during night-time gamma radiography using a projection container with a 721 GBq (19.5 Ci) irdium-192 source. Following an exposure, a radiographer forgot to retract the radioactive source before placing new films and moving the end of the projection tube for a new exposure. He also forgot to observe the safety controls and procedures provided for the container.
The sequence of events was as follows:
- The incident occurred at 2:00 a.m. It is common for the radiography to take place at night – so as to avoid any clashes with normal daytime work on site (and thus sped up the process). The incident took place during the 12th shot: the repetitive nature of the work may have contributed to the failure to retract the source to the safe position.
- Operator A did not positively verify the actual return of the source to the container. This can be done by checking that the remote control timer has returned to zero, that there is a yellow “source returned” indicator on the container, and finally by mechanically locking the source in the container.
- An electronic dosemeter was worn, but it was underneath work clothing (in fact, the risk of contamination in the work area required the wearing of a paper coverall). The dosimeter did not have an audible alarm to advise the person involved of the increase in dose rate: it only emitted flashes (which would not have been visible) when a dose of 1 mSv was reached. The operator did not check the reading on the dosemeter.
- There is a safety system on the container, whereby it has to be re-set before the next exposure in order to release the source. It is then that the operator realized that the source did not re-enter the container. The other safety device is an indicator on the container that shows:
- Green: Source retracted, device locked, key removed (storage position).
- Yellow: Source retracted, device locked, key in place, shutter closed.
- Red: Shutter open, source exposed or ready to be exposed.
- Red with white dot: Incomplete retraction of source holder.
However, the operator did not check these indicators during the incident.
Radiological consequences
The radiographer received a whole body dose of 15 mSv. It is not uncommon for this radiography to undertaken using a source with up to six times the activity in this incident – thus a dose of up to 90 mSv could have been received.
In order to determine the dose received by the hands, the person was subject to a range of medical examinations which did not reveal any effects.
Lessons to be learned from the incident
For each and every radiography exposure it is essential that the return of the source to the safe position is verified. This container was fitted with special safety indicators, but these are not sufficient, by themselves, to prevent incidents. The source should always be locked in the container and a measurement must be made with a suitable dose rate monitor after every exposure.
If workers have to wear additional clothing such as coveralls, it must be ensured that personal electronic dosemeters can still be read. In any case, such dosemeters should have an audible alarm function to indicate the presence of high dose rates and whenever doses above a pre-determined level are exceeded.
There are lessons in terms of repetitive night working, i.e. in which the risk of operator error can be greatly increased. Employers should be encouraged to take such factors into account.
Even experienced and trained radiographers need reminding of the potential radiation risks associated with poor working procedures. Refresher training should be arranged at regular intervals.
pdf Persons exposed due to failing to wind in NDT source between shots (95 kB)