Published: May 06, 2019
This is taken from the IAEA News Channel (Nuclear & Radiological Events)
A radiographer working in the permanent shooting room at one of the licensee’s facilities, cranked out a 2.48 TBq (67 Ci) Ir-192 source for an exposure and did not crank the source back into the exposure device before setting up for the next exposure. Upon badge processing, it was determined that the radiographer received an exposure of 81.49 mSV (8.149 rem). The calculated exposures to the left hand and right hand were 396.84 mSv (39.684 rem) and 93.38 mSv (9.338 rem), respectively. The radiographer is being monitored by a medical physician. The primary cause of the incident appears to be human error. In addition to not cranking the source back at the end of the exposure, the radiographer failed to observe the radiation actuated visible alarm; bypassed the audible alarm feature of the shooting room; and failed to observe his survey meter upon entry into the shooting room. All safety features were operating properly at the time of the incident. The dose received by the radiographer exceeds the U.S. statutory limit for whole body dose of 0.05 Sv (5 rem). NRC EN53994
The incident log for the above event can be read at the following link: IAEA News Channel - Overexposure to Radiographer
Having recently returned from a client with a fully functioning interlocked radiography bay ‘enclosure radiography’, I picked this entry on the IAEA news event page and read with dismay. As always, not all the detail is present, but the report highlights a catalogue of basic errors. Initially I thought ‘permanent shooting room’ must indicate a fully interlocked radiography system, but the more I read the above entry this seemed little more than a shielded area which then relies on human factors for radiation safety (which appear to have failed). In the UK, the above description of the facility (brief as it is), would not meet the requirements of enclosure radiography, and would instead be classed as ‘site radiography’ (although a reclassification would unlikely lead to any safer working conditions given the circumstances of this case).
Before analysing this incident further, let me briefly describe the safety features that were present in the industrial radiography bay I have just visited. The following features were observed (working from the inside outwards).
- Ir-192 source projector connected to an electrically driven wind-out motor (interlocked to the bay access system and controlled from outside the bay)
- Emergency pull wire system around the bay interior (connected to the source wind out system)
- Radiation source status lights (indicating “safe” – green, “radiation imminent” – amber and “radiation exposure” - red). These lights are connected to the source control system AND an independent radiation monitor mounted to the wall
- Independent radiation monitor – independent from the door interlock and source wind-out system (therefore a diverse safety system that will indicate radiation status independently from the source control)
- Interlocked personnel entry door (two diverse interlocks connected to the source control system)
- Door control motor disabled by removing castell key (this being needed to power up the source control system)
- Active signage on the outside of the facility (as noted above for the interior of the facility)
- An internal and external radiation imminent audible alarm
- Source wind out control system panel -which can only be activated once all interlocks have been set
With respect to the above incident, the following declarations are particularly relevant:
1) Cranking the source back at the end of the exposure appears to be a manual action - since forgetting to do so leads to an unsafe condition. In the client radiography bay described, the source would be forced to return to the shielded position before the door could be opened. [Note: in the event of a power failure the source can be manually wound back in].
2) Failure to observe the radiation actuated visible alarm. The report does not provide enough detail to know if this failure could be down to inadequate visibility of the alarm, and its not clear if an audible indication was also given (also see below). Either way, the radiographer has failed to react to warning signs that radiation was still present.
3) Bypassed the audible alarm. This probably means that there is an audible alarm for the duration of the radiation exposure - probably loud to. It is possible that the radiographer muted this alarm (either using a mute button if fitted, or by making a modification or simply covering up the alarm to deaden the sound). This is clearly bad practice but also may be down to bad design - an obnoxious audible alarm that is too loud may well be muted if the user finds it unpleasant or annoying.
4) Failed to observe survey meter upon entry - it is not clear if the monitor was taken into the shooting room switched off, or switched on but not observed, or just not taken in at all.
Items 1-4 are a catastrophic failure of bad design (if the shooting room was designed for enclosure radiography conditions), inadequate engineering and safety features, inadequate management and supervision, and inadequate training in radiation safety practice. In addition, an alarming personal dose meter would have potentially mitigated the dose received once the individual had entered the room by then providing a warning of radiation still being present.
By industrial radiography standards, 2.48 TBq (67 Ci) of Ir-192 is a high activity source (High Activity Sealed Source - HASS). The dose rate expected at 1m from the source (un- collimated) would be about 280 mSv/h, tending towards 2800 Sv/h at 1cm (i.e > 3000 Sv/h on the source). The potential source handling exposure (i.e. extremity rate to the hands) would be of the order of 50 Sv/min or 0.8 Sv/second. Given that the right hand calculated exposure was nearly 400 mSv it is very unlikely that the individual came close to actually handling the source (or source within its collimator). This is very fortune since handling for only a few seconds would have likely resulted in deterministic effects to the fingers (i.e. radiation burns). The whole body dose of 81mSv would indicated a resident time of perhaps 17 minutes at 1m from the source. Obviously actual distance would vary with time, however at a distance of 10cm from the source the whole body dose rate is nearly 8mSv per second so its unlikely they could have come this close given that the whole body recorded dose of 81mSv would be reached in just over 10 seconds.
Overall the dose assessment shows that the individual was very lucky not to receive a dose of radiation leading to deterministic injury or even death.