| Accident scenario |
On August 9, 1989, three workers at
a CANDU nuclear power station in Ontario were involved in an overexposure
incident. The accident took place when a crew of workers was removing an
irradiated cobalt adjuster rod from a nuclear reactor. The rod was contaminated
with cobalt 60, a pure gamma emitter, and was later estimated to contain
approximately 396,900 Ci. During adjuster rod removal, an adapter “donut”
or shielded transition piece is used to connect the reactivity mechanism
deck on which the work is carried out to the shielded flask into which
the contaminated rod is ultimately transported. A crane is used to place
the donut and the flask in position, and then a crank is turned to winch
the irradiated adjuster rod into the shielded flask. There were two donuts
available for the job: one was a shielded donut, and the other was an identical
hollow rehearsal donut. After the exposure incident, it was discovered
that the rehearsal donut had been used for this job. Consequently, the
workers were exposed to an inadequately shielded cobalt 60 gamma source
while the contaminated adjuster rod was being cranked into the containment
flask. This resulted in radiation exposure which was maximal next to the
unshielded donut at a height of approximately 30 cm, and then diminished
with distance and height from the donut.
The three individuals involved in this
incident were all wearing thermoluminescent dosimeters on their ankles,
arms, head, and body (to estimate gonad dose). During the procedure, portable
gamma meters were used to measure the radiation fields. When the adjuster
rod was being cranked into the containment flask, it was noted that the
high-range gamma meter went off scale, an event which had never happened
during adjuster rod removal and the workers became alamed. However, they
completed the job in a few seconds so that the contaminated adjuster rod
was properly stored in the shielded flask. Subsequently, they informed
their supervisor and their dosimetry results were obtained as soon as possible. |
| Clinical findings |
| Complete blood counts, including
red blood cells, lymphocytes, neutrophils, and platelets, were done on
each of the three workers for 4 consecutive weeks, commencing on August
l6 (1 week after the exposure incident) and ending September 5. No sustained
reduction in lymphocyte counts or other blood counts was seen in any of
the three exposed employees. |
| Chromosomal dosimetry |
| Lymphocytes from a peripheral blood
sample are analyzed to determine the presence of chromosome aberrations
typically associated with radiation -centric rings and dicentrics. Three
different laboratories were used to carry out the cytogenetic analysis
; these included two university-affiliated laboratories- one in the United
States (Laboratory A) and one in Canada (Laboratory BLand a Canadian government
laboratory (Laboratory C). |
| Reference |
| House RA, Sax SE, Rumack ER
and Holness DL: Medical management of three workers following a radiation
exposure incident. Am. J. Indust. Med., 22: 249-257, 1992. |
| Chromosome aberration analysis |