6.2 Exposed Populations
53
Radiation exposure to emergency responders
Let us assume that a terrorist group has set off a radiological dispersal device
consisting of 1000 Ci (37 TBq) of Co-60. The force of the explosion distributes
the majority of the radioactivity throughout an area about 100 m in radius,
contaminating an area of about 7.8 acres (3.1 ha). The Incident Commander
and/or Health and Safety Officer might be interested in determining how long
emergency response personnel can safely work on this site.
For the sake of simplicity we will assume that the contamination is spread
evenly across this site; spreading 37 × 10
12 Bq across about 31,000 m
2 produces
an activity concentration of 1.2 × 10
9 Bq per square meter. Federal Guidance
Report #15 [10] notes a dose conversion factor for Co-60 distributed as contamination on an infinite plane will produce a radiation dose rate of 1.54 × 10
−15
Sv s
−1 Bq
−1 m
2 . Multiplying these two terms shows us that this level of contamination will produce a radiation dose rate of about 1.85 × 10
−6 Sv s
−1 or 6.7
× 10
−3 Sv hr
−1 (6.7 mSv hr
−1 or 670 mrem hr
−1 in American units).
In the United State emergency responders have a dose limit of 0.25 Sv
in order to save property and a recommendation that individuals be limited to
twice this dose (0.50 Sv) to save lives. This means that, if necessary, a firefighter
could be permitted to work in this area for as long as 37 h fighting fires to save
structures or could be engaged in life-saving activities for slightly more than
three days. Note that these times refer to times working in the contaminated
area, which means that each firefighter could make repeated entries over the
course of several days or longer (assuming that they would exit the area from
time to time to change air supplies, rest, eat, rehydrate, and so forth).
Considering that the minimum radiation dose needed to cause radiation
sickness is about 1 Sv (100 rem) it seems unlikely that any firefighter in this
scenario would be exposed to enough radiation to cause illness. Over a longer
term, assuming a risk coefficient of 5% per Sv of exposure, these personnel
would have an increased lifetime cancer mortality risk of no more than 2.5%.
For members of the public who shelter in a stable and safe location until the
emergency phase ends the dose (and resulting risk) would be significantly lower
than to firefighters.
However, those members of the public who are not proximate to the release of
radioactivity and those who are indoors (and who remain indoors until the dust has
settled) are likely to receive little radiation exposure with the exception of that which
they receive during evacuation from the affected area.
53
Radiation exposure to emergency responders
Let us assume that a terrorist group has set off a radiological dispersal device
consisting of 1000 Ci (37 TBq) of Co-60. The force of the explosion distributes
the majority of the radioactivity throughout an area about 100 m in radius,
contaminating an area of about 7.8 acres (3.1 ha). The Incident Commander
and/or Health and Safety Officer might be interested in determining how long
emergency response personnel can safely work on this site.
For the sake of simplicity we will assume that the contamination is spread
evenly across this site; spreading 37 × 10
12 Bq across about 31,000 m
2 produces
an activity concentration of 1.2 × 10
9 Bq per square meter. Federal Guidance
Report #15 [10] notes a dose conversion factor for Co-60 distributed as contamination on an infinite plane will produce a radiation dose rate of 1.54 × 10
−15
Sv s
−1 Bq
−1 m
2 . Multiplying these two terms shows us that this level of contamination will produce a radiation dose rate of about 1.85 × 10
−6 Sv s
−1 or 6.7
× 10
−3 Sv hr
−1 (6.7 mSv hr
−1 or 670 mrem hr
−1 in American units).
In the United State emergency responders have a dose limit of 0.25 Sv
in order to save property and a recommendation that individuals be limited to
twice this dose (0.50 Sv) to save lives. This means that, if necessary, a firefighter
could be permitted to work in this area for as long as 37 h fighting fires to save
structures or could be engaged in life-saving activities for slightly more than
three days. Note that these times refer to times working in the contaminated
area, which means that each firefighter could make repeated entries over the
course of several days or longer (assuming that they would exit the area from
time to time to change air supplies, rest, eat, rehydrate, and so forth).
Considering that the minimum radiation dose needed to cause radiation
sickness is about 1 Sv (100 rem) it seems unlikely that any firefighter in this
scenario would be exposed to enough radiation to cause illness. Over a longer
term, assuming a risk coefficient of 5% per Sv of exposure, these personnel
would have an increased lifetime cancer mortality risk of no more than 2.5%.
For members of the public who shelter in a stable and safe location until the
emergency phase ends the dose (and resulting risk) would be significantly lower
than to firefighters.
However, those members of the public who are not proximate to the release of
radioactivity and those who are indoors (and who remain indoors until the dust has
settled) are likely to receive little radiation exposure with the exception of that which
they receive during evacuation from the affected area.