Hi All,
Reality Check - "dirty bombs"
Comments continue to circulate about the possibility for terrorists to
explode a 'dirty bomb' or a radiological bomb'. Indeed, even Warren
Buffett, the 'Sage of Omaha' has the other day been reported as saying that
a radiological attack is not a question of 'if', but of 'when'.
There is however very little written about just what the possible health
impact of such an action might be.
It is not difficult to make some very general estimates of what the
radiation consequences might be, and the findings that I here present
indicate that the health effects would probably be very small, although the
disruption and the cleanup task might be a serious cost.
Let us assume that a stolen source wrapped with explosives has been
detonated in a large office block or shopping mall. There would certainly
be serious damage from the explosive, and injuries resulting from flying
debris, building collapse and so forth.
But I thought I might try to estimate potential radiation doses and hence
health risks from such a release...
So, here's a little calculation of possible doses to emergency responders or
to members of the public who might remain trapped in the building:
Example 1.
Assume a stolen Cs-137 source of Activity 100 millicuries (= 3.7 GBq).
Assume it is exploded and all the caesium disperses into a building of
volume 500,000 cubic metres as a fine respirable powder.
This implies an in-air concentration of about 8000 Bq/m3.
IAEA BSS (Safety Series 115) Tables II-III and II-VII indicate dose per
unit activity by inhalation for Cs-137 is in the order of 5 * 10-9 Sv/Bq.
So a person breathing the contaminated air for 1 hour without respiratory
protection will incur about (1 m3 * 8000 * 5 * 10-9 ) Sv or 40
microsieverts.
This amount is essentially trivial in this context.
Example 2.
Assume that an Iridium-192 source of Activity 1 Terabecquerel (approx 30
Curies) is dispersed in the same building.
The airborne activity concentration will then be 2 * 106 Bq/m3 and the total
activity breathed in in one hour will be approx 2 * 106 Bq.
This will incur (using data from IAEA BSS as above) a dose of (2 * 106 * 5
* 10-9) = 10 mSv, which is about equivalent to a 'CAT scan'. In other
words, not that much.
Please note that in actual fact, it will be quite difficult to get all of
the radioactive material into the air in respirable form: it will want to
plate out on its surroundings. So the above calculations are almost
certainly unduly pessimistic, and exaggerating the actual doses that would
be incurred.
So we can say that any injuries or deaths will be from the explosive blast,
mechanical injury, or any resulting fire, or from the panic, eg in hasty and
ill-disciplined (not to mention ill-advised) evacuation traffic accidents)
rather than from the radiation dose.
Responding emergency agencies should be informed beforehand, i.e., NOW, that
dose control requirements are thus NOT an overriding issue, but rather the
control of panic. Responders should be reassured, NOW, that 'moonsuits are
not required', but rather, that first aid and fire control are the essential
requirements.
Mark Sonter
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