Re: Skin dose from enriched uranium

[Al Tschaeche <antatnsu@pacbell.net>]

Quastel wrote:
> 
> Dear Radsafers,                                         August 16, 1997
> 
>        A local author is writing a thriller involving terrorists with
> atomic bombs available to them. He asked me what mass of enriched uranium,
> if handled with bare hands, could cause radiation burns to the skin. 

The skin dose rate from unirradiated uranium, not uranium ore, of any
enrichment is about 200 mrem/hr.  The dose to cause skin "burns",
depending on the area irradiated, is greater than 1500 rem.  The mass of
uranium is not a factor unless it is very small.  If it is very small,
e.g. a gram or so, there is no effect even if the person were to hold it
for a very long time.  Therefore, a person would need to hold the
uranium in his hand for 1500000/200 = 7500 hours (a little less than a
year) to get a dose that might cause a burn, if the dose were given all
at once.  Because the dose is spread out over so long a time, it would
take longer (e.g. a higher dose) to produce an effect, because the skin
has a repair mechanism. I don't think the author will have a person
holding uranium continuously for such a long time. 

Since the material in question is enriched uranium, please advise the
author that he better not have his hero hold too big a mass, or the hero
will be holding a critical mass and will probablly be killed by the
fission radiation.  

Also,
> what mass of unshielded enriched uranium would cause sufficient acute whole
> body exposure to cause a lethal acute radiation syndrome?

Any mass of enriched uranium less than a critical mass cannot cause
sufficient acute whole body exposure to cause a lethal acute radiation
syndrome, or any other effect for that matter.  The gamma surface dose
rate from uranium is a few mrem/hr.  It takes more than 100 rem to
create any ARS symptoms in the vast majority of people.  So it would
take 100000/5 = 20000 hours (~ 2.5 years) to get 100 rem at the surface
of the uranium.  The farther away from the surface one gets, the lower
the dose rate.  I would say that it is not possible to get a lethal dose
from any less-than-critial mass of unirradiated uranium of any
enrichment.  Again, if he holds a critical mass (assuming proper
moderation and reflection), he will probably be killed by the fission
radiation as the Russian was who died recently.

Hope this helps.  Al Tschaeche antatnsu@pacbell.net