Re: Plutonium vs Natural Thorium

["Jerry Cohen" <jjcohen@prodigy.net>]

In a previous somewhat facetious posting, I asked how, if one really wanted
to identify the most toxic or hazardous substance, what is the most
appropriate measure for making such a determination?  Is there  any clear
unambiguous answer?  Any ideas?         jjcohen@prodigy.net


-----Original Message-----
From: Bradshaw, Keith <Keith.Bradshaw@nnc.co.uk>
To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
Date: Wednesday, May 10, 2000 6:22 AM
Subject: RE: Plutonium vs Natural Thorium


>
>
>> -----Original Message-----
>> From: Leo  M. Lowe [SMTP:llowe@senes.on.ca]
>> Sent: Tuesday, May 09, 2000 10:47 PM
>> To: Multiple recipients of list
>> Subject: Plutonium vs Natural Thorium
>>
>> Previous posts have noted that:
>>
>> 1) natural thorium (Th-232) has a lower ingestion ALI than Pu-238 or
>> Pu-239,
>>    but also
>> 2) one would have to inhale 10 grams of thorium vs only 28 micrograms of
>> Pu-239    to get 1 Sv.
>>
>> Would not a better comparison be to natural Th-230 in the U-238 series?
>> The ICRP inhalation dose conversion factor for Th-230 is almost the same
>> as
>> that for Pu-239 i.e. 1.4E-05 Sv/Bq versus 1.6E-05 for Pu-239 (inhalation
>> class S).  Also, the specific activity of Th-230 (77 000 y) in only about
>> a
>> factor of three smaller than that of Pu-239 (24 100 y).
>>
>> On this basis, one would need to inhale about 100 ug of Th-230 to get 1
>> Sv.
>>  This type of calculation could be extended to Ra-226 which is even more
>> "toxic" (on a per gram basis) than Pu-239.
>>
>> Just think, all this radioactivity is in our own backyards!
>>
>>
> OK here's a league table of radiotoxicity by ingestion on a mass
>basis:
>
> Mass for 1 Sv
>
> U-232    3.8 Microgram
> Pu-238  6.7 Microgram
> Am-241  39.5 Microgram
> Ra-226  97.6 Microgram
> Pu-240   595 Microgram
> Pu-239  1.76 Milligram  (note unit change)
> Th-230  6.38 Milligram
> Th-232  1.075 Kilogram  (yes KILOGRAM)
>
> In practice, Pu is a mixture of 239,240, 238 and 241 isotopes, with
>239 predominant.  Th-230 will probably be accompanied by Th-232 which would
>act to dilute the thorium specific activity (even though it comes from the
>U-238 decay chain, U containing minerals also contain some  nat-Th).  U-232
>is synthetic but again there's probably a few microgram of nat-U in
>everyone's daily diet which would dilute the U specific activity.  So the
>element with the greatest mass-for-mass toxicity might be said to be
>americium, in this way of looking at things.
>
> Note this is strictly for fun (!!!).  Please do not base anything on
>these figures.  Also, I don't pretend to have necessarily found the most
>radiotoxic-by-mass nuclide here.  I have not got time.   What you want is
>high dose coefficient coupled to short half-life.
>
> Dose coeffs from IAEA safety series 115 Table II-VI (ingestion,
>members of the public).
>
> Personal views only
>
> Regards
>
> keith.bradshaw@nnc.co.uk
>
> END
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> END
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