RE: DU - request for scientific (!) information

["Christoph Hofmeyr" <Christoph_Hofmeyr@nnr.co.za>]

Franz, Ruth, Keith, Activists and Radsafers,
Reminder: The 0.0055% by mass of U-234 in natural uranium is one of the
progeny in the decay chain of U-238, and in equilibrium therefore accounts
for about the same activity as the 99.3% of U-238.  U-235 contributes only
about 2% of the total radio-activity, but the term depletion refers to this
(fissile) isotope.  Uranium enrichment results in a small quantity of
enriched and a large stock of depleted uranium.  The fate of the U-234
depends on the enrichment method used, but with currently popular
technologies it is depleted even stronger than U-235 in depleted uranium.
   The dose conversion coefficients  for inhalation expressed per activity
   (Bq) are very similar between the relevant Uranium isotopes 234, 235 and
   238, (but of course very different on a per mass basis).
   The conversion coefficients (Sv/Bq) increase by a factor of 10 to 15
   (depending on particle size) between the most soluble (F(ast)) and least
   soluble (S(low)) chemical forms, previously termed D and Y.
   The oxides are relatively insoluble and therefore inhalation of a given
   quantity imparts a larger dose to the lung than a more soluble form.
   Radiologically the oxide is therefore more hazardous than soluble forms.
   Conversely, soluble forms are more toxic.
   My understanding is that the reduced dose coefficients for uranium
   result from the 'new' lung model (post ICRP61) which happened to be
   published after the Gulf war (historical coincidence).
   Now to the chemistry:  A dissolved concentration of 1 mgU/kg of body
   mass is considered lethal, i.e. 70 mg of depleted uranium (70 kg
   individual) representing roughly 1500 Bq, which is relatively
   inconsequential from a radiation perspective.  The chemical toxicity of
   soluble U is apparently such that  an excretion rate of 3 mgU/day was
   considered the sustainable limit, while 10 mg/day caused severe kidney
   damage in the short term.
   Due to the slow uptake,  inhalation of the insoluble oxide causes a
   relatively very weak signal in the urine and presumably the toxic impact
   is accordingly of a lower but more extended nature for a given intake.
   This, together with the higher applicable commited dose conversion
   factor, would tend to emphasise the radiation hazard.
   It would seem only a qualitative comparison is possible between
   radiation and toxicity for a range of exposures.



Chris Hofmeyr
chofmeyr@nnr.co.za


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