[ RadSafe ] Fractionation of Uranium Isotopes in Nature

John Andrews andrewsjp at chartertn.net
Fri Mar 10 19:53:57 CST 2006

Orthen, Rick wrote:

>To add to Bob's spot-on evaluation:
>The State of California and the USEPA have suggested other
>mass-to-activity conversion factors (0.79 pCi/ug and 1.3 pCi/ug,
>respectively) reflecting higher relative abundances of 234U to 238U.
>The mass of uranium in water is largely determined by 238U, due to its
>longer half-life, while the total activity in the water is determined by
>the activity of all uranium isotopes. In natural water, the 234U is
>slightly more soluble causing the activity ratio of 234U to 238U to vary
>from 1:1 to more than 20:1. Consequently, accurate conversion from mass
>to activity or vice versa requires knowledge of the concentration of
>each of the three uranium isotopes.
>Rick Orthen
>CEC Inc.
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I am not an expert on the solubility of the uranium isotopes, but I 
thought that the difference in the ratios of U-234 to U-238 in water was 
due to the different solubility of uranium and thorium because the U-238 
decays to Th-234, then to Pa-234 before becoming U-234.  In this case, I 
believe that the thorium is seriously less soluble than the uranium 
under most environmental conditions.  This leads to a different ratio 
after the natural separation occurs.  Please correct me if I have it wrong.

John Andrews, Knoxville, Tennessee

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