[ RadSafe ] Fractionation of Uranium Isotopes in Nature

[John R Johnson]

John Andrews

I'm not an expert in environmental chemistry, but I've been told by experts
that the reason 234U is higher than U238 in ground water is that the decay
of U238 "dislodges" the atom from its crystalline structure. After it decays
through 234Th (halflife=24.1 days) and 234Pa (halflife=6.75 hours) to 234U,
it is more "available" to the ground water than it was as 238U.

John Johnson

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl]On
Behalf Of John Andrews
Sent: March 10, 2006 5:54 PM
To: Orthen, Rick
Cc: Leo M. Lowe; radsafe at radlab.nl
Subject: Re: [ RadSafe ] Fractionation of Uranium Isotopes in Nature


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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