[ RadSafe ] Fractionation of Uranium Isotopes in Nature

Bob Shannon bobcat167 at earthlink.net
Fri Mar 10 13:37:06 CST 2006

The EPA drinking water regulations assume secular equilibrium of the
U-234:U-238 (i.e., 1:1) and suggest a conversion factor of 0.67 pCi/ug

The user is warned however to enjoy this value with extreme caution. The
conversion is not meant to provide a conservative (i.e. high-biased)
estimate of the total uranium alpha activity present - rather quite to the

The factor was designed to provide a low-biased estimate of uranium
activity. Based on its intended use, the outcome IS, however, conservative
(i.e. leads to a high bias). This is because the activity determined by the
conversion is meant to be subtracted from the gross alpha result to
determine compliance with the maximum contaminant level for gross alpha (15
pCi/L). Using a higher specific activity (e.g., U-234:U-238 at 1.3) would
lead to a non-conservative correction of alpha.

The moral of the story is: if you need to accurately know that total U
activity, a mass measurement is not enough. You must measure activity or
isotopic concentrations. 

You can either check this at http://www.epa.gov/OGWDW/methods/rads.html
(handy short version) of go to the CFR.

Bob Shannon

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of Leo M. Lowe
Sent: Friday, March 10, 2006 10:08 AM
To: radsafe at radlab.nl
Subject: [ RadSafe ] Fractionation of Uranium Isotopes in Nature

Further to the recent discussions on the chemistry of natural uranium, DU 
or enriched uranium, the fractionation of uranium isotopes does occur in 
nature.  For example, despite the equal radioactivity concentrations of 
U-238 and U-234 in rocks and soils, the U-234/U-238 ratio in groundwater 
generally unity, and can reach values of 10 or more.  The US EPA drinking 
water guidelines use a U-234/U-238 ratio of 1.3 when converting between 
mass and activity concentrations of uranium in water.

Of course, this should not be taken to imply that the chemistry or 
biokinetics of natural uranium, DU  and enriched uranium differ 
significantly when discussing the potential non-radiological health impacts 
of uranium.

Leo M. Lowe, Ph.D., P.Phys.

SENES Consultants Limited
llowe at senes.ca
Tel: (905) 764-9380
Fax:(905) 764-9386

This transmission is intended only for the addressee and may contain 
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