Re: tritium stratification

[js_dukelow@ccmail.pnl.gov]

On Wed, 21 Feb 1996, Gary Schroeder asked:

>  Does anyone know if it's possible for tritiated water to "stratify if
>  kept in a large pond that accepts ordinary rainwater?  We're talking 
>  about a total pond depth of approximately 7 ft. (pond is stagnant, 
>  artificial, and lined with plastic).  Everything tells me that the 
>  concentrations of tritium  ...  should be completely homogeneous, but I 
>  have data that suggests otherwise.

A quick swing through Bird, Stewart, and Lightfoot, _Transport Phenomena_  
and Streeter's _Handbook of Fluid Dynamics_ strongly suggests that you 
should believe your data.

HTO, having molecular weight of 20 vs 18 for ordinary water, should move 
preferentially more rapidly 

  1)  down a temperature gradient (thermophoresis),
  2)  up a pressure gradient,
  3)  toward the center of the earth, and 
  4)  down its own concentration gradient.

Effects 1), 2), and 3) will move the HTO toward the bottom of any pond 
with a normal temperature gradient.  In the equilibrium condition, the 
concentration of HTO at the bottom will be enough greater that the return 
"flow" of HTO down the concentration gradient (and toward the surface of 
the pond) will exactly balance the Effect 1), 2), and 3) flows. 

Effects 1), 2), and 3) are quite small in a liquid, so the resulting 
concentration gradient should also be quite small.  These phenomena are 
roughly 4 orders of magnitude more important in a mixture of gases, which 
is probably why hydrogen and helium in the atmosphere tend to leak off 
into space.  Also, it would appear that the same thing will happen in a 
mixture of water and C2H5OH ; that is, the concentration of alcohol is 
slightly greater in the bottom of the bottle (unless these effects are 
overwhelmed by phenomena related to the size of the molecules or to 
binding between the alcohol molecules and the surrounding water). 

The physical reason for Effects 1) and 2) is that the higher mass of the 
HTO molecule means that molecular collisions will produce a tighter 
velocity distribution with a smaller mean velocity for HTO molecules, by 
comparison with H2O.

Best regards.

Jim Dukelow
Pacific Northwest National Laboratory
Richland, WA

js_dukelow@pnl.gov

These thoughts are mine and have not been reviewed and/or approved by my 
management or by the U.S. Department of Energy.