RE: Tritium Bubblers

["Wood, Michael" <woodm@crl5.crl.aecl.ca>]

     Thanks to everyone that responded to my request for information on
     Flow Through Ion Chambers for Tritium.  We also have a need for a
     tritium Bubbler, hopefully one that can handle relatively large flow
     rates (50 lpm) for extended periods (e.g., 1 week).  If you know of
     specific vendors that can provide such instruments, please contact me
     directly.


Jerry

 Do you really need 50 lpm?  Life would be a lot easier if you could sample 
the exhaust at a few lpm.  If you know your stack factor, all you need is a 
couple of standard gas washing bottles, a pump, a flow meter and a flow 
integrator (a commercial gas meter is a great choice - inexpensive accurate 
and not effected by power failures).  Using 200 mL of distilled water in 
each bubbler and a flow rate of ~200 mL/min should be fine for a one week 
sampling time.  If sensitivity is a problem you may want to go to us a drier 
(rather than bubblers) to strip out the HTO, however that is more work 
and/or more costly - depending on if you regenerate or dispose ($$$) of your 
desiccants after use.


In addition:

Ric and I recently prepared the following response to questions posed by 
staff at a power reactor.  We felt that this information would also be 
relevant to you inquiry f interest to others on the list.



Question #1.

What is an accepted value for transfer of tritium into water solution when 
run through a bubbler (collection efficiency)?

Answer #1.

We are assuming that you are interested in tritiated water vapor (ie. HTO, 
T2O or DTO).  Tritium in its elemental form is not soluble in water and 
therefore has to be oxidized before it can be trapped in water.  Solubility 
of other chemical forms of tritium will vary significantly.

The transfer of tritiated water vapor into water can easily be greater than 
99%.  However this transfer is dependent on the flow rate of gas through the 
bubbler, the resonance time of the vapor in the bubbler and, to a lesser 
extent, the temperature and humidity.

For a standard 250 mL gas washing bottle (without frit) filled with 200 mL 
of tritium-free (initially) H2O, greater than 99% trapping efficiency can be 
expected with flow rates up to about 6 L/min, assuming an air sample of ~300 
L or less.   For larger air samples a second bubbler should be used to 
collect the exhaust from the first.  This arrangement increases the total 
collection efficiency and provides a means of estimating the efficiency of 
the sampling system.

We have prepared the following table using Osborne s bubbler model 
(reference 1 below) for a guide.

Conditions:    Temp=25C,
          RH=50%,
          isotope effect =0.92,
          initial bubbler     volume=200 mL (tritium-free water)
          single bubbler

Volume of air  Sample time         Calculated Bubbler
Sampled (L)         (hours)    @2 L/min efficiency (%)

20             0.08                99.9
100            0.83                99.5
300            2.5                 98
500            4.2                 97
1000           8.3                 95
2000           16.7                89
4000           33.3                79
6000           50                  68
10000               83                  45

Question #2.

What is the maximum concentration of tritium that can be extracted into the 
water of a bubbler before the above efficiency is reduced?

Answer #2.

The specific activity of the bubbler solution can reach greater than 100% 
(typically 110%) of  the specific activity of the tritiated water vapor 
being sampled.  See reference 2 below.  However, as the specific activity of 
the solution approaches that of the source, the overall collection 
efficiency goes down as the exchange is driven by the concentration gradient 
between the source (water vapor)  and the sampling sink (water), and this 
gradient is decreasing.


Question #3.

 What criteria is there on bubblers as far as need to diffuse the air being 
drawn through it to ensure high collection efficiency?

Answer #3

Fritted bubblers increase the efficiency of a bubbler for trapping HTO, 
however we have found that they are not necessary for typical sampling 
conditions, as outlined in our answer to question #1.  They are usually just 
an unwelcome flow restriction !


Question #4.

What is the technical basis for the above?  Are there any
technical papers that discuss this?  Etc.

Answer #4.

Our responses above are primarily based on Osborne s work.  However, over 
the past 15 years we have used many different bubbler systems to trap HTO 
and carried out efficiency measurements on our sampling systems.  Our 
experience is in very good agreement with reference 1 below.

References

1/ Osborne, R.V. Sampling for tritiated water vapor. In: Proceedings of the 
3rd International Congress, 9-14 September, Washington, DC. International 
Radiation Protection Association; CONF-730907-P2; 1973:1428-1433.

2/ Sepall, O.; Mason, S.G. Vapor/liquid partition of tritium in tritiated 
water, Can. J. Chem. 38,2024-2025:1960.


Mike Wood and Ric Surette
Radiation Biology and Health Physics Branch
Chalk River Laboratories, Chalk River.
AECL
..