Re: Tritium and other isotopes in freezers and refrigerators

[dj_strom@ccmail.pnl.gov]

During my years as a campus RSO, we found that any contamination in frost-free 
refrigerators-freezers (R-Fs) would show up in the drain pan underneath the 
unit.  Most R-Fs have a pan or drip tray underneath them that can be removed by 
snapping out the grill below the door.  There is usually a fan that blows warm 
air that has been heated by the evaporator coils over the drip tray, to 
evaporate the water that collects there each time the unit defrosts.  The crud 
that builds up in the tray provides an integrated sampler for whatever's loose 
in the R-F.  H-3 in frost either exchanges with hydrogen on the dirt, or was not
water in the first place (e.g., organically-bound tritium, OBT).  Other isotopes
would show up in the drip tray, too; we found C-14 and I-125.

We generally found that, if the drip tray was clean, so was the R-F (I can 
recall no exceptions to this).  If you count your wipe of crud from the drip 
tray by liquid scintillation, be aware that significant chemiluminescence 
usually accompanies this kind of dirt.  For us, the drip tray was a "must 
survey" location on quarterly lab surveys, along with centrifuges, vortex 
stirrers, and water baths.

Dan Strom <dj_strom@pnl.gov>
Pacific Northwest Laboratory