Re: tritium oxidation rates -Reply

[FRAMEP@ORAU.GOV]

Franz Schoenhofer wrote in part;

>I suppose that the 1.1% is for cold platinum? (Sorry I am at home and do
>not have our Health Physics at hand.) If yes, what would the rate of a
>heated catalyst be, as proposed in the original message?

The table provided by Okada and Momoshima provides oxidation rates
that might be found in environmental/workplace conditions. The specific
conditions that produced  a 1.1% oxidation rate would have to be found
in the original source for the 1.1% number i.e., Eakins and Hutchinson
1973. 

I would be very surprised if it has any relevance to the catalytic
convertors used in air sampling where the air is intentionally drawn
through a mesh or screen of palladium.

ASTM D3442 recommends 5 grams of palladium at 500 degrees C at 100
to 500 ml/min,  or 10 grams of copper at 600 degrees C. Apparently some
people have used up to 50 grams of platinum (not palladium). We used to
run with as little as 1 gram of palladium, but that was more related to the
cost of palladium than anything else.  I remember that after packing the
palladium catalyst in the tube furnaces, we sent them to our machine
shop so that they could weld on a particular fitting. When we got them
back , we were told that the conscientious shop guys had noticed how
plugged up the furnaces were. They proceeded to clean them out. As I
recall a couple of thousand dollars worth of palladium went into the trash
(most of which we recovered).

Those temperatures are what would be needed if organic tritium were
present but lower temperatures can be used if only HT is the problem
(e.g. 400 degrees using copper). With a carrier gas and palladium, HT
can even be converted into HTO at room temperatures. The hydrogen
gas carrier that is added upstream of the furnace can be used to
calculate conversion and collection efficiencies. See Wood et al Health
Physics 65(6):610-627 for a nice summary which I am mostly parroting

Our palladium was a powder that was distributed in quartz wool. A wire
or sponge form of the catalyst is more common. Apparently palladium
coated silica gel or molecular sieve is sometimes used as a combination
oxidizer collector.

It was once pointed out to me that running a catalytic furnace at 500
degrees (or similar temperature) carries a greater risk of harm than the
tritium we are trying to oxidize.

Paul Frame
Professional Training programs
ORISE
framep@orau.gov
www.orau.gov/ptp/ptp.htm