Re[2]: Tritium Exit Signs

[bkolb@arinc.com]

> devices rupturing or leaking?  Is there potential for mass contamination
> if the thing was seriously damaged?  Perhaps any research that has been
> done into this area could be shared with us.  Also, are there any non-
> electrical alternatives to the tritium exit sign.
>
> After 7 years or so, the tritium signs are marked to "expire" because of
> decay.  - How are these signs disposed of?
> _ Does anyone have a rotation program for these things when they
> expire?  Is it really necessary?


I've read through brochures and independent studies done on these items and
came away with the feeling that even if they break, they present less of a
radiation hazard than a cross-country flight in terms of potential exposure
(far less in terms of actual mortality).  The tubes can be individually
removed and must be sent back to the manufacturer for disposal.  New tubes
are shipped in special containers and are replaced by hand.  Radiation at
the surface of these devices is extremely small since it consists of very
weak beta that is almost entirely absorbed by the glass.  I once tried to
buy a replacement tube but the manufacturer was pretty wary and wouldn't
sell me a new one unless I returned the old one.  Similar lights have been
used for exit signs in aircraft, movie theaters, industrial plants, and
public facilities.  One local theater has hundreds of aisle lights that are
2 Ci apiece.  These are basically the same thing but embedded in a heavy
acrylic housing that can withstand being walked on, kicked, and vacuumed.
These must be returned to the manufacturer for disposal and have an
estimated life of 10-15 years.  Smaller versions of these tubes are used in
consumer items such as watches and rifle sights.

There is no non-electrical alternative except ZnS luminous paint, which
isn't as bright, has to be recharged by bright light, and fades more or
less rapidly depending on how it is doped.  Interestingly,
electroluminescent signs have a life of just 8 years, whereas tritium
lights last 10, 12, 15, or 20 years depending on which you buy.

The following is from literature provided to me by one tritium 
light company.

"Although highly improbable, if an installer dropped an exit sign 
from an extreme height, and a tube in the module did break, an 
accident scenario could be predicted from:
     1) How many tubes broke (1-12)?
     2) How large a volume of air and how many air changes per 
        minute?
     3) What was the residence time for the installer?

If one tube broke in the drop, 1.2 Ci of tritium gas would be 
released to the atmosphere.  With a tritium content of 1%, there 
would be 0.012 Ci of oxide.  All tubes would release a total of 
0.08 Ci.  All calculations are for tritium oxide.

Dispersion of the oxide-laden air through a room is quite rapid.  
The larger the room, the less concentrated the oxide would become.  
A typical small room could be assumed to be a hallway 10 feet high 
by 30 feet long by 10 feet wide (3,000 cubic feet or 84.96 cubic 
meters).  For a worst case scenario assume that there is no 
ventilation in ;the hallway, and that 12 tubes released 0.08 Ci of 
oxide into the 3,000 cubic feet.

A typical installation takes 5 minutes; therefore, for a scenario, 
assume that the sign was broken, diffusion into the hallway 
occurred, and that the installer remained in the hallway for the 
full 5 minutes.

If 0.08 Ci of oxide diffused into a 84.96 cubic meter room, the 
concentration would be 9.4xE10 Ci/cc.  The average breathing rate 
for a standard 70 kg person is 20820 cc/min, and the oxide is 
ingested both through the lungs and exposed skin, so the rate of 
ingestion of tritium oxide is assumed to be twice the breathing 
rate.  In a typical 5 minute exposure an average person would 
ingest 39.1 microcuries/min (0.0000391 Ci/min) or 195.9 
microcuries total; therefore, the total dosage is 0.033 rem for 12 
broken tubes.  If a more reasonable accident is assumed (1 broken 
tube), the rem dosage would be 0.005 rem or 5 millirem."

One should probably balance this against situations in theaters 
and high rises where a fire and subsequent electrical failure 
could plunge occupants into complete darkness.  Tritium devices 
are built to ANSI N540 standards which specify 22 drop tests of a 
production model onto a steel pad from heights of one and two 
meters and having steel bars dropped on them from heights of 6 
feet.  The glass tubes are made of refractory borosilicate glass 
to withstand fires and all tubes are leak tested for anything in 
excess of 50 nanocuries.

bill