Re: H-3 Exit Signs

[/I=R/S=WALKER/O=AECB.CCEA@mhs-atomcon.attmail.com]

     Tritium-powered exit signs are initially filled with anywhere from 10 
     to 20 curies of molecular tritium gas, but over time, the phosphoric 
     acid used to bind the phosphor powder to the glass tubes radiolyzes, 
     and a small quantity of the far more hazardous tritiated water, HTO, 
     is formed (ICRP 30 assigns a DAC to tritiated water which is four 
     orders of magnitude less than that of molecular tritium gas).  Tests 
     on aged signs conducted at Brookhaven indicate that the quantity of 
     HTO formed could equal several percent of the original HT charge.  The 
     porous ZnS phosphor particles suck HTO up like a sponge, and when the 
     tubes are broken, the resulting contamination can be problematic.
     
     There was an incident here in Canada a couple of years ago during 
     which some tritium-powered heliport landing lights were vandalized by 
     teenage children, who subsequently smeared the phosphor over their 
     clothing, and ingested some by hand-to-mouth transfer.  Urinalysis 
     over the following several weeks showed that the most exposed 
     individual received a whole body committed dose of about 3 mSv.
     
     Admittedly, the broken sign in the university library isn't likely to 
     result in uptakes like these, but finite risk is nonetheless 
     associated with these types of devices.  The decision of whether or 
     not to install tritium-powered exit signs should be taken only after 
     weighing this risk against the benefit of having illuminated exit 
     signs during an emergency which don't rely on outside sources of 
     power.