Answer to Question #11050 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

How can we monitor and measure the committed effective dose due to tritium gas inhalation? From literature, I’ve found that fraction of tritium gas when inhaled is converted to HTO (tritiated water) in body fluids and if we measure the tritium concentration in urine, which dose conversion factor is to be used–HTO or HT gas (tritium gas), for dose estimation?

A

There are a number of companies that provide commercial versions of monitors for measuring airborne tritium. You can find some on the Internet by searching under “tritium air monitor.” Many use flow-through ionization chambers and commonly allow for corrections for background, including radon, and sometimes for other nontritium radionuclides.

If all of the airborne tritium is in the form of the elemental gas (HT), I would be inclined to assess dose according to the model recommended by the International Commission on Radiological Protection (ICRP) in Publication 68, Dose Coefficients for Intakes of Radionuclides by Workers, 1995. The model for dose from tritium gas, HT, does account for a small portion of the tritium intake (0.01 percent) being converted to tritiated water in the body, and this small amount of HTO does dominate the committed dose. The effective dose to the whole body as a result of an intake of I Bq of HT is given (as per ICRP 68) as

E = 1.8 x 10-15 Sv Bq-1 I.

The conversion factor of 1.8 x 10-15 Sv Bq-1 is appreciably greater than what would apply if only the dose impact from HT gas decay in the lungs were considered with no accounting for the HTO dose.

George Chabot, PhD

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