Tritium Probes

TRITIUM MONITORING

DOE-HDBK-1079-94

Tritium Primer

transferred to the body by skin contact or inhalation if it becomes airborne. As a result, loose

contamination is routinely monitored by smears, which are wiped over a surface and then

analyzed by liquid scintillation or proportional counting.

The smears are typically small round filter papers used dry or wet (with water, glycol, or

glycerol). Wet smears are more efficient in removing tritium, and the results are more

reproducible, although the papers are usually more fragile when wet. However, results are only

semiquantitative, and reproducibility within a factor of 2 agreement (for wet or dry smears) is

considered satisfactory. Ordinarily, an area of 100 cm2 of the surface is wiped with the smear

paper and quickly placed in a vial with about 10 mL of liquid scintillation cocktail, or 1 or 2 mL

of water with the cocktail added later. The paper must be placed in liquid immediately after

wiping because losses from evaporation can be considerable, especially if the paper is dry. The

efficiency of the liquid scintillation cocktail is only slightly affected by the size of the swipe.

Foam smears are also available commercially. These smears dissolve in most cocktails and do

not interfere significantly with the normal counting efficiency.

Smears may be counted by gas-flow proportional counting. However, because of the inherent

counting delays, tritium losses before counting can be significant. Moreover, counting

efficiencies may be difficult to determine and may vary greatly from one sample to the next.

Another drawback is potential contamination of the counting chamber when counting very "hot"

smears. For all of these reasons, a liquid scintillation spectrometer is the preferred system.

An effective tritium health physics program must specify the frequency of routine smear surveys.

Each facility should develop a routine surveillance program that may include daily smear surveys

in laboratories, process areas, step-off pads, change rooms, and lunchrooms. In many locations

within a facility, weekly or monthly routine smear surveys may be sufficient. The frequency

should be dictated by operational experience and the potential for contamination. In addition

to the routine survey program, special surveys should be made following spills or on potentially

contaminated material being transferred to a less controlled area to prevent the spread of

contamination from controlled areas.

The surface contamination levels acceptable for the release of materials from radiological areas

may be found in the DOE Radiological Control Manual and DOE Order 5400.5.

Tritium Probes

In general, the total tritium contamination on a surface can be measured only by

destructive techniques. When tritium penetrates a surface even slightly, it becomes

undetectable because of the weak energy of its beta particles. With open-window probes

operated in the Geiger Mueller (GM) or proportional regions, it is possible to measure

many of the betas emitted from the surface. Quantifying that measurement in terms of

the total tritium present is difficult because the history of every exposure is different.

Consequently, the relative amounts of measurable and unmeasurable tritium are different.

Tritium

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