Differential Air Monitoring

TRITIUM MONITORING

DOE-HDBK-1079-94

Tritium Primer

Differential Air Monitoring

Because HTO is more toxic than HT (10,000 to 25,000 times greater), it may be

desirable to know the relative amounts of each species following a significant release into

a room or to the environment. In the case of stack monitoring, discrete samples of the

stack effluent should be taken using bubblers or desiccants with a catalyst for oxidizing

the HT. Another technique for differential monitoring uses a desiccant cartridge in the

sampling line of an ionization chamber monitor. The result is a measurement of the HT

concentration. Without the cartridge, the total tritium concentration is measured.

Subtraction of HT from the total produces the HTO concentration. The technique may

be used with two instruments or one instrument in which the desiccant cartridge is

automatically switched in and out of the sampling line.

Another technique uses a semipermeable membrane tube bundle in the sampling line to

remove the HTO (preferentially over the HT), which is directed to an HTO monitor.

After removing the remaining HTO with another membrane dryer, the sampled air is

directed to the HT monitor. Although this technique is slower than the one requiring a

desiccant cartridge, it does not require a periodic cartridge replacement. Furthermore,

it can be adapted to measure tritium in both species in the presence of noble gases or

other radioactive gases by adding a catalyst after the HTO dryers, followed by additional

membrane dryers for the HTO. However, because of its slow response, it is more

suitable for effluent or stack monitoring than for room monitoring. Because significant

releases into a room are quite rare, it is easier to treat any such release as one of HTO

than use complicated techniques for continuous differential monitoring.

Discrete Air Sampling

Discrete sampling differs from real-time monitoring in that the sampled gas (usually air)

must be analyzed for tritium content (usually by liquid scintillation counting). The usual

technique is to flow the sampled air through either a solid desiccant (molecular sieve,

silica gel, or Drierite) or water or glycol bubblers. For low-flow rates (about 0.1 to 1

L/min), bubblers may be used. Bubblers are more convenient for sampling, but are less

sensitive than the solid desiccant cartridges if the water in the desiccant is recovered by

heating. Glycol or water may be used, but glycol is preferred for long-term sampling.

In any case, the collected water is then analyzed for HTO. For differential monitoring

of HTO and HT, a heated catalyst (usually a palladium sponge) is used between the HTO

desiccant cartridge or bubblers and the HT cartridge or bubblers. This is currently the

preferred method for monitoring stacks for reporting purposes. In a different

arrangement, palladium is coated on the molecular sieve in the HT cartridge to oxidize

the HT into HTO, which is then absorbed by the molecular sieve. However, this

technique is used primarily for environmental monitoring.

Another technique for sampling HTO in room air is to use a "cold finger" to freeze HTO

out of the air. An alcohol and dry ice mixture in a stainless steel beaker works well. To

Tritium

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