Answer to Question #9059 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

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

Q

What are the optimal conditions for using bubbling potential tritiated air through a bubbler system? At a nuclear power plant, we currently use a 100 ml single bottle bubbler with a pump flow rate of 500 cc/min. We do not limit the total volume passed through the water, but it is usually on the order of 15,000 cc. I'm concerned about saturation point of the water and the collection efficiency of this sampling method.

Specifically: (1) What is the optimal flow rate? (2) What is the optimal total volume through the bubbler water? (3) Should more than one bottle be used to ensure the collection efficiency is as assumed? (4) What readily available reference material is available on this subject?

A

The optimum conditions for collecting tritiated water vapor from air using a bubbler system may vary, depending on environmental conditions and specific facility requirements. Thus, if you are concerned with average concentrations over extended periods, it may be desirable to reduce the sample flow rate to minimize saturation effects and evaporation effects. If you require an estimate of air concentrations within a short time, a higher flow rate and shorter sampling time may be necessary to obtain sufficient tritium for measurement. Additionally, the choice of liquid media may vary with conditions. If the air environment is on the humid side, water may be a better choice as the medium. If the air is noticeably dry, a solvent such as ethylene glycol may hold some advantages, especially if longer sampling intervals are anticipated. This is because the ethylene glycol has a lower vapor pressure than water and will not evaporate as quickly. If the air is very humid, however, the glycol may saturate with water more quickly than desired, leading to a reduction in tritium collection efficiency. Another variable is volume of the collecting fluid. Volumes vary from about 25 cc to about 200 cc in the most common systems with which I am familiar. Larger liquid volumes support higher air flow rates and/or longer sampling times.

Thus, some bubbler systems may use 30 cc of ethylene glycol and operate at an air flow rate between 50 and 100 cc/min and maybe intended to run continuously for a week or two. Others may use 150 to 200 ml of water as collector and be designed for air flow rates close to 5 liters/min with operating times of 8 to 12 hours.

Your specification of 100 ml of water and a flow rate of 500 cc/min is reasonable, depending somewhat on environmental conditions and operating time. You should have no problem with an air volume sample of 15,000 cc.

If you have never done any sampling with two bubblers in series, I would definitely recommend that you do so. While I expect that you will find good results, with collection efficiencies between 90% and 100%, the series collection will provide a solid answer and allow you to produce documentation that may be useful in the highly regulated environment of nuclear power reactors.

In my opinion some of the best papers written on the topic of tritium collection in bubbler systems date back a number of years. One publication by R.V. Osborne I think is especially helpful. It is Osborne RV, Sampling for tritiated water vapor, Proceedings of the Third International Congress of the International Radiation Protection Association, published as Report CONF-730907-P1 (National Technical Information Service, US Department of Commerce, Springfield, Virginia), 1973.

You can find other references in more common sources—e.g., search the internet or if you are a member of the Health Physics Society, go to the Members Only section and review articles available online in the Health Physics Journal. Use "bubbler" as a search term and you will find some related papers. Good luck.

George Chabot, PhD, CHP

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