Answer to Question #9722 Submitted to "Ask the Experts"

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

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Q

For radiation monitoring of nuclear power plants, the beta activity in air is monitored. The detector is placed in front of a filter for beta and/or alpha particles. The air is sucked in using a pump of certain capacity so that there is particulate collection of radioactivity on paper. What is the criterion or philosophy for selecting the speed of the pump for measurement of activity in Bq m-3? What is the effect of pump speed, say 5 liters per minute and 50 liters per minute?

A

The pump-driven volume flow rate through the filter is a critical parameter in air sampling because the airborne radioactivity collection rate is directly proportional to the volume flow rate. If C is the airborne concentration of the particulate radionuclide of interest (e.g., Bq m-3), F is the pump induced volume flow rate through the collection filter (e.g., m3 min-1), and R is the filter collection efficiency for the particulates of interest (often about 1.0 for high efficiency glass fiber or membrane filters), the radioactivity collection rate, Ar, of the specific radionuclide on the filter paper is given by

Ar = CFR.

Thus, for the flow rates that you quote, the activity collection rate at 50 liters per minute would be 10 times greater than that at 5 liters per minute. It is easy to show that for a radionuclide with a radioactive decay constant, λ, that is sampled from the air for a period T, the activity, A(T) present on the filter at the end of the sampling interval would be     

A(T) = (CFR/λ)(1-e-λT).

It is clear that the activity collected is also proportional to the sampling flow rate. This has potentially important implications when airborne concentrations are low and/or when it is desired to obtain a sample with sufficient activity to yield a readily measurable response within a relatively short time.

Another observation that is sometimes useful is to note that the time rate of change of activity on the filter during collection is given by

dA/dt = CFR – λA.

This represents the slope of the activity collection vs. time curve. If the output of the detector that is monitoring the filter is displayed on a recorder, as is often the case with constant air monitors, when the observation period is short compared to the half-life of the radionuclide being collected, the slope is approximated by dA/dt = CFR; when the flow rate and collection efficiency are known, this slope can be used to estimate the activity of the radionuclide of interest. This may allow for a rapid estimation of the airborne activity, much faster than waiting for the activity to reach a saturation level on the filter. It is clear that, for a given concentration and filter efficiency, the slope will increase with increasing flow rate, thus also making for increased sensitivity if this method is used.

Most constant air particulate monitors used in nuclear facilities have relatively high flow rates, often in the range from about 14 liters min-1 to 140 liters min-1. The high flow rates are advantageous in providing good sensitivity to airborne contaminants.

I hope this answer is sufficient for your needs.

George Chabot, PhD, CHP

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