Answer to Question #10173 Submitted to "Ask the Experts"

I would like to know the criteria for selecting the flow rate of continuous air monitors (CAM). Is there any standard available? What factors should be considered for selecting the flow rate and why? Is 50 liters per minute adequate?

There are various designs of constant air monitors intended for airborne radioactivity assessment. Among the most common have been simple air particulate monitors that use a simple filter to remove radioactive particulates from air that is actively drawn though the filter. The filter may be monitored for radioactivity by viewing it continuously with a thin-window GM (Geiger-Mueller) detector or other appropriate detector. More specialized monitors include moving filter tape systems that are often used in monitoring airborne effluents from facility stacks, systems equipped with solid-state detectors to allow energy discrimination, especially in monitoring alpha emitters, gaseous collection and monitoring systems as might be used for airborne radioiodine assessment, and others. For our purposes here, I will assume a simple particulate monitor of the fixed filter type.

The flow rate at which a constant monitor operates may be determined by several factors. These include simple physical factors, such as the capacity of the pumping system and the resistance of the filter to air flow (i.e., the pressure drop across the filter). These considerations seem straightforward, but they can be important, and some monitors have failed to operate appropriately because of poor design in these regards.

I recall a number of years ago evaluating a CAM from a well-known manufacturer. The specifications indicated that the unit would allow about 85 liters per minute of air flow when used with the filters recommended by the manufacturer; the filters were claimed to have been greater than 90% efficient for particulates of interest. When we evaluated the system we found, indeed, that the unit produced the 85 liters per minute flow rate when the specified filter was used, but when we evaluated the collection efficiency of the filter, we found that it retained only about 10% of the particulate activity. When we replaced the filter with a common high-efficiency glass fiber filter, the pump motor rapidly overheated and failed. We had to replace the motor with a higher-horsepower one in order to make the system useful.

Assuming that the pumping system and filter are capable of performing adequately, the other major considerations that impact one's specification of desired flow rate include possible regulatory requirements and facility administrative requirements. For example, it is a usual regulatory requirement to limit airborne activity concentrations to which workers may be routinely exposed to values not exceeding the derived air concentration (DAC). Additionally, your facility may establish an action level such that you must take remedial action if the airborne concentration reaches some specified fraction of the applicable DAC or when a potential for a specified worker exposure exists. There may be a specification or desire that the CAM achieve a given count rate, possibly associated with activation of an alarm, within a specified time interval when the concentration is at a specified action level, or there may be a requirement that the system alarm at a time when a particular exposure, in number of DAC-hrs, has accrued. The flow rate of the sampling system affects the activity collection rate on the filter, and this clearly affects the amount of time before the CAM will reach a particular count rate.

Assuming a constant air concentration of a single radionuclide of interest, the rate of change of activity on the collection filter is given by

dA/dt = CFR – LA, (1)

where C is the activity concentration, F is the CAM flow rate, R is the filter collection efficiency for the particulate activity of interest, and L is the decay constant for the radionuclide. For a sampling period that extends from t = 0 to t = T, the integral activity on the filter at time T is A(T)

A(T) = (CFR/L) (1-e^-LT). (2)

A simple example of how a specified exposure might be used to establish an alarm point for a CAM might be helpful. If we suppose that a facility has established an action level of 2 DAC-hrs for a radionuclide that has a DAC of 1.85 × 10^-4 Bq cm^-3, then 2 DAC-hrs represents (for reference man with an inhalation rate of 20 liters per minute) an intake by man of 444 Bq. If the half-life of the radionuclide is 10 days, the decay constant would be 6.93 × 10^-2 d^-1 or 4.81 × 10^-5 min^-1. The time required for a reference individual to inhale 2 DAC-hrs of the radionuclide is given by

T = (2 hrs)(DAC/C), (3)

which is the value of T we will use in equation 2.

Below are calculated values of T and A(T) and expected count rate associated with a potential inhalation of 2 DAC-hrs of the above radionuclide for air concentrations ranging from 1.85 × 10^-6 Bq cm^-3 to 1.85 × 10^-2 Bq cm^-3. For the calculations, the decay constant is 4.81 × 10^-5 min^-1, F is 5 × 10⁴ cm³ min^-1 (50 Lpm), and R is taken as 1.0. The activity A(T) is calculated from equation 2. The expected CAM count rate associated with the filter activity is based on an assumed detection efficiency of 0.10 counts per disintegration of the radionuclide of interest. The additional significant figures for column 3 and 4 values are simply to show the trend in filter activity and count rate with concentration.

As you might expect, the amount of activity on the filter and the associated count rate that would relate to 2 DAC-hours of potential exposure increase somewhat as concentration increases because of the reduced effect of radioactive decay on the collected filter activity for the shorter sampling times. For longer half-life radionuclides, one might expect more equal activities and count rates.

The question as to whether a particular flow rate is adequate will depend on what your response criteria are for the CAM. If you require that the time to alarm at a given concentration be no greater than a particular value, then you can use approaches such as the above to estimate the required flow rate. In some instances, one may not be able to meet specified criteria—e.g., the required flow rate may be so high as to be unachievable. In such instances, changes in equipment or performance criteria may be necessary.

Typical constant air monitors used to assess ambient airborne radioactivity levels may vary considerably in design, depending on their intended purposes. To my knowledge, there are no regulations or standards that specify required flow rates for CAMs. Because it is often the intent to obtain a fast response to airborne activity increases in the workplace, relatively high flow rates are common for these constant air monitors. Flow rates from about 28 Lpm to more than 280 Lpm are representative. The higher flow rates are often associated with sampling devices that have large active areas and/or lower pressure drops. The 50 liters per minute that you inquire about is certainly suitable for many applications, but you must consider the specific aims of the monitoring and the criteria for detection in order to make a legitimate appraisal of your system. Complications that might have to be considered in your determinations include contributions to detector response from radon progeny and from other possible radionuclides in the air.

There are a number of references that deal with the topic of air sampling. Two commonly cited sources that are available on the internet are US Nuclear Regulatory Regulatory Guide 8.25 and US Nuclear Regulatory Commission NUREG-1400.

I hope the above is helpful to you.

George Chabot, PhD, CHP