Size -Selective Devices

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

atmosphere without passing through a tube, orifice, or other obstruction. This precludes loss of the

radionuclide to surfaces upstream from the filter. The holder should face downward to avoid collection of

large, non- inhalable particles, unless a different position is required. Closed-face cassettes are

recommended for small PAS, to protect the filter from direct contamination. Research studies of

commonly-available types of closed-face cassettes with 4-mm inlets indicate that these designs have good

particle -collection characteristics (at a flow rate of 2 L min-1) and reduce sample contamination problems.

Other closed-face filter inlet diameters, geometries, and flow rates may also be acceptable, but have not

been characterized.

The filter should receive adequate support so that it is not stretched or torn by the pressure drop

caused by the flow of sample air. The filter holder should be free of air leakage around the filter as well as

into or through the holder's component parts. Metallic filter holders are generally more reliable and

durable than plastics. Finally, filter-changing and holder replacement should be convenient and positive.

Size -Selective Devices

Size-selective devices fall into two categories: respirable -fraction samplers and instruments for

measuring particle -size distributions. A respirable -fraction sampler collects a range of particle sizes, with

collection efficiency decreasing for larger particle sizes. Particles that penetrate the size-selector

represent those that would deposit in the pulmonary region of the lungs. A particle -size distribution

instrument collects all particles with classification of particle size. Size-distribution data can be used to

calculate the expected deposition of particles throughout the respiratory tract.

Particle -Sizing Devices. Particle -size distribution measuring devices are typically more complex

and require more sample analysis than a size-selective sampler. The major advantage in using these

devices is that the size distribution of airborne contaminants is useful for estimating regional deposition of

inhaled particles in the respiratory tract. This information is more accurate than that provided by a simple

size-selective sampler, especially if a large part of the airborne material has particle size less than about 2

m. Particle -size measurement should be performed only by properly trained individuals, as an

investigative tool for evaluating the health hazard posed by a process or procedure suspected of generating

airborne contamination.

The cascade impactor is the most commonly available particle -sizing device. Aerosol passing through

a cascade impactor is forced through a series of increasingly rapid changes of velocity. The inertia of the

particles causes them to deviate from the direction of the airstream at locations where the particle speed

and direction are changing most rapidly. Particles of different aerodynamic size deflect to different extents

so that larger particles contact the surface of the collection stage. The quantity of material deposited on

each stage is measured and the size distribution calculated for the sampled aerosol.

There are some drawbacks to the use of impactors. Cascade impactors subdivide the sample so that

more sensitive assay methods may be required for successful use. There is a limit to the mass of material

that can be collected on each stage before overloading; inactive dust particles contribute to this mass, but

not to the analyte. Each stage of the impactor is a separate fraction of the sample which must be

analyzed. This multiplies sample number-capacity requirements of the activity measurement system.

Careful calibration of a precisely controlled airflow rate is required for accurate particle -size

measurement.

Optical particle -sizing instruments, such as a laser particle -size spectrometer, have the advantage of

giving practically real time information. Most of these instruments give only an optical particle size,

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