Answer to Question #9967 Submitted to "Ask the Experts"

I wish to conduct environmental monitoring to determine the airborne concentration of radioiodine (¹²⁵I) in our waste storage facility. What are the monitoring practices currently used for such applications?

Depending on the physical and chemical characteristics of the waste at your site and the specific processes being conducted at your facility (e.g., volume reduction, repackaging, etc.), airborne radioiodine may be in various forms. These might include particulate iodine, gaseous elemental iodine, and volatile organic iodine compounds. The sampling methods used for radioiodines depend, at least in part, on the expected forms of the iodine and the presence of possible interfering radionuclides.

For particulate iodine, a high efficiency particulate filter, such as a glass fiber filter, will effectively remove the particulates from the airstream as air is drawn through the filter. For the gaseous iodine forms, various collection cartridges have become popular. Among the most popular of these are activated charcoal cartridges in which the charcoal has been impregnated with triethylene diamine (TEDA) and zeolite (an alumino silicate material that has a microporouous structure and strong ion exchange properties) that has been impregnated with silver. Both cartridge types will collect elemental and organic forms of radioiodine. The silver-zeolite cartridge has a low retention for noble gases, and this can be an advantage in some environments, such as operating nuclear reactors, in which radioactive noble gases are commonly present in the air. Their presence on air collection media may make it more difficult to assess radioiodines on the same media. When either of these cartridge types is used, the normal procedure is to use a high efficiency particulate filter upstream of the cartridge to remove particulate iodine as well as to remove other airborne radioactive contaminants that might otherwise be collected on the cartridge and interfere with iodine analysis.
 

Iodine air filters and cartridges may be used as grab samplers in which cases the devices have air drawn through them with an appropriate pump for a predetermined time. The collection device is then removed from the sampling system and counted on an appropriate laboratory system. In the case of ¹²⁵I, perhaps the major radiations used for detection and measurement purposes are the low energy x rays and gamma rays, from about 27 keV to 35 keV, emitted during the electron capture decay process. It is also possible to use collection cartridges or filters in continuous monitoring systems in which the collection device is held in proximity to an appropriate radiation detector which feeds a suitable readout and/or recording system. Detectors, such as high purity germanium detectors, capable of good gamma discrimination may be used in conjunction with a multichannel analyzer, or even a single channel analyzer, to identify and quantify the photon emissions from the ¹²⁵I.

There is more that could be said about this topic, but you can find many more details by consulting the internet. If you Google "radioiodine air sampling" and/or related topics you will find a number of links to useful information. I wish you well in your establishment of a suitable monitoring system for your facility.

George Chabot, PhD, CHP