Airborne particulate radioactivity monitoring

Continuous particulate air monitors (CPAMs) have been used for years in nuclear facilities to assess airborne particulate radioactivity (APR). In more recent times they may also be used to monitor people in their homes for the presence of manmade radioactivity. These monitors can be used to trigger alarms, indicating to personnel that they should evacuate an area. This article will focus on CPAM use in nuclear power plants, as opposed to other nuclear fuel-cycle facilities, or laboratories, or public-safety applications.

In nuclear power plants, CPAMs are used for measuring releases of APR from the facility, monitoring levels of APR for protection of plant personnel, monitoring the air in the reactor containment structure to detect leakage from the reactor systems, and to control ventilation fans, when the APR level has exceeded a defined threshold in the ventilation system.

CPAMs use a pump to draw air through a filter medium to collect airborne particulate matter that carries very small particles of radioactive material; the air itself is not radioactive. The particulate radioactive material might be natural, e.g., radon decay products ("progeny", e.g., ²¹²Pb), or manmade, usually fission or activation products (e.g., ¹³⁷Cs), or a combination of both. There are also "gas monitors" which pass the air through a sample chamber volume which is viewed continuously by a radiation detector. Radionuclides that occur in the gaseous form (e.g., ⁸⁵Kr) are not collected on the CPAM filter to any appreciable extent, so that a separate monitoring system is needed to assess these nuclide concentrations in the sampled air. These gas monitors are often placed downstream of a CPAM so that any particulate matter in the sampled air is collected by the CPAM and thus will not contaminate the gas monitor's sample chamber.

In monitoring, the region of deposition of this material onto the filter medium is continuously viewed by a radiation detector, concurrent with the collection. This is as opposed to a sampling system, in which the airborne material is collected by pumping air, usually at a much higher volumetric flowrate than a CPAM, through a collection medium for some period of time, but there is no continuous radiation detection; the filter medium is removed periodically from the sampler and taken to a separate radiation detection system for analysis.

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