Stack monitoring—taking samples from exhaust ducts

Limits for the emission of radioactive aerosols from nuclear facilities into the environment must be met. Monitoring with respect to the given limit is frequently done by measuring the activity concentration inside exhaust ducts.

This requires probes which have been designed to take a representative aerosol sample without producing deposition losses. This can be realized only by iso-kinetic sampling where the velocity of the air stream at the inlet of the probe is the same as the one in the duct.

The velocity of the air at the probe will be defined by the vacuum system of the connected monitor. Therefore, the flow rate of a monitor must be adjustable over a wide range.

The requirements for such monitors are defined by the normatives EN 60761-1 and EM 60761-2 while the ISO 2889 shows the design and implementation of probes and lines.

The aerosol monitors Aer 5200 (models S/GS) and Aer 5400 fulfill these requirements. Both instruments are equipped with a closed air loop and a filter stepping unit. Outstanding filter life times are possible by the large open filter area. Sampling and activity detection are carried out simultaneously. The natural radon background is compensated by the spectroscopic analysis. The flow regulation circuit provides a constant flow rate even if the filter becomes loaded or in case of abrasion of the pump. The set point of the flow rate can be adjusted by the instrument configuration. The air stream is smoothly guided from the side towards the filter. This avoids impaction of aerosols by strong change of the flow direction at the detector edge.

To achieve high mechanical stability, filter tapes are reinforced by fabric creating a rough backside. Thus, some kind of micro channels will arise if the filter is simply guided over a metal filter support. In our monitors we use a mechanism which presses the filter against a soft rubber sealing to avoid any bypassing of the airstream.

The filter tape can be accessed easily (no hermetic cassette) allowing to cut out an activated filter if necessary. No tools are required to replace the filter tape.

The Aer 5200 is a mobile unit with integrated pump. It is the right choice, if the monitor must be relocated frequently.

The Aer 5400 comes in an IP54 stainless steel enclosure and has been designed for wall mounting. The flow regulation is realized by an electronic valve which allows the user to connect the monitor either to a pump or a central vacuum system. A trolley for monitor and pump is also available.