Study on the representativeness of airborne effluent sampling in the stack of a high-temperature gas-cooled pebble-bed modular reactor

•The transport behaviors of airborne radioactive substances in the stack of HTR-PM have been investigated.•The well-mixed effluent location which can obtain a representative sample in the HTR-PM stack has been determined.•Methods for the improvement of flow mixing in the stack have been discussed and presented. Monitoring the airborne radioactive effluent from the stacks of nuclear power plants is very important. In order to obtain reliable and accurate measurements, the samples acquired from the stack must be representative of the whole. Thus, sampling should be performed at a location where the airflow is well mixed. We studied the transport behavior of airborne radioactive substances in the stack of a high-temperature gas-cooled reactor pebble-bed module (HTR-PM) demonstration power plant using a computational fluid dynamics method. The distributions of effluent flow velocity and concentration have been determined and compared to the mixing uniformity criteria of the ISO-2889 standard, then the source of uniformity is analyzed from vorticity distribution, particle trajectory, cyclone angle and coefficient of variation. We have determined the well-mixed location within the HTR-PM stack where a representative sample can be obtained. We discussed methods for the improvement of flow mixing and analyze the key factors involved.