A Simple Small Scale Method to Determine the Re-suspension Rate for Non-fixed Surface Contamination

In the frame of the Co-ordinated Research Program of IAEA “Radiological aspects of package and conveyance non-fixed contamination” it has been identified that there is a lack of knowledge concerning the airborne release from surfaces with non-fixed contamination of radioactive materials during normal transport conditions. This has a direct effect on the models and parameters underlying the non-fixed contamination limits for packages and conveyances. We have developed a simple and practicable methodology to investigate the suspension of non-fixed particle bound contamination from surfaces due to effects such as wind and vibration affecting packages during normal transport conditions. This method is based on the formation of a defined contamination layer on small test surfaces by bringing them into either liquid or airborne suspensions of surrogate particles. A time and particle size resolved detection technique covering the particle size range between 1 and 100 µm is employed to measure the particle resuspension rate from the test surface exposed to controlled hydrodynamic and/or vibrational forces inside a narrow flow channel (4 mm channel height). All particles suspended from the test surface and incorporated into the airflow are size classified and counted by a clean room particle counter. The re-suspension process is recorded in detail in order to distinguish between short term and long term effects as well as to perform statistical analysis of the re-suspension pattern enabled by the possibility of detecting single resuspension events. For particles in the respirable size range the detection limit for the re-suspension rate is 10-6 [1/h]. The detection limit is determined by the maximum tolerable particle coverage of the investigated surface and decreases for larger particle diameters. The method is very versatile and allows for easy variation of particle and surface properties. Measurements made with various test samples reveal consistent results which show an initially increased resuspension rate as a short term effect followed by a much reduced long term rate. The long term re-suspension rate increases with flow velocity and decreases with particle size. For a high flow velocity of 30 m/s corresponding to a wall shear stress of 1 [N/m²] the long term re-suspension rate for 4 µm alumina particles deposited from the airborne state onto a smooth glass surface is 1 10-4 [1/h] with an estimated reproducibility of a factor of 2. The same particles deposited from an aqueous suspension show a re-suspension rate of at least a factor of 10 lower magnitude due to the remaining liquid forces between particles and surface even after overnight drying of the test samples. This causes also a considerably larger variation of results obtained in repeated experiments.