Experimental and Analytical Investigations of Aerosol Processes—Wash-Out and Wash-Down

Timely evolution of radioactive airborne aerosols, the mass of particles deposited on structures or transported with condensing water into the sump, and any influence of safety systems on the aerosol distribution will have a crucial influence on the potential aerosol source term into the environment. Different phenomena affect the main aerosol processes, and important ones besides gravimetrical and diffusive settling require experimental investigations to improve and validate modeling assumptions. Experimental investigations in the THAI (Thermal-hydraulics, Hydrogen, Aerosol, Iodine) test facility have been performed to investigate (a) insoluble silver aerosol wash-down behavior from vertical steel and horizontal decontamination paint-coated surfaces by condensing steam (test AW-3, supported by a laboratory-scale wash-down test series), and (b) the depletion of the airborne aerosol concentration by wash-out due to the use of a nuclear power plant typical water spray system (test AW-4). The paper discusses experimental findings of both tests supported by analytical analyses using the containment code system COCOSYS developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS). To validate and further improve the new model Abwaschmodell für unlösliche Aerosole (AULA) in COCOSYS, used for the wash-down of insoluble aerosols from containment typical structures, the AW-3 laboratory tests related to the AW-3 test were used. Building upon these results, the AW-3 wash-down test is simulated. The results of the calculation for the AW-3 test show that the wash-down of insoluble silver particles at least qualitatively resembles the experimental results, though generally the washed-down aerosol mass is lower compared to the experiment. In test AW-4, it was in question if the modeling of aerosol wash-out with spray systems is adequately treated by assuming monodisperse spray droplets or if a droplet distribution has to be applied. Posttest calculation of AW-4 indicates that the wash-out of CsI aerosols by spray systems can be captured qualitatively. However, it is also shown that the calculated wash-out rate is too large and the depletion of the CsI aerosols during the dry phase is underestimated.