Resuspension models for monolayer and multilayer deposits of graphite dust

Abstract Graphite dust that will be generated in a multi-pass pebble-bed HTR (high temperature reactor), for example the Chinese HTR-PM and the South African PBMR, during normal reactor operation will be deposited inside the primary system and will become radioactive due to sorption of fission products. A significant amount of radioactive dust may be resuspended and released from the reactor cooling system in case of a depressurization accident. Therefore, accurate particle resuspension models are required for HTR/PBMR safety analyses. A review of available resuspension models applicable for monolayer and multilayer deposits is presented in this paper. It is demonstrated that for both multilayer and monolayer deposits, the main problem is the lack of data on adhesion forces and particle-to-particle contact forces. For monolayer deposits, a simple resuspension model, based on a moment balance, referred to here as KS-MB, is proposed and compared with several available resuspension models and available experimental data. It is concluded that a key factor in successful resuspension predictions is a good knowledge of the adhesion force distribution for dust particles deposited on rough surfaces. We demonstrate that relatively simple, quasi-static models, such as the KS-MB model, are as useful as the more complicated dynamic models for resuspension calculations in lack of precise data concerning adhesion forces. For multilayer deposits, resuspension modelling is even more complex. Several models exist, but there is no sufficiently extensive validation. Furthermore, the models may even give contradicting trends of the resuspension rates. The KS-MB resuspension model applicable for multilayer deposits is proposed here and validated against available experimental data. It is concluded that important factors are deposit structure as well as adhesion forces and particle-to-particle contact forces. Furthermore, it is not possible to positively identify the trend of resuspension rates in multilayer deposits. The effect of the multiple layers is overwhelmed by uncertainties in the adhesion force definitions. The main recommendation from the current work is that further measurements of adhesion forces, preferably done for the actual materials and conditions (temperatures, pressures) of the analyzed system (for example HTR-PM) are crucial for development of models and accurate prediction of resuspension.