Modeling of Hydrodynamic Characteristics and Iodine Removal Efficiency in Wet Scrubber for Filtered Containment Venting System

Abstract

Iodine is a hazardous substance that can be released from the nuclear reactors during severe accident like in Fukushima Daichi, Chernobyl and Three Mile Island. In order to reduce the risk associated with accidental release of iodine mitigation techniques need to be applied and optimized. In this regard Filtered Containment Venting System is a widely proposed and suitable technique for the removal of iodine from vent gas in case of severe accident. The main part in FCVS system is the venturi scrubber where harmful gaseous substances are removed by contact with scrubbing liquid by the process of mass transfer. To enhance the mass transfer, the scrubbing solution is often composed of chemicals that react with the gaseous substances thereby increasing the removal efficiency of the gaseous pollutants. In this research the performance of a venturi scrubber has been studied for the removal of iodine from vent gas through chemical absorption using scrubbing solution of sodium thiosulfate. Computational fluid dynamics has been used to conduct this study. The parameters studied are hydrodynamics, iodine removal efficiency and enhancement in mass transfer rate due to chemical reaction with thiosulfate. The 2-D geometry and mesh has been generated in ICEM CFD. The CFD simulations and post-processing have been performed in ANSYS FLUENT. The Eulerian-Eulerian approach has been used for modeling the multiphase flows and k-ε turbulence model for modeling the turbulence. The mass transfer of iodine, chemical reaction of iodine and thiosulfate and the enhancement in mass transfer due to chemical reaction have been modeled using a User-Defined Function based on the two-film theory for mass transfer. The results show that the pressure drop in the venturi scrubber increases with decreasing droplet diameter. This increase is significant when droplet diameter is changed from 10 microns to 1 micron. The mass transfer rate is observed to be enhanced significantly with the chemical reaction thus increasing the iodine removal efficiency. The iodine removal efficiency reaches almost 100% for droplet diameters of 1-micron and decreases significantly by increasing the droplet diameter to 10 microns or greater.