Experimental and theoretical considerations for designing an optimal activated charcoal bed for ²²⁰Rn mitigation

In our earlier publications we experimentally demonstrated the ²²²Rn hold up and ²²⁰Rn mitigation characteristics of coconut-shell based activated charcoal bed. It was also reported that the excellent adsorption property of activated charcoal can be gainfully employed for the mitigation of ²²⁰Rn in industrial workplaces through a suitably designed bed system. In this paper, we report the development of a theoretical model for aiding in the design of an industrial scale charcoal bed for adsorbing ²²⁰Rn from affluent gas streams of thorium processing facilities. The model also takes into consideration the flow rate and pressure drop across the system vis-a-vis mitigation factor for the optimal design of the ²²⁰Rn mitigation system. The model consists of two parts. First is the transport model that treats adsorption as being driven by inter-and intra-grain diffusion along with convective transport and radioactive decay of the gas in a porous adsorbing matrix, to obtain general relationships between the mitigation factor and the system parameters. Second is the optimization model that predicts the optimal dimensions of the system for achieving a given mitigation factor for thoron under the constraints of specified flow rate and pressure drops. Based on these calculations, the adsorber bed was designed and dimensions were optimized to obtain the pre-specified mitigation factor (∼10⁴) for practically relevant pressure drop and flow rate situations. Laboratory experiments were also carried out to validate the theoretical predictions and the details are presented in this paper. (author)