Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 2):...

Exposure due to thoron (²²⁰Rn) gas and its decay products in a thorium fuel cycle facility handling thorium or ²³²U/²³³U mixture compounds is an important issue of radiological concern requiring control and mitigation. Adsorption in a flow-through charcoal bed offers an excellent method of alleviating the release of ²²⁰Rn into occupational and public domain. In this paper, we present the design, development, and characterization of a Thoron Mitigation System (TMS) for industrial application. Systematic experiments were conducted in the TMS for examining the ²²⁰Rn mitigation characteristics with respect to a host of parameters such as flow rate, pressure drop, charcoal grain size, charcoal mass and bed depth, water content, and heat of the carrier gas. An analysis of the experimental data shows that ²²⁰Rn attenuation in a flow through charcoal bed is not exponential with respect to the residence time, L/U_a (L: bed depth; U_a: superficial velocity), but follows a power law behaviour, which can be attributed to the occurrence of large voids due to wall channeling in a flow through bed. The study demonstrates the regeneration of charcoal adsorption capacity degraded due to moisture adsorption, by hot air blowing technique. It is found that the mitigation factor (MF), which is the ratio of the inlet ²²⁰Rn concentration (C_in) to the outlet ²²⁰Rn concentration (C_out), of more than 10⁴ for the TMS is easily achievable during continuous operation (>1000 h) at a flow rate of 40 L min^-1 with negligible (<1 cm of water column) pressure drop. The Thoron Mitigation System based on adsorption on charcoal bed offers a compact and effective device to remove ²²⁰Rn from affluent air streams in a space constrained domain. The prototype system has been installed in a thorium fuel cycle facility where it is being evaluated for its long-term performance and overall effectiveness in mitigating ²²⁰Rn levels in the workplace.