Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 2): Development, characterization, and performance evaluation K. Sudeep Kumara a , B.K. Sahoo b , J.J. Gaware b , B.K. Sapra b , Y.S. Mayya a, c , N. Karunakara a, * a Centre for Advanced Research in Environmental Radioactivity, Mangalore University, Mangalagangothri, 574199, Mangalore, India b Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India c Department of Chemical Engineering, Indian Institute of Technology-Bombay, Mumbai, 400 076, India article info Article history: Received 21 October 2016 Received in revised form 2 March 2017 Accepted 12 March 2017 Keywords: 220 Rn Activated charcoal Thoron Mitigation System (TMS) Mitigation factor (MF) abstract Exposure due to thoron ( 220 Rn) gas and its decay products in a thorium fuel cycle facility handling thorium or 232 U/ 233 U 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 220 Rn 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 220 Rn 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 220 Rn attenuation in a flow through charcoal bed is not expo- nential 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 220 Rn concentration (C in ) to the outlet 220 Rn concentration (C out ), of more than 10 4 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 220 Rn 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 per- formance and overall effectiveness in mitigating 220 Rn levels in the workplace. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Exposure of population to radon isotopes ( 222 Rn and 220 Rn) occurring at elevated levels in indoor environment is of considerable concern to public health professionals. It is also a major source of health hazard to workers at the front and back end of a nuclear fuel cycle (WHO, 2009; IAEA, 2005). A large number of past studies (Stranden et al., 1979; Miles, 1998; Srivastava, 2005; Darby et al., 2005; Krewski et al., 2005; Zhang et al., 2007) in the natural envi- ronment have generally focused on 222 Rn and neglected 220 Rn. This was under the assumption that due to its short half-life (55.6 s) only a small fraction of 220 Rn is released into living spaces, thereby contributing negligibly to inhalation doses. There is however an emerging interest on 220 Rn - partly due to detection of non- negligible 220 Rn in dwellings in various parts of the world (Imoto et al., 2001; Mayya et al., 2012; Rosaline et al., 2015), and also due to pursuit of thorium-based fuels for nuclear power generation. The thorium fuel cycle has attracted special attention in recent times due to several potential advantages over a uranium fuel cycle, such as, greater abundance of thorium on Earth, superior physical and nu- clear fuel properties, and reduced nuclear waste production (IAEA, 2005). Exposure to 220 Rn is a matter of concern in facilities, which handle either thorium directly or deal with 233 U/ 232 U bearing products (e.g., irradiated thoria in a power reactor). It may be noted * Corresponding author. E-mail addresses: drkarunakara@gmail.com, karunakara_n@yahoo.com (N. Karunakara). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2017.03.016 0265-931X/© 2017 Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity 172 (2017) 249e260