Modelling the effect of ventilation on the concentration and distribution of radon in closed environments
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Date
2023
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University of Zululand
Abstract
Epidemiological studies show that radon (222Rn and 220Rn) is the second most important cause of lung cancer after smoking. Prolonged exposure to high levels of this radioactive gas in various indoor environments is linked to multiple lung cancer-related deaths around the world. It was classified as a human carcinogen by The International Agency for Research on Cancer. One of the most effective techniques used to mitigate radon is adequate ventilation, which can be natural (e.g., natural winds) or mechanical (e.g., fans). Recently, Computational Fluid Dynamics (CFD) has become an efficient and powerful tool for studying radon concentration and distribution in ventilated environments. This is mainly because these codes are time and cost-effective. The current work aimed to study the effects of natural ventilation in the Huguenot second tunnel and mechanical ventilation in the Steenkampskraal monazite mine on the concentration and distribution of radon using a CFD package called ANSYS FLUENT based on realistic values for the radon exhalation and ventilation rates. The radon measurements were conducted at three points along the tunnel and two points in the mine using a combination of the RAD7, Airthings, and Electret Ion Chambers (EICs). The exhalation rates in the tunnel were measured with EIC flux monitors, and in the mine, it was measured using the accumulator method. Additionally, the wind speed and direction in both environments were measured with an anemometer and wind vane. These measurements were conducted in the tunnel in July 2021 (winter) and November 2021 (spring). In the mine, measurements were collected in August 2021. During the CFD analyses, two setups (or cases) were considered for each geometry (mine or tunnel). Additionally, for the tunnel in each case, two sets of simulations were conducted, one for July and one for November, each based on measurements of these respective months. For tunnel Case 1, the exhalation distribution in the walls was based on an average, and in Case 2, it was based on the tunnel geology. Whereas for the mine, Case 1 had no ventilation, and Case 2 had ventilation. Various approximations were made to simplify these models (e.g., simplified geometry and constant wind directions). The tunnel simulation for both Cases and months showed that the 222Rn concentration increases in the flow direction and that the levels depend on the exhalation rate, tunnel geometry (e.g., length), and natural wind velocity (or wind speed and magnitude). This behaviour was further supported by the measurements, which agreed with the simulations, with some relative deviation as low as 4%. This suggested that the tunnel model can be used in future works with only minor improvements. On the other hand, the simulation for the mine with no ventilation showed that the 220Rn concentration decreases exponentially from the source wall's normal direction towards the centre, a behaviour noted in the literature. This highlighted the importance of monitoring points for representative values when studying 220Rn. When there was mechanical ventilation, 220Rn was moderately mitigated. This was because the 220Rn distribution field changed from being inhomogeneous when there was no ventilation to being mostly homogenous when there was ventilation, which reduced the 220Rn by about half in some of the near-wall regions. This overall 220Rn behaviour was in line with the measured general behaviour and existing literature. However, there was a disagreement between the simulated and measured concentration at the monitoring points in the current work. This suggested that the mine model still needs further improvement (e.g., improved geometry).
Description
A Thesis submitted to the Faculty of Science, Agriculture and Engineering in fulfilment of the requirements for the Degree of Master of Science in the Department of Physics and Engineering at the University of Zululand, South Africa [2023].