**3.5 Finding 5: in a fully developed cave, undercut level ventilation increases the radon concentration in the production drift**

**Figure 10** shows the velocity contours through a section of the model (the production drift and draw points). The velocity magnitude through the first and second drift is similar; however, it is higher in the third drift due to the influence of the undercut ventilation. Based on the proximity of the undercut drift to the third drift, most of the airflow from the undercut duct flows out through the draw bells in the third drift, creating a significant difference in the magnitude of velocity.

The static pressure decreases down the production drift due to wall friction and shock losses through the draw points. The drawpoint—drift configuration is similar to a "T-junction" with orientation, which introduces flow separation [15]. Therefore, the number of drawbells (shock loss sources) in each drift affects the total pressure drop in the drift. In addition, for each drift, the distance between the inlet and the first draw bell affects the developing flow due to momentum loss from the drawbells. Therefore, the static pressure is maximum in the third drift based on the distance.

In addition, we analyzed the static pressures in the localized regions (**Figure 11a**) and the shock losses for both sides of the second drift (**Figure 11b**). We observe a notable variation in pressure around region "B" compared to region

**Figure 10.** *The magnitude of velocity through the production drift.*

**Figure 11.**

*(a) Static pressure contours through the production drift; (b) localized pressure distribution from a section of the second drift.*

"A"; hence the shock loss is greater for region "B" due to the orientation of the drawbells. Since air flows from right to left, the flow makes a 56° turn to get into drawbells around region "B," and a 24° turn into region "A." Therefore, as the bend angle increases, shock loss increases [16], hence the airflow pressure is more efficient with drawbell that requires a less angle of airflow rotation (Region A of the second drift, and the first drift).

**Figure 12** shows the working level through the three production drifts, and based on the number of radon sources, the second drift (with 13 sources) has the largest region with high concentrations compared with the first drift (with seven sources), and the third drift (with six radon sources).

The concentrations are lowest in the third drift due to the combined effects of the pressure distribution, the number of radon sources, and the undercut ventilation. However, there are small regions with a high concentration around the last drawbell due to the impact of undercut ventilation. Most of the airflow from the undercut duct flows out through the third drift with a high radon concentration from the cave. **Figure 13** shows the working level through a section of the cave along with the undercut drift. The positive pressurization due to the undercut ventilation effectively prevents the influx of radon into the undercut drift. Therefore, personnel working in the undercut drifts are not exposed to high concentrations with this ventilation design.

**Figure 14a** shows a plot of radon concentration along the center of the first, second, and third production drifts to understand the changes in radon

### **Figure 12.**

*Radon concentration through the production drift presented in working levels to represent the concentration of harmful radon daughters.*

### **Figure 13.**

*Radon concentration through a section of cave presented in working levels.*

### **Figure 14.**

*(a) Growth of radon along the center of the production drift for the first, second, and third, after 9 hours. (b) Radon concentration at the end of the production drifts w.r.t time.*

concentration through the production drift. The concentrations increase nonlinearly as the number of radon sources increases down the drifts. However, there is a drop in concentration for the first drift due to the distance from the last source of radon, which is the farthest. Hence, based on the drift layout/configuration, specific locations other than the outlet might have higher radon concentrations. As observed in **Figure 14a**, the non-linear increase is due to the differences in radon sources (drawbells) compared to the wall sources, which increases linearly from previous studies [17]. For further investigation, **Figure 14b** compares the concentration at the outlet of the three production drifts at different times. The concentrations increase with time, but the rate decreases after about 4 hours as the flow develops. The results presented in **Figure 14a** and **b** are from the center of the production drifts; however, radon concentration varies across the drift cross-section for a fixed location.

## **3.6 Finding 6: with undercut ventilation, negative pressure on the cave top effectively reduces radon concentration through the production drifts**

In some instances, a regulator is used along with the exhaust fan to maintain negative pressure on top of the cave to remove pollutants from the cave due to the pressure difference. We conducted an independent study to understand the impact of negative pressure (300 Pa) [18] on top of the cave for the result presented in **Figure 12**. **Figure 15** shows a significant reduction in radon concentration through

**Figure 15.** *Effect of negative pressure (boundary condition for cave top) on radon concentrations in the production drifts.*

the production drift. Therefore, imposing a negative pressure on the cave top, positive pressurization of the production, and undercut drifts effectively reduce radon concentration through the production drifts.
