Radon daughter and thoron daughter deposition velocity and unattached fraction under laboratory-controlled conditions and in underground uranium mines

The deposition velocity and unattached fraction of the short-lived decay products of thoron and radon have been measured in a large (26 m 3) radon/thoron calibration facility and in underground uranium mines. Measurements were conducted under various environmental conditions of relative humidity, temperature, aerosol concentration, and air velocity (0–1.2 m s −1). Unattached fractions were determined using wire screens. Plate-out experiments, carried out to determine deposition velocities, were done using circular stainless steel plates attached to walls. The average RaA unattached fraction in the aerosol concentration range 3.5 × 10 2–1.5 × 10 3 cm −3 was about 0.5. Substantially lower values were obtained for RaB and RaC. Similarly, the average ThB unattached fraction in the same aerosol concentration range was approx. 0.1. Significantly lower values were found for ThC. The unattached fractions in underground uranium mines were considerably lower than the above values, i.e. < 0.01, an experimental result which is consistent with the much higher aerosol concentration in underground working areas. (Experimentally determined radon and thoron daughter deposition velocities were in the range 0.06–0.6 cm/s.) These values are substantially lower than theoretical estimates by Jacobi (0.5–1.5 cm/s) and values used by other authors in the past. Our radon daughter data are, however, in agreement with recent laboratory work by several authors. Deposition velocity and unattached fraction data are relevant for the calculation of daughter product plate-out on mine walls, and hence, for realistic determination of airborne radiation levels in underground uranium mines. This study is of particular interest for some Canadian mines, and other mines, where the presence of thoron and its daughter products is appreciable.