Radon and radon progeny in the Carlsbad Caverns

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Aerosol Science and Technology




Measurements were made in July 1994 to determine air exchange rate, aerosol characteristics, radon concentrations, and radon progeny activity size distributions in the Carlsbad Caverns. The measured radon concentrations were stable at a level of 1821 ± 55 Bq m?3 (mean ±SD). Using a SF6 trace gas method, it was determined that stagnant air in the Caverns was exchanged once every 18 days. The stagnant air was a key factor in maintaining stable environmental conditions and radon concentration. The low air exchange and few aerosol sources inside the Caverns also contributed to the low aerosol concentrations of between 200 and 400 cm ?3—orders of magnitude lower than mining, indoor, and outdoor environments. The alpha spectrum showed radon progeny but no thoron progeny, which may be a result of the long transport time for radioactive gases to diffuse from the deep rock to the cave. The activity size distribution of radon progeny showed typical bimodal distributions with higher unattached fractions [69–99% for 218Po and 25–59% for potential alpha energy concentration (PAEC)] than other natural environments. The high unattached fraction was attributed to the extremely low aerosol concentration. The total PAEC was 4.36 × 10–6 ± 0.51 × 10–6 J m–3 (mean ± SD). Considering the seasonal variation in radon concentration, the estimated cumulative exposure of 1.65 working level months (WLMs) for a worker spending 2000 h in the Carlsbad Caverns with the observed radon concentration seems high, but it is still below the recommended occupational exposure limit for underground uranium miners. However, because of the higher unattached fraction found in the Caverns, calculated doses are much higher than would be expected in a uranium mine under the same WLMs and may exceed the recommended annual dose limits for uranium miners.