Abstract
Abstract. Postojna Cave is the largest of 21 show caves in Slovenia. The radon concentration there was measured continuously in the Great Mountain hall from July 2005 to October 2009 and ranged from about 200 Bq m−3 in winter to about 3 kBq m−3 in summer. The observed seasonal pattern of radon concentration is governed by air movement due to the difference in external and internal air densities, controlled mainly by air temperature. The cave behaves as a large chimney and in the cold period, the warmer cave air is released vertically through cracks and fissures to the colder outside atmosphere, enabling the inflow of fresh air with low radon levels. In summer the ventilation is minimal or reversed and the air flows from the higher to the lower openings of the cave. Our calculations have shown that the effect of the difference between outside and cave air temperatures on radon concentration is delayed for four days, presumably because of the distance of the measurement point from the lower entrance (ca. 2 km). A model developed for predicting radon concentration on the basis of outside air temperature has been checked and found to be successful.
Highlights
Radon (222Rn), a radioactive gas with a half-life of 3.82 days, is released from minerals into the pore space of rocks and subsequently into the atmosphere (Nazaroff, 1992)
Radon concentration in the Great Mountain hall was found to be in the range from about 200 Bq m−3 to about 3 kBq m−3
The seasonal pattern is mainly the result of air movements caused by differences between external and internal air densities, controlled to a large extent by air temperature; this corresponds well with the processes characteristics of horizontal caves (Hakl et al, 1996)
Summary
Radon (222Rn), a radioactive gas with a half-life of 3.82 days, is released from minerals into the pore space of rocks and subsequently into the atmosphere (Nazaroff, 1992). Seasonal changes in natural ventilation often cause large temporal variation in radon levels, most commonly characterised by high summer and low winter concentrations (Kowalczk and Froelich, 2010; Perrier and Richon, 2010; Przylibski, 1999; Tanahara et al, 1997; Wilkening and Watkins, 1976). Our intention was to establish a model of radon concentration in the cave air on the basis of temperature differences between the cave air and the outside air. Such a model could reduce the number of measurements in the cave needed for dose estimates for the personnel working there
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