A COMPREHENSIVE CHARACTERISTIC OF 222Rn ACTIVITY CONCENTRATION CHANGES AND IONISING RADIATION EXPOSURE IN NEWLY DISCOVERED PARTS OF BEAR CAVE IN KLETNO, POLAND

Abstract

The article presents the results of research into 222Rn activity concentration changes and ionising radiation exposure in newly discovered parts of Bear Cave in Kletno, unknown until 2012. 222Rn activity concentration was registered at three measurement points located inside over 500-m-long newly discovered cave passages. The air in these parts of the cave is characterised by a stable temperature of about 5°C and a relative humidity of 100%. The measurements started in September 2013 and were completed by mid-July 2016. The measured data were recorded continuously, including hourly registration of 222Rn activity concentration with the use of three Polish semiconductor detectors SRDN-3a. Three data sets were obtained. Two of them, containing the same numbers of data, were recorded between 4 September 2013 and 10 July 2016 by SRDN-3a detectors no 7 and 9 located at sites no 1 and no 3, respectively. The third set comprises data registered between 27 October 2014 and 10 July 2016 by SRDN-3a probe no 8 at site no 2. Distinct daily changes in 222Rn activity concentration in the new parts of Bear Cave occur in early spring (from March to May) and in autumn (October). This observation confirms the notion that radon concentration changes depend on natural convective air exchange between the cave and the atmosphere. It has been shown that daily radon concentration changes in other seasons are irregular and rather small. The highest values of 222Rn activity concentration inside the cave are more frequent in daytime, i.e. from the morning to midday. The recorded annual mean values of 222Rn activity concentration in successive measurement years are comparable for all the three sites and range from 2690 to 3179 Bqm-3 in 2013, from 2676 to 2973 Bqm-3 in 2014, from 2587 to 2917 Bqm-3 in 2015 and from 2581 to 3719 Bqm-3 in 2016. These values exceed those recommended as maximum allowable radon concentrations in such spaces (0.5-1.5 kBqm-3) and are many times as high as the values recommended for workplaces (0.3 kBqm-3). The recorded values of 222Rn activity concentration in the air of the new parts of Bear Cave in Kletno result in a risk of exposure to a radiation dose higher than the national standard of 1 mSv per year adopted for the general public. In the most unfavourable conditions, it appears as early as after 25 hours spent inside the cave. The minimum effective dose to which a person staying inside the cave is exposed within 1 hour is 0.002 mSv, and the maximum dose is 0.04 mSv. The results of the conducted measurements demonstrate the necessity of introducing appropriate solutions, crucial in terms of radiological protection and legislation, that would minimise the risk of exposure to ionising radiation in the new parts of Bear Cave.