Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 ANALYSIS OF LONG-TERM MEASUREMENTS OF RADON IN A DOLOMITE CAVE Kateřina Rovenská1,2 and Lenka Thinová2 1 National Radiation Protection Institute, Bartoškova 28, 140 00 Praha 4, Czech Republic 2 Czech Technical University, Faculty of Nuclear Sciences and Physical Engineering. Břehová 7, 115 19 Praha 1, Czech Republic Abstract Measurement of the radon concentration has been performed in the Bozkov dolomite cave since 2002. Radon concentration was obtained by two means: continuous measurement by Radim3 monitor in 30-minute interval and 6-month average by LR115 SSNTD in the diffusion chamber placed at 8 points along the cave tour route. The radon concentration shows diurnal, seasonal, and yearly variations. The concentration maximum in the caves, in contrast to the dwellings, is in the summer time. At the same time, high variability of radon concentration occurs. Statistical analysis of the long-time series of radon concentrations was performed; the meteorological data were taken into account. In-situ and laboratory gamma spectrometric measurements are also included in this paper. The annual effective dose from radon for the cave guide (year 2006, working time spent in the cave 414 h) was 3.05 mSv; radon concentration used was obtained by SSNTD as described above. Introduction Public open caves are underground workplaces with a high probability of elevated radon concentration (thousands of Bq/m3). Thus, caves constitute a special case for radiation protection in workplaces. Several papers focusing on the dose assessment for cave guides have been published (Rovenská, 2008). To fulfill the Czech national radiation protection standards and methodology, the radon concentration in the public open cave is measured by SSNTD Kodak LR 115 with the concentration integrated over a6 month time interval. The dose from radon has been evaluated on the basis of SSNTD results. As far as one is interested in the dynamics of radon in the cave, one needs continuous record of radon concentration and other quantities that may influence the concentration. The development of the radon concentration time series is influenced by the temperature difference between the outside and the inside air, changes in pressure, strength of radon source, velocity and direction of air flow and other possible factors (i.e., local releasing of air pockets with high concentration, etc.) Study of the data could help the better understanding of the processes in the cave. Bozkov dolomite caves were chosen for this kind of measurement, which started in 2002. Cave description Bozkov dolomite caves (BDC), originated in Silurian period, are one of 13 of the public open caves in the Czech Republic and the only cave system in the North Bohemia accessible to the public. The BDC are situated on the northern slope of the Bozkov village in the hilly landscape of the Krkonoše foothills. Caves originated in karstic mass in the area of Zelezny Brod crystalline regions. The entrance into the underground space was discovered by dolomite miners in the 1940s. Attractive items of the sightseeing route include underground lakes with crystal-clear blue- 1 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 greenish water. The caves and their surroundings is a protected area; protected both by nature and by landscape protection law. The average temperature in the cave is between 7.5 – 9 °C, the relative humidity is near 100%. The cave tour is 350 m long and takes 45 minutes. Figure 1 shows the map of the cave on which measurement points are depicted with the red points. The BDC has been continually monitored since 2002. Figure 1: Map of the Bozkov dolomite cave, red points are SSNTD measuring points Methods of measurement Radon Radon has been monitored in two ways in the cave. Continuously, by a Radim3 continuous monitor with 30-minute response intervals and as an average for 6 months by SSNTD (also called alpha track detectors). Radim3 consists of a small diffusion chamber and semiconductor detector. Radon concentration is determined by spectrometric measurement of the RaA alpha activity. Statistical error is equal to ± 20%. A power supply is necessary for operating the monitor. Blackouts often stopped the data acquisition and it was necessary to start the device manually. The effect of humidity is compensated up to 90% RH by the device itself. In case of an averaged measurement of radon concentration, the cave has been monitored by SSNTD using Kodak LR115 foil as free detectors and since 2004 enclosed in the plastic diffusion chamber called Ramarn. There are 8 SSNTDs for summer starting on 1st of April and ending on 31st of October and 4 SSNTDs for winter season (lower concentration) placed along the cave tour route. The concentration was integrated per 6 months according to the summer and winter use in the cave. 2 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 Meteorological data Meteorological data were obtained from the Liberec meteo-station, which is owned and operated by the Czech Hydro-meteorological Institute. The Liberec station is 20 km, straightline, from the Bozkov village. Results The acquired data are not complete in time because of the power supply blackouts as mentioned above. The most consistent data were obtained from the place called Lake and Hell. The results of analysis of the data from these two places are described in this section. Annual variations The annual variation is demonstrated on the Fig. 2 which compares the daily radon concentration average from the Lake for the years 2002 – 2006. Summer averages measured by SSNTD for these years are shown in the Fig. 3, from which the variation is also visible. Lake - Daily average of radon concentration 10000 2002 2003 2004 2005 2006 9000 8000 End of the Summer season 7000 Beginning of the Summer season CRn (Bq/m3) 6000 5000 4000 3000 2000 1000 0 29.12 7.2 18.3 27.4 6.6 16.7 25.8 4.10 13.11 23.12 time Figure 2: Interannual variation of radon concentration 3 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 SSNTD season average radon concetration 14000 Summer/2001 Summer/2003 Summer/2005 12000 Summer/2002 Summer/2004 Summer/2006 CRn (Bq/m3) 10000 8000 6000 4000 2000 0 Chapel Midnight Crossing Hell Muggy way Helmet November way Lake position Figure 3: Interannual variation measured with SSNTD, circles - free SSNTD, squares - SSNTD in diffusion chamber (Thinová, 2008) Seasonal variations The highest radon concentration in the cave is found in the hot summer time. Assuming that in the lower inaccessible parts of the cave some of the radon sources (sediments and rocks with high content of 226Ra) are present, the increase in concentration is caused by the stack effect1 determined by outside and inside temperature difference. A one-year progress of the radon concentration and temperature difference is shown in the Fig. 4. Figure 4: Seasonal variation of radon concentration corresponding to the temperature difference 1 Stack effect is caused by the cold cave air escaping into the outer atmosphere. 4 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 Diurnal variations Equally to the seasonal variations the diurnal ones are influenced mainly by the temperature difference. This dependence is depicted in following six graphs (see Fig. 5 – 10). Table 1 shows the parameters of measured intervals. The temperature difference was obtained as a difference between outside and inside temperature. Table 1: Description of measured intervals in the Lake Season AvgCRn (Bq/m3) AvgTempIn (°C) AvgTempOut (°C) Avg Temp difference Summer 4220 10.7 21 10.0 Autumn 2850 10.8 9.3 -1.5 Winter 1660 9.7 -7.3 -17.0 Lake 2003 - Radon concentration, Outside Pressure C tempOut 4400 25 4100 °C C Rn (Bq/m3) 4200 20 4000 3900 C Rn (Bq/m3) 30 4300 15 3800 3700 6.8.03 12:00 7.8.03 0:00 7.8.03 12:00 8.8.03 0:00 8.8.03 12:00 9.8.03 0:00 C 4500 35 10 9.8.03 12:00 985 980 4200 975 4100 970 4000 965 3900 960 3800 955 3700 6.8.03 12:00 950 7.8.03 0:00 7.8.03 12:00 8.8.03 0:00 8.8.03 12:00 9.8.03 0:00 9.8.03 12:00 tim e Figure 5: Summer season, Temperature difference high Figure 6: Summer season, influence of pressure Lake 2003 - Radon concentration, Outside Temperature C 990 4300 tim e 4000 Tout 4400 hPa Lake 2003 - Radon concentration, Outside Temperature 4500 tempOut Lake 2003 - Radon concentration, Outside Pressure 14 4000 3500 12 3500 3000 10 3000 2500 8 2000 6 1500 4 1500 2 1000 19.11.03 0:00 C pressOut 980 2500 970 °C CRn (Bq/m3) °C CRn (Bq/m3) 975 2000 965 1000 19.11.03 12:00 20.11.03 0:00 20.11.03 12:00 21.11.03 0:00 21.11.03 12:00 22.11.03 0:00 19.11.03 12:00 20.11.03 0:00 20.11.03 12:00 tim e tempOut Lake 2003 - Radon concentration, Outside pressure pressOut 990 1800 -2 985 1700 1700 1600 1500 -6 1400 -8 1300 -10 1200 -12 1100 31.1.03 0:00 31.1.03 12:00 1.2.03 0:00 1.2.03 12:00 2.2.03 0:00 2.2.03 12:00 3.2.03 0:00 -14 3.2.03 12:00 time Figure 9: Winter season, Temperature difference high CRn (Bq/m3) -4 °C CRn (Bq/m3) C 1900 0 1800 1000 30.1.03 12:00 960 22.11.03 0:00 Figure 8: Autumn season, influence of pressure Lake 2003 - Radon concentration, Outside Temperature C 21.11.03 12:00 time Figure 7: Autumn season, Temperature difference near zero 1900 21.11.03 0:00 980 1600 975 1500 970 1400 965 1300 1200 960 1100 955 1000 30.1.03 12:00 hPa 19.11.03 0:00 950 31.1.03 0:00 31.1.03 12:00 1.2.03 0:00 1.2.03 12:00 2.2.03 0:00 2.2.03 12:00 3.2.03 0:00 3.2.03 12:00 time Figure 10: Winter season, influence of pressure 5 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 These figures show that the time shift between changes in temperature and radon concentration is in the range of 2 – 12 hours with the average of 7 hours. The correlation is strongly dependent on the properties of period for which the correlation is evaluated. Measurement in high relative humidity The following section will show the comparison of measurement with Radim3 placed in a plastic box with desiccant and in the free air in the cave. The data were acquired at the same time and at the same place (Hell). As can be seen from Fig 11, the plastic box caused smoothing of the brief radon peaks. On the other hand, in the period without the sudden concentration peaks, the results from free Radim3 and enclosed Radim3 in the plastic box are in a good agreement (Fig. 12). Comparison of measurement with free Radim3 and Radim3 in plastic box Comparison of measurement with free Radim3 and Radim3 in plastic box 25000 2300 Free Radim3 1900 CRn (Bq/m3) CRn (Bq/m3) 2100 Radim3 in box 20000 15000 10000 1700 1500 1300 1100 900 5000 Free Radim3 700 0 13.2.04 12:00 14.2.04 0:00 14.2.04 12:00 15.2.04 0:00 15.2.04 12:00 16.2.04 0:00 Radim3 in box 500 19.12.03 20.12.03 20.12.03 21.12.03 21.12.03 22.12.03 22.12.03 23.12.03 23.12.03 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 16.2.04 12:00 time time Figure 11: Smoothing of peaks Figure 12: Good agreement of the measured data The average concentration for the period shown in Fig. 11 is 3420 Bq/m3 for the free Radim3 and 3067 Bq/m3 for the enclosed Radim3. The averages for the day after the peak maximum (second day of the interval shown) are 2287 Bq/m3 for the free Radim3 and 3190 Bq/m3 for the enclosed Radim3. Radon is diffusing through the plastic box and the peak is smoothed and delayed. The average concentration for the period shown in Fig. 12 is 1444 Bq/m3 for the free Radim3 and 1384 Bq/m3 for the enclosed Radim3. Comparison of the results of continuous measurement average and SSNTD results Because of the inconsistent variation of the data, it was not possible to substitute for missing data in a reliable way. Therefore, the comparison between the radon concentrations obtained by SSNTDs and from time series of continuous measurements is not relevant. Gamma spectrometry analysis Gamma spectrometry was performed on rock and sediments samples. Table 2 shows the spectrometry results. These values are little higher than the average content in other public open caves. Table 2: Description of measured intervals (Thinová, 2007) Activity (Bq/kg) 40 228 226 K Th Ra Bedrock Phyllite 01 385.0 ± 13.6 2.2 ± 0.6 3.3 ± 0.4 Phyllite 02 261.1 ± 8.2 3.0 ± 0.3 7.7 ± 0.2 Sediments Hell Crossing Lake 468 378 363 15 20 13 51 29 31 6 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 Radon concentration in water samples Radon concentration in water was measured by Radim4. The measured activities are 7.6 Bq/l in average. This value is one of the highest among the water samples taken in public open caves. On the other hand, the average concentration in drinkable water in the Czech Republic is 15 Bq/l. Summary The processing of long time series of radon concentration showed the variability of concentration in year, during day and among the years. Therefore, it is not possible to substitute the missing data by the data from other period or other years. The strong correlation between concentration and temperature was shown in summer season when the temperature difference is very high. On the other hand the concentration in the cave is stable during cold months. This study is a preliminary one. It is still necessary to compare the data with the air flow measurement, measurement of the seismicity and other measurable quantities. 7 Proceedings of the American Association of Radon Scientists and Technologists 2008 International Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008 References 1. Thinová L: Final report for the project VaV 12/2006: Correction of dose assessment for the underground workers, Praha 2007, in Czech. 2. Rovenská K., Thinová L.: Assessment of the dose from radon and its decay products in the Bozkov dolomite cave, in: Radiation Protection Dosimetry, 2008, doi:10.1093/rpd/ncn114. 3. Thinová L., Burian I.: Effective dose assessment for workers in caves in the Czech Republic – experiments with passive radon detectors, in: Radiation Protection Dosimetry, 2008, doi:10.1093/rpd/ncn118. 8