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Radon in Finnish mines – regular monitoring since 1972
657
Radon in Finnish mines – regular monitoring since 1972 M. Annanmäki, E. Oksanen, E. Venelampi, M. Markkanen Radiation and Nuclear Safety Authority – STUK P.O. Box 14, FIN-00881, Helsinki, Finland
Radon measurements in Finnish underground mines were started in 1972. At that time there were 23 operating underground mines in Finland. Some of the mines were old, having a lot of underground abandoned spaces not in active use. Today there are 8 underground mines in operation, most of them being small in size. The first radon measurements were made in order to find out radon levels in the mines and to estimate the extent of a possible radon problem. Radon measurements were made in work places, as well as, in areas not under work but in which high radon concentrations were expected to occur. Since then regular measurements in all underground mines have been made in order to control radon concentration in work places and to ascertain that the limits set for radon are not exceeded, and in some cases to evaluate the doses to miners. In most cases radon daughter concentration has also been measured together with radon. During the last few years only radon concentrations have been measured. In 1975, a limit for radon was set at 1100 Bq m−3 (30 pCi L−1 ), the value being the radon in equilibrium with its short-lived daughter products. In 1992, an action level of 400 Bq m−3 for radon, an average over the annual working cycle, was adopted. In 1975 the mean effective dose for a miner was 3.5 mSv, in 1985 2.4 mSv, in 1995 1.7 mSv and in 2001 0.9 mSv. The decrease in radon concentration has been partly due to countermeasures made to decrease high radon concentrations but mostly because many of the large, poorly ventilated old mines have now been closed. The modern mines are small in size and it is technically easier to arrange appropriate ventilation in them. The equilibrium factor has varied from 0.2 to 0.9 depending on the mine and the year. The average equilibrium factor when all the mines are taken into account is 0.6. The alpha energy concentration due to thoron 220 Rn (212 Pb) was measured in 8 different mines from a total of 103 filter samples. The average alpha energy concentration due to 220 Rn was 0.1 µJ m−3 , and the highest measured value was 0.5 µJ m−3 . 1. Introduction The significance of radon (222 Rn) as a source of radiation exposure was realised in the 1950s when it was discovered that exposures due to radon (specifically to its decay products 218 Po, RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07080-9
© 2005 Elsevier Ltd. All rights reserved.
658
M. Annanmäki et al.
Fig. 1. Number of workers in Finnish underground mines. 214 Pb, 214 Bi
and 214 Po) could explain the excess in lung cancers detected among miners. The first results indicating a significant excess of lung cancer among uranium miners were obtained in the 1960s in the United States. High concentrations of radon exist also in nonuranium mines, in various underground excavation works and other underground workplaces. Today, the epidemiological studies on uranium and non-uranium miners form the main source of information for estimating the risk (the dose) caused by exposures to radon [1]. Radon measurements in the Finnish underground mines were started in 1972. The first radon measurements were made in order to find out radon levels in the mines and to estimate the extent of a possible radon problem. Radon measurements were made in work places, as well as in areas not in active use but in which high radon concentrations were expected to occur. Since 1972 radon measurements have been made regularly in all underground mines every year or every second year, depending on the radon concentration measured in earlier years. The number of underground mineworkers in Finland is presented in Fig. 1. In the early 1970s there were 23 operating mines and the number of underground workers was about 1300. Today there are only 8 mines, most of them being small in size and employing about 400 workers in total. The minerals mined are mostly copper, zinc, gold and limestone. Earlier iron was also mined. During the years different equipment has been used to measure concentrations of radon and its short-lived daughter products. However, the measurement methods themselves remained the same. Radon was measured with grab sampling using Lucas-type chambers [2]. The shortlived daughter products were measured with the Kusnetz method [3]. In 1983 and 1984 the alpha energy concentration due to 220 Rn (212 Pb) was also measured in most of the mines from the same filter samples which were used to measure the alpha energy concentration due to 222 Rn.
2. Material and methods 2.1. Regulation of radon in mines The initiative to start radon measurements in 1972 came from the Ministry of Trade and Industry, which is in charge of the safety of mines. First regulations for controlling radon in
Radon in Finnish mines – regular monitoring since 1972
659
underground mines were issued in 1975. At that time, a limit for radon was set at 1100 Bq m−3 (30 pCi L−1 ), defined as the Equivalent Equilibrium Radon concentration (EEC), i.e., radon in equilibrium with its short-lived daughter products. A major revision of the Finnish Radiation Act took effect in 1992 [4]. Since then the Act has covered all work activities, not only mines, involving significant occupational and public exposures to natural radiation in accordance with the principles set out in the 1990 ICRP Recommendations. The Act and other provisions issued by virtue of it cover the activities falling under the scope of Title VII of the new BSS Directive [5,6]. In 1992, an action level of 400 Bq m−3 for radon was adopted. The action level is defined as an annual average concentration during working hours. Steps must be taken to reduce the radon concentration if the action level is exceeded. If countermeasures do not work, individual doses of miners must be assessed. In this case, the maximum permissible radon concentration is 3000 Bq m−3 leading to an effective dose of 20 mSv, the dose limit for workers. The individual doses are assessed by measuring the radon concentrations in different working areas and by recording the working hours of the miners in those areas. 2.2. Radon and radon daughter monitors used During the last 30 years several types of equipment have been used to measure radon. However, the measurement method itself, measuring the activity of an air sample (grab sampling) using Lucas-type chambers, has remained the same [2]. Silver activated zinc sulfide is used to detect the alpha-activity of radon (222 Rn) and radon daughters (218 Po, 214 Po). In the beginning, there was only one measurement head and the air samples were taken into gas bottles and the air sample was then transferred into the measurement head for its measurement. Later on about 50 detector chambers were used into which air samples were taken directly. The construction of these detectors was our own. There were also several counting systems (a photo multiplier and an electronic counter) into which the chambers could be connected for the measurement. Since 1998, a commercially available monitor with several detector chambers has been used (Pylon AB 5). The detectors were regularly calibrated in a radon chamber maintained at STUK. The reference devices used for calibration purposes were regularly checked against a standard radon concentration produced into a steel drum by emanating radon from a known amount of radium (226 Ra). In 1990, a major study was carried out to check the validity of the calibration system and to recalibrate the radon monitors. As a consequence of the study, the calibration factors of the monitors were changed upwards by about 5% [7]. STUK has participated in several intercomparisons for radon measurements. The first was organised with the Swedish National Institute of Radiation Protection (SSI) in 1978. The second, organised in 1982, was made together with the Swedish National Institute of Radiation Protection, the National Institute of Radiation Hygiene (SIS, Denmark) and the National Institute of Radiation Hygiene (NRPA, Norway). In 1987 the third CEC intercomparison of active and passive detectors for the measurement of radon and radon decay products was organised by the National Radiological Protection Board (NRPB, UK) in which STUK also participated [8]. In the 1990s, intercomparisons with the Swedish National Institute of Radiation Protection were also organised. All these intercomparisons showed that the differences between the instrument calibrations of the laboratories used as reference were small.
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M. Annanmäki et al.
Several instruments have been used to measure radon daughter concentration, but the method used has been the same. A dust sample is collected on a glass fibre or membrane filter and, after a sampling period of 5 to 10 minutes, the alpha activity collected on the filter is measured using a silver activated zinc-sulfide or a surface barrier detector. The results are expressed in units of µJ m−3 and are calculated using the so-called Kusnetz method [3]. Some of the instruments have been our own construction but since 1986 commercially available equipment has been used (Pylon WL-1000C Working Level Monitor). In the third CEC intercomparison exercise the results obtained with the radon daughter monitors were also compared and STUK’s results were in good agreement with the reference instrument [8]. 2.3. Dose assessment Measurements were made in all operating mines every year or every second year, depending on the concentrations detected in earlier years. These consecutive measurements were made during different seasons so as also to consider, in the long run, possible seasonal changes in ventilation conditions of the mine. The number of radon measurements per mine varied from about 10 to 30 depending on its size and on the number of different working areas. The results were compiled so that an average concentration, considered representative for the whole mine, was derived by calculating an arithmetic mean of concentrations detected in different working areas of the mine. This average concentration was used for comparison with the action level and for assessing the workers doses. The effective dose (E) was calculated using the equation [9]: E = 7.78 × 10−9
Sv ·F ·T ·C Bq h m−3
(1)
where C is the mean radon concentration (Bq m−3 ) in the working places of a mine, F is the equilibrium factor (value 0.64 was used), T is the annual working hours (value 1600 h was used). In all dose assessments presented in this paper, the values F = 0.64 (mean value for all mines during 1972–1992) and T = 1600 h were used. This gives a direct correspondence between the radon concentration and the effective dose: 100 Bq m−3 ↔ 0.8 mSv. In cases where no radon measurements were made in a particular year, the mean radon concentration was calculated as the mean of the previous and following years.
3. Results Regular radon measurements have been made in all underground mines for 30 years in order to ascertain that the action levels set for radon are not exceeded. In most cases also radon daughter concentration has been measured simultaneously. Since 1997 only radon concentrations have been measured. Where action levels were exceeded, the results were used to assess radiation doses received by the workers to ensure that the dose limits were not exceeded. In the 1970s the radon concentrations were less than 400 Bq m−3 in about 50% of all the measured places of work in the mines. Concentrations exceeded 2000 Bq m−3 in about
Radon in Finnish mines – regular monitoring since 1972
661
20% of the measurements. The highest detected concentration in a working area was about 37 000 Bq m−3 . Even higher concentrations were detected in some poorly ventilated unused areas of the mines. Today (results of 2000–2001) the mean radon concentration in all the mines is about 110 Bq m−3 (weighted by the number of workers in each mine) and individual results exceeding 400 Bq m−3 are rather rare. The average individual dose to underground miners was calculated in the following way. First, a mine-specific individual dose was calculated using equation (1). Then the average of the mine-specific doses was calculated using the number of workers in each mine as a weighting factor. The results are presented in Fig. 2. The collective dose of Finnish miners was derived by summing up the product of the minespecific individual dose and the number of workers for all mines operating during the year in question. The results are presented in Fig. 3. Also a mine-specific mean value for the equilibrium factor F was calculated. Measurements were made annually in 2–15 different mines. Usually about 10 measurements were made in each mine and the average of these results was taken as the mine specific value. In the calculations only measurements made at working areas were taken into account. The distribution of mine-specific equilibrium factors is presented in Fig. 4. There were no measurements in 1979.
Fig. 2. Average individual dose received in Finnish underground mines.
Fig. 3. Collective annual dose received in Finnish underground mines.
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Fig. 4. Equilibrium factor F in Finnish underground mines. The “Minimum” and “Maximum” values are the lowest and the highest mine-specific values. The “Average” is the average of all the mine-specific values determined during the year in question.
The alpha energy concentration due to thoron 220 Rn (212 Pb) was measured in 8 different mines from the total of 103 filter samples. The average alpha energy concentration due to 220 Rn was 0.1 µJ m−3 , and the highest measured value was 0.5 µJ m−3 .
4. Discussion Radon concentrations in mines and doses caused by radon have decreased significantly since the early 1970s. The decrease in radon concentration has been partly due to countermeasures made to decrease high radon concentrations but mostly because many of the large, poorly ventilated old mines have now been closed. The modern mines are small in size and it is technically easier to arrange appropriate ventilation in them. Good general ventilation is also required by the use of diesel-powered machinery. In addition, remedial actions have been taken wherever regular control measurements have shown concentrations exceeding the action level. However, sometimes the reduction of radon concentration is technically difficult, especially if the radon originates from radon-rich bedrock-water entering the cavities in large amounts. The most common remedial measures used are increasing the ventilation and preventing the water entering the mine. The very high individual and collective doses calculated for the years 1972–1973 were partially caused by the former lead mine in Kosrnäs whose ore contained significantly elevated levels of uranium (238 U). The radon concentrations were in the order of 10 000– 20 000 Bq m−3 . The mine was closed soon after the regular radon monitoring started. There was also one large mine in which the ventilation was not operating correctly resulting in concentrations as high as 10 000–50 000 Bq m−3 . Action was taken to improve the situation resulting in significant reduction in radon concentrations. The results of the very early years might also be slightly biased by the fact that during that time many measurements were made in poorly ventilated spaces of old large mines and later on it was difficult to identify whether they really were regular working areas or not. There was a slight increase in both the individual and collective doses in the late 1980s. The statistics show that the increase was caused by two large old mines which were then closed
Radon in Finnish mines – regular monitoring since 1972
663
in 1988 and 1989, respectively. During the last few years of operation these mines exploited some residual ores of the older parts of the mine. It is assumed that ventilation in these parts of the mines was not as good as in the newer parts. In the late 1990s there was once again a slight increase in the average individual dose. This was caused by a mine operating only for some five years. The concentrations in this particular mine were in the order of 1000–2000 Bq m−3 and temporally concentrations up to about 7000 Bq m−3 were detected in some working areas. The reduction of radon concentration was technically almost impossible, because the radon originated from radon-rich bedrockwater entering the cavities in large amounts. The equilibrium factor F varied from 0.2 to 0.9, depending on the mine and the year. The mean value for the equilibrium factor for all the mines has mostly been between 0.6 and 0.7. The average of all the annual mean values for the years 1972–1992 is 0.64. The equilibrium factors in the mines are somewhat higher than the value 0.4 commonly used for dose assessments [1]. This should be taken into account whenever assessing the miners’ doses. The alpha energy concentrations due to thoron 220 Rn (212 Pb) were so low that it was concluded that thoron does not cause significant exposures and regular monitoring was not deemed necessary. It can be concluded that radon in the Finnish mines is not at the moment a major problem. The average concentration in all the mines is about 110 Bq m−3 (results for the years 2000–2001) and very seldom do individual results of different working areas exceed the action level of 400 Bq m−2 . The average in mines is almost the same as the average in Finnish dwellings! Monitoring of normal above ground workplaces has showed, e.g., that in certain areas of Finland the arithmetic mean radon concentration was as high as 500 Bq m−3 [10]. In some of the most radon prone areas in Finland, up to 10–30% of aboveground workplaces and 30–40% of single-family houses may exceed the action level of 400 Bq m−3 [9]. Considering also the number of workers involved, there is no doubt that radon in ordinary above ground workplaces, i.e., not in the underground mines, is the most important source of occupational radon exposure in Finland. In fact, radon in above ground workplaces is by far the most important source of all occupational exposures in Finland [9]. However, the sudden increases in radon concentration detected in some mines in the late 1980s and late 1990s demonstrate clearly that regular monitoring is needed in order to detect circumstances and tendencies which might lead to significantly high exposures to the miners as was the case in the early 1970s when the regular monitoring was started.
Acknowledgements During the last 30 years several persons have participated in the radon measurements in mines. Special thanks are due to Mr. Heimo Kahlos who first ordered investigation of the possible occurrence of radon in Finnish mines and assessment of whether or not countermeasures were needed. He was the person who was in charge of these measurements in the beginning. Thanks also to Ms. Ritva Paatelainen, who for several years assisted in measurements and recorded all the measurement data into a data file for later use. We are grateful to Ms. Kyllikki Aakko, who in 1990 made a special study on the calibration of the measurement method used for radon measurements.
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References [1] ICRP Publication 65: Protection against radon-222 at home and at work, Ann. ICRP 23 (2) (1993). [2] H.F. Lucas, Improved low-level alpha-scintillation counter for radon, Rev. Sci. Instrum. 28 (1957) 680–683. [3] H.L. Kusnetz, Radon daughters in mine atmospheres – a field method for determining concentrations, Am. Ind. Hyg. Assoc. Quart. 17 (1956) 85–88. [4] Radiation Act, The Statutes of Finland 592/91, Helsinki, 1991. [5] Council Directive 96/29/Euratom of 13 May 1996 laying down basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionising radiation, Official J. Eur. Commun. Ser. L 159 (29.6.1996). [6] Recommendations for the implementation of Title VII of the European Basic Safety Standards Directive (BSS) concerning significant increase in exposure to natural radiation sources, Radiation Protection 88, European Commission, Luxembourg, 1997. [7] K. Aakko, Calibration of ZnS 50 radon monitor of STUK, MSc thesis, University of Jyväskylä, 1990 (in Finnish). [8] J.C.H. Miles, J. Sinnaeve, Results of the third CEC intercomparison of active and passive detectors for the measurement of radon and radon decay products, Report EUR 11882 EN, Commission of the European Communities, Luxembourg, 1988. [9] M. Markkanen, M. Annanmäki, E. Oksanen, Radon in workplaces, Kerntechnik 65 (1) (2000). [10] M. Annanmäki, E. Oksanen, M. Markkanen, Radon at workplaces other than mines and underground excavations, Environ. Int. 22 (Suppl. 1) (1996) S769–S772.
Radon in Finnish mines – regular monitoring since 1972 M. Annanmäki, E. Oksanen, E. Venelampi, M. Markkanen Radiation and Nuclear Safety Authority – STUK P.O. Box 14, FIN-00881, Helsinki, Finland
Radon measurements in Finnish underground mines were started in 1972. At that time there were 23 operating underground mines in Finland. Some of the mines were old, having a lot of underground abandoned spaces not in active use. Today there are 8 underground mines in operation, most of them being small in size. The first radon measurements were made in order to find out radon levels in the mines and to estimate the extent of a possible radon problem. Radon measurements were made in work places, as well as, in areas not under work but in which high radon concentrations were expected to occur. Since then regular measurements in all underground mines have been made in order to control radon concentration in work places and to ascertain that the limits set for radon are not exceeded, and in some cases to evaluate the doses to miners. In most cases radon daughter concentration has also been measured together with radon. During the last few years only radon concentrations have been measured. In 1975, a limit for radon was set at 1100 Bq m−3 (30 pCi L−1 ), the value being the radon in equilibrium with its short-lived daughter products. In 1992, an action level of 400 Bq m−3 for radon, an average over the annual working cycle, was adopted. In 1975 the mean effective dose for a miner was 3.5 mSv, in 1985 2.4 mSv, in 1995 1.7 mSv and in 2001 0.9 mSv. The decrease in radon concentration has been partly due to countermeasures made to decrease high radon concentrations but mostly because many of the large, poorly ventilated old mines have now been closed. The modern mines are small in size and it is technically easier to arrange appropriate ventilation in them. The equilibrium factor has varied from 0.2 to 0.9 depending on the mine and the year. The average equilibrium factor when all the mines are taken into account is 0.6. The alpha energy concentration due to thoron 220 Rn (212 Pb) was measured in 8 different mines from a total of 103 filter samples. The average alpha energy concentration due to 220 Rn was 0.1 µJ m−3 , and the highest measured value was 0.5 µJ m−3 . 1. Introduction The significance of radon (222 Rn) as a source of radiation exposure was realised in the 1950s when it was discovered that exposures due to radon (specifically to its decay products 218 Po, RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07080-9
© 2005 Elsevier Ltd. All rights reserved.
658
M. Annanmäki et al.
Fig. 1. Number of workers in Finnish underground mines. 214 Pb, 214 Bi
and 214 Po) could explain the excess in lung cancers detected among miners. The first results indicating a significant excess of lung cancer among uranium miners were obtained in the 1960s in the United States. High concentrations of radon exist also in nonuranium mines, in various underground excavation works and other underground workplaces. Today, the epidemiological studies on uranium and non-uranium miners form the main source of information for estimating the risk (the dose) caused by exposures to radon [1]. Radon measurements in the Finnish underground mines were started in 1972. The first radon measurements were made in order to find out radon levels in the mines and to estimate the extent of a possible radon problem. Radon measurements were made in work places, as well as in areas not in active use but in which high radon concentrations were expected to occur. Since 1972 radon measurements have been made regularly in all underground mines every year or every second year, depending on the radon concentration measured in earlier years. The number of underground mineworkers in Finland is presented in Fig. 1. In the early 1970s there were 23 operating mines and the number of underground workers was about 1300. Today there are only 8 mines, most of them being small in size and employing about 400 workers in total. The minerals mined are mostly copper, zinc, gold and limestone. Earlier iron was also mined. During the years different equipment has been used to measure concentrations of radon and its short-lived daughter products. However, the measurement methods themselves remained the same. Radon was measured with grab sampling using Lucas-type chambers [2]. The shortlived daughter products were measured with the Kusnetz method [3]. In 1983 and 1984 the alpha energy concentration due to 220 Rn (212 Pb) was also measured in most of the mines from the same filter samples which were used to measure the alpha energy concentration due to 222 Rn.
2. Material and methods 2.1. Regulation of radon in mines The initiative to start radon measurements in 1972 came from the Ministry of Trade and Industry, which is in charge of the safety of mines. First regulations for controlling radon in
Radon in Finnish mines – regular monitoring since 1972
659
underground mines were issued in 1975. At that time, a limit for radon was set at 1100 Bq m−3 (30 pCi L−1 ), defined as the Equivalent Equilibrium Radon concentration (EEC), i.e., radon in equilibrium with its short-lived daughter products. A major revision of the Finnish Radiation Act took effect in 1992 [4]. Since then the Act has covered all work activities, not only mines, involving significant occupational and public exposures to natural radiation in accordance with the principles set out in the 1990 ICRP Recommendations. The Act and other provisions issued by virtue of it cover the activities falling under the scope of Title VII of the new BSS Directive [5,6]. In 1992, an action level of 400 Bq m−3 for radon was adopted. The action level is defined as an annual average concentration during working hours. Steps must be taken to reduce the radon concentration if the action level is exceeded. If countermeasures do not work, individual doses of miners must be assessed. In this case, the maximum permissible radon concentration is 3000 Bq m−3 leading to an effective dose of 20 mSv, the dose limit for workers. The individual doses are assessed by measuring the radon concentrations in different working areas and by recording the working hours of the miners in those areas. 2.2. Radon and radon daughter monitors used During the last 30 years several types of equipment have been used to measure radon. However, the measurement method itself, measuring the activity of an air sample (grab sampling) using Lucas-type chambers, has remained the same [2]. Silver activated zinc sulfide is used to detect the alpha-activity of radon (222 Rn) and radon daughters (218 Po, 214 Po). In the beginning, there was only one measurement head and the air samples were taken into gas bottles and the air sample was then transferred into the measurement head for its measurement. Later on about 50 detector chambers were used into which air samples were taken directly. The construction of these detectors was our own. There were also several counting systems (a photo multiplier and an electronic counter) into which the chambers could be connected for the measurement. Since 1998, a commercially available monitor with several detector chambers has been used (Pylon AB 5). The detectors were regularly calibrated in a radon chamber maintained at STUK. The reference devices used for calibration purposes were regularly checked against a standard radon concentration produced into a steel drum by emanating radon from a known amount of radium (226 Ra). In 1990, a major study was carried out to check the validity of the calibration system and to recalibrate the radon monitors. As a consequence of the study, the calibration factors of the monitors were changed upwards by about 5% [7]. STUK has participated in several intercomparisons for radon measurements. The first was organised with the Swedish National Institute of Radiation Protection (SSI) in 1978. The second, organised in 1982, was made together with the Swedish National Institute of Radiation Protection, the National Institute of Radiation Hygiene (SIS, Denmark) and the National Institute of Radiation Hygiene (NRPA, Norway). In 1987 the third CEC intercomparison of active and passive detectors for the measurement of radon and radon decay products was organised by the National Radiological Protection Board (NRPB, UK) in which STUK also participated [8]. In the 1990s, intercomparisons with the Swedish National Institute of Radiation Protection were also organised. All these intercomparisons showed that the differences between the instrument calibrations of the laboratories used as reference were small.
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M. Annanmäki et al.
Several instruments have been used to measure radon daughter concentration, but the method used has been the same. A dust sample is collected on a glass fibre or membrane filter and, after a sampling period of 5 to 10 minutes, the alpha activity collected on the filter is measured using a silver activated zinc-sulfide or a surface barrier detector. The results are expressed in units of µJ m−3 and are calculated using the so-called Kusnetz method [3]. Some of the instruments have been our own construction but since 1986 commercially available equipment has been used (Pylon WL-1000C Working Level Monitor). In the third CEC intercomparison exercise the results obtained with the radon daughter monitors were also compared and STUK’s results were in good agreement with the reference instrument [8]. 2.3. Dose assessment Measurements were made in all operating mines every year or every second year, depending on the concentrations detected in earlier years. These consecutive measurements were made during different seasons so as also to consider, in the long run, possible seasonal changes in ventilation conditions of the mine. The number of radon measurements per mine varied from about 10 to 30 depending on its size and on the number of different working areas. The results were compiled so that an average concentration, considered representative for the whole mine, was derived by calculating an arithmetic mean of concentrations detected in different working areas of the mine. This average concentration was used for comparison with the action level and for assessing the workers doses. The effective dose (E) was calculated using the equation [9]: E = 7.78 × 10−9
Sv ·F ·T ·C Bq h m−3
(1)
where C is the mean radon concentration (Bq m−3 ) in the working places of a mine, F is the equilibrium factor (value 0.64 was used), T is the annual working hours (value 1600 h was used). In all dose assessments presented in this paper, the values F = 0.64 (mean value for all mines during 1972–1992) and T = 1600 h were used. This gives a direct correspondence between the radon concentration and the effective dose: 100 Bq m−3 ↔ 0.8 mSv. In cases where no radon measurements were made in a particular year, the mean radon concentration was calculated as the mean of the previous and following years.
3. Results Regular radon measurements have been made in all underground mines for 30 years in order to ascertain that the action levels set for radon are not exceeded. In most cases also radon daughter concentration has been measured simultaneously. Since 1997 only radon concentrations have been measured. Where action levels were exceeded, the results were used to assess radiation doses received by the workers to ensure that the dose limits were not exceeded. In the 1970s the radon concentrations were less than 400 Bq m−3 in about 50% of all the measured places of work in the mines. Concentrations exceeded 2000 Bq m−3 in about
Radon in Finnish mines – regular monitoring since 1972
661
20% of the measurements. The highest detected concentration in a working area was about 37 000 Bq m−3 . Even higher concentrations were detected in some poorly ventilated unused areas of the mines. Today (results of 2000–2001) the mean radon concentration in all the mines is about 110 Bq m−3 (weighted by the number of workers in each mine) and individual results exceeding 400 Bq m−3 are rather rare. The average individual dose to underground miners was calculated in the following way. First, a mine-specific individual dose was calculated using equation (1). Then the average of the mine-specific doses was calculated using the number of workers in each mine as a weighting factor. The results are presented in Fig. 2. The collective dose of Finnish miners was derived by summing up the product of the minespecific individual dose and the number of workers for all mines operating during the year in question. The results are presented in Fig. 3. Also a mine-specific mean value for the equilibrium factor F was calculated. Measurements were made annually in 2–15 different mines. Usually about 10 measurements were made in each mine and the average of these results was taken as the mine specific value. In the calculations only measurements made at working areas were taken into account. The distribution of mine-specific equilibrium factors is presented in Fig. 4. There were no measurements in 1979.
Fig. 2. Average individual dose received in Finnish underground mines.
Fig. 3. Collective annual dose received in Finnish underground mines.
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Fig. 4. Equilibrium factor F in Finnish underground mines. The “Minimum” and “Maximum” values are the lowest and the highest mine-specific values. The “Average” is the average of all the mine-specific values determined during the year in question.
The alpha energy concentration due to thoron 220 Rn (212 Pb) was measured in 8 different mines from the total of 103 filter samples. The average alpha energy concentration due to 220 Rn was 0.1 µJ m−3 , and the highest measured value was 0.5 µJ m−3 .
4. Discussion Radon concentrations in mines and doses caused by radon have decreased significantly since the early 1970s. The decrease in radon concentration has been partly due to countermeasures made to decrease high radon concentrations but mostly because many of the large, poorly ventilated old mines have now been closed. The modern mines are small in size and it is technically easier to arrange appropriate ventilation in them. Good general ventilation is also required by the use of diesel-powered machinery. In addition, remedial actions have been taken wherever regular control measurements have shown concentrations exceeding the action level. However, sometimes the reduction of radon concentration is technically difficult, especially if the radon originates from radon-rich bedrock-water entering the cavities in large amounts. The most common remedial measures used are increasing the ventilation and preventing the water entering the mine. The very high individual and collective doses calculated for the years 1972–1973 were partially caused by the former lead mine in Kosrnäs whose ore contained significantly elevated levels of uranium (238 U). The radon concentrations were in the order of 10 000– 20 000 Bq m−3 . The mine was closed soon after the regular radon monitoring started. There was also one large mine in which the ventilation was not operating correctly resulting in concentrations as high as 10 000–50 000 Bq m−3 . Action was taken to improve the situation resulting in significant reduction in radon concentrations. The results of the very early years might also be slightly biased by the fact that during that time many measurements were made in poorly ventilated spaces of old large mines and later on it was difficult to identify whether they really were regular working areas or not. There was a slight increase in both the individual and collective doses in the late 1980s. The statistics show that the increase was caused by two large old mines which were then closed
Radon in Finnish mines – regular monitoring since 1972
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in 1988 and 1989, respectively. During the last few years of operation these mines exploited some residual ores of the older parts of the mine. It is assumed that ventilation in these parts of the mines was not as good as in the newer parts. In the late 1990s there was once again a slight increase in the average individual dose. This was caused by a mine operating only for some five years. The concentrations in this particular mine were in the order of 1000–2000 Bq m−3 and temporally concentrations up to about 7000 Bq m−3 were detected in some working areas. The reduction of radon concentration was technically almost impossible, because the radon originated from radon-rich bedrockwater entering the cavities in large amounts. The equilibrium factor F varied from 0.2 to 0.9, depending on the mine and the year. The mean value for the equilibrium factor for all the mines has mostly been between 0.6 and 0.7. The average of all the annual mean values for the years 1972–1992 is 0.64. The equilibrium factors in the mines are somewhat higher than the value 0.4 commonly used for dose assessments [1]. This should be taken into account whenever assessing the miners’ doses. The alpha energy concentrations due to thoron 220 Rn (212 Pb) were so low that it was concluded that thoron does not cause significant exposures and regular monitoring was not deemed necessary. It can be concluded that radon in the Finnish mines is not at the moment a major problem. The average concentration in all the mines is about 110 Bq m−3 (results for the years 2000–2001) and very seldom do individual results of different working areas exceed the action level of 400 Bq m−2 . The average in mines is almost the same as the average in Finnish dwellings! Monitoring of normal above ground workplaces has showed, e.g., that in certain areas of Finland the arithmetic mean radon concentration was as high as 500 Bq m−3 [10]. In some of the most radon prone areas in Finland, up to 10–30% of aboveground workplaces and 30–40% of single-family houses may exceed the action level of 400 Bq m−3 [9]. Considering also the number of workers involved, there is no doubt that radon in ordinary above ground workplaces, i.e., not in the underground mines, is the most important source of occupational radon exposure in Finland. In fact, radon in above ground workplaces is by far the most important source of all occupational exposures in Finland [9]. However, the sudden increases in radon concentration detected in some mines in the late 1980s and late 1990s demonstrate clearly that regular monitoring is needed in order to detect circumstances and tendencies which might lead to significantly high exposures to the miners as was the case in the early 1970s when the regular monitoring was started.
Acknowledgements During the last 30 years several persons have participated in the radon measurements in mines. Special thanks are due to Mr. Heimo Kahlos who first ordered investigation of the possible occurrence of radon in Finnish mines and assessment of whether or not countermeasures were needed. He was the person who was in charge of these measurements in the beginning. Thanks also to Ms. Ritva Paatelainen, who for several years assisted in measurements and recorded all the measurement data into a data file for later use. We are grateful to Ms. Kyllikki Aakko, who in 1990 made a special study on the calibration of the measurement method used for radon measurements.
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References [1] ICRP Publication 65: Protection against radon-222 at home and at work, Ann. ICRP 23 (2) (1993). [2] H.F. Lucas, Improved low-level alpha-scintillation counter for radon, Rev. Sci. Instrum. 28 (1957) 680–683. [3] H.L. Kusnetz, Radon daughters in mine atmospheres – a field method for determining concentrations, Am. Ind. Hyg. Assoc. Quart. 17 (1956) 85–88. [4] Radiation Act, The Statutes of Finland 592/91, Helsinki, 1991. [5] Council Directive 96/29/Euratom of 13 May 1996 laying down basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionising radiation, Official J. Eur. Commun. Ser. L 159 (29.6.1996). [6] Recommendations for the implementation of Title VII of the European Basic Safety Standards Directive (BSS) concerning significant increase in exposure to natural radiation sources, Radiation Protection 88, European Commission, Luxembourg, 1997. [7] K. Aakko, Calibration of ZnS 50 radon monitor of STUK, MSc thesis, University of Jyväskylä, 1990 (in Finnish). [8] J.C.H. Miles, J. Sinnaeve, Results of the third CEC intercomparison of active and passive detectors for the measurement of radon and radon decay products, Report EUR 11882 EN, Commission of the European Communities, Luxembourg, 1988. [9] M. Markkanen, M. Annanmäki, E. Oksanen, Radon in workplaces, Kerntechnik 65 (1) (2000). [10] M. Annanmäki, E. Oksanen, M. Markkanen, Radon at workplaces other than mines and underground excavations, Environ. Int. 22 (Suppl. 1) (1996) S769–S772.