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Radon concentration in the tunnels of a hydroelectric power station under construction in Italy, a case study
215
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy, a case study S. Verdelocco, D. Walker, P. Turkowsky Joint Research Centre, Via E. Fermi 1, 21020 Ispra (Va), Italy
The purpose of this study was to measure the radon concentration in air, in the tunnels of a hydroelectric power station in Piedmont, Italy. Both active and passive methods were used for radon measurements. In addition gamma spectrometry was performed on various rock samples. The results showed how radon concentration in tunnels is influenced by the ventilation rate both for active and for passive measurements. It was clear that in this type of environment the only solution to the problem of reducing the radon concentration in air was to use a good ventilation system that should always be in service during periods of work. The source of radon in the tunnels was not attributed to the rock type (metamorphic rocks), as was verified with spectrometry analysis.
1. Introduction The purpose of this study was to measure the radon concentration in air, in the tunnels of a hydroelectric power station in Piedmont, Italy. This power station is the only hydroelectric installation under construction today in Italy and is predominantly underground. The necessity for monitoring arose from the presence in this area of various uraniferous outcrops identified by drilling and mineral research performed in the past. For this reason it could not be excluded that some uraniferous deposits might be encountered while boring the tunnels. The geology of the area is predominantly characterised by metamorphic rocks; these kinds of rocks usually rich in uranium minerals could also be a radon source. The study commenced in 1998 and was completed in 2000. Since the site where the monitoring was performed presented anomalous environmental characteristics (water, dust, humidity, vibrations, etc.), it was necessary to adapt the methodology used, choosing the equipment that could give the best results considering the type of environment. RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07024-X
© 2005 Elsevier Ltd. All rights reserved.
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S. Verdelocco et al.
2. Geology of the area The geology of the area is characterised by outcrops belonging to the Ambin Massif (northwest zone) and to the Piemontese Area (central-east zone). The sequences, both of the Ambin Massif and of the Piemontese Area, are comprised of metamorphic rocks such as: quartzite, micaceous quartzite, dolomitic and calcitic marble, gneiss, mica schist, carbonatic schist, and calcareous schist. The superficial coverings in the area are drift deposits, fluvial deposits, recent alluvial deposits, colluvium, and detrital-colluvial covering. The structural setting is characterised by folds (Alpine compressional phase) normal faults and discontinuities (regional stretching stress).
3. Materials and methods Both active and passive methods were used for radon measurements. The Genitron Alphaguard monitor was used for active measurements. Regular checks on the air radon concentration, at the tunnel work-faces, were performed during working periods. The passive method, Karlsruhe diffusion chambers sealed in polyethylene, was used to measure the longterm radon concentration including non-working periods. In addition gamma spectrometry was performed on various rock samples. 3.1. Working methods and problems related to the conditions in the tunnels The Alphaguard instrument was chosen for real time radon measurements as its characteristics best met the frequently severe conditions in the tunnels. Notwithstanding the fact that it uses an ionisation chamber, hence normally considered delicate, it remains unaffected by high humidity. It is also capable of measuring the very high concentrations that may be found in these areas. In all tunnels the measurements were performed as close as possible to the work face (being the area where the workers pass most of there time) depending on the state of work. For the tunnels excavated using the TBM (tunnel boring machine) the monitor was placed on the machine head as close as possible to the cutting tool. On a number of occasions the measurements were performed with the machine boring with subsequent very high vibrations. On other occasions they were performed in the presence of gas and dust due to drilling and coating operations. The measurements were always performed for at least half an hour for each measurement site. The monitor was always covered by its protective bag and frequently with an umbrella. The instrument case was always used as a stand. The use of dosimeters for long term radon and ambient measurements presented problems other than those linked to humidity and dust but rather problems related to the continuous advancement of the work face. Initially the dosimeters were attached to the walls or placed in the wall supports. The area was highlighted with red paint or brought to the attention of the workers. Frequently after one or two months the dosimeters were lost, either cemented into the wall covering or buried under the floor covering, or simply damaged. Because of this highly visible steel plates were manufactured with supports for the dosimeters. These plates were fixed to the tunnel walls. In this way the visibility of the plates was guaranteed and they were made easy for the workers to move and reposition safely. For the tunnels excavated using a TBM the situation was made
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy
217
easier by positioning the dosimeters in the TBM control room. In this way the measurements could continue while the work face advanced. A further problem, related to the presence of water, mud and dust, was resolved by placing the dosimeters in polythene bags impermeable to water but permeable to radon. These dosimeters were changed at each visit and therefore remained exposed for a period of at least one month.
4. Results Measurements were made with the active monitor, with the ventilation in and out of service. In general with the ventilation in service the radon concentration in air was below 100 Bq m−3 , whilst with the ventilation out of service radon concentrations of over 10 000 Bq m−3 were measured. Figure 1 shows the instantaneous measurement results of all the tunnels monitored. Measurements performed with passive detectors showed the same results: radon concentration below 1000 Bq m−3 during normal periods and over 10 000 Bq m−3 during periods in which the construction site was closed. Figure 2 shows some of the long-term measurement results. The radon concentration measured with dosimeters reflects the mean concentration for the complete exposure period. These measurements should be considered excessive from a precautionary point of view because they include periods when work was suspended and the site was closed (i.e., during Christmas holidays). In this case the absence of recirculation of the air present in the tunnels aggravates the accumulation of radon. It should be noticed that the high concentration measured in tunnel 5 were due to the fact that the drilling (and the ventilation) was stopped for a period because of technical problems. Figure 3 shows the difference on radon concentrations for two tunnels measured in the same day with working and non-working ventilation.
Fig. 1. Instantaneous measurement results.
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Fig. 2. Long term radon results.
Fig. 3. Radon concentrations in two different tunnels with working and non-working ventilation.
It was verified that radionuclides of the uranium and thorium series contained in the rock samples analysed were in equilibrium. Measurements on rock samples showed that the concentrations of uranium-238 and thorium-232 were lower than 1 Bq g−1 . According to the legislative decree which was in force at the time the measurements were performed (d.lgs. 230/1995 point 1.3 attachment I) these are the Italian limits above which the materials must
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy
219
Fig. 4. Results of gamma spectrometry measurements performed on rock samples taken in 1998.
Fig. 5. Results of gamma spectrometry measurements performed on rock samples taken in 1999.
be considered radioactive [1]. Figures 4 and 5 show results of gamma spectrometry measurements performed on rock samples taken respectively in 1998 and 1999. 5. Conclusion The results showed how radon concentration in tunnels is influenced by the ventilation rate both for active and for passive measurements. It was clear that in this type of environment
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S. Verdelocco et al.
the only solution to the problem of reducing the radon concentration in air was to use a good ventilation system that should always be in service during periods of work. The radon source in the tunnels was not attributed to the rock type (metamorphic rocks), as was verified with the spectrometry analysis. It is probable that the large volumes of water in the tunnels acted as a conduit for the radon, since the highest radon concentrations were measured in tunnels with a large volume of water flowing through faults and fractures. With regard to the technical aspects of the monitoring campaign, it is difficult to define a standard measurement routine, because it will depend on the work programme for the site and on the decisions of the local authorities. With regard to the progression of the measurements, the number performed depends in first instance on the length of the tunnels and the excavation rate. The tunnels in which the most measurements were performed were those with extended excavation times due to problems encountered during excavation (meeting a fault plane, water sources, collapse of rock wedges, etc.), or excessive length. Meanwhile those tunnels with the smallest number of measurements were the shortest or those that encountered the least problems. The occasional gaps in the results obtained may be alleviated by the following conditions: information provided to the workers on measurements in progress, positioning of dosimeters in an area that guarantees their safety (often difficult to identify). Dosimeters may be damaged by the progression of the excavation and the difficult conditions such as dust, mud, water, cement and excavation using explosives. With regard to the measurement methods it was found to be essential for the accuracy of the measurements to use both active and passive methods. With the active method it is possible to have a real time measurement of the radon concentration and evaluate the efficiency of protective measures during actual periods of work (ventilation strength). Alternatively the passive method being an integrated measurement provides the mean radon concentration during a given time period, including periods when work was suspended and radon was allowed to accumulate.
References [1] Decreto legislativo 17 marzo 1995, n. 230, supplemento ordinario alla GURI 136 del 13 giugno 1995.
Further reading [1] T. Domanski, W. Chruscielewski, M. Hofman, Monitoring the exposure to radon decay products in mine air using passive track detectors, Health Phys. 40 (1980) 211. [2] 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. [3] Genitron Instruments, Report of the radon monitor AlphaGUARD in mines. [4] S.E.A. Impianto Idroelettrico Pont Ventoux-Susa, Nota Geologica Informativa, Commento Alle Tavole Geologiche Doc. SV9714RGD00. [5] http://members.tripod.it/PontVentoux/. [6] S. Verdelocco, D. Walker, P. Turkowsky, C. Osimani, Misure di radon-222 e radioattività ambientale nell’impianto idroelettrico di Pont Ventoux-Susa, Piemonte, Rapporto della Commissione Europea EUR 19656 IT, 2000.
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy, a case study S. Verdelocco, D. Walker, P. Turkowsky Joint Research Centre, Via E. Fermi 1, 21020 Ispra (Va), Italy
The purpose of this study was to measure the radon concentration in air, in the tunnels of a hydroelectric power station in Piedmont, Italy. Both active and passive methods were used for radon measurements. In addition gamma spectrometry was performed on various rock samples. The results showed how radon concentration in tunnels is influenced by the ventilation rate both for active and for passive measurements. It was clear that in this type of environment the only solution to the problem of reducing the radon concentration in air was to use a good ventilation system that should always be in service during periods of work. The source of radon in the tunnels was not attributed to the rock type (metamorphic rocks), as was verified with spectrometry analysis.
1. Introduction The purpose of this study was to measure the radon concentration in air, in the tunnels of a hydroelectric power station in Piedmont, Italy. This power station is the only hydroelectric installation under construction today in Italy and is predominantly underground. The necessity for monitoring arose from the presence in this area of various uraniferous outcrops identified by drilling and mineral research performed in the past. For this reason it could not be excluded that some uraniferous deposits might be encountered while boring the tunnels. The geology of the area is predominantly characterised by metamorphic rocks; these kinds of rocks usually rich in uranium minerals could also be a radon source. The study commenced in 1998 and was completed in 2000. Since the site where the monitoring was performed presented anomalous environmental characteristics (water, dust, humidity, vibrations, etc.), it was necessary to adapt the methodology used, choosing the equipment that could give the best results considering the type of environment. RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07024-X
© 2005 Elsevier Ltd. All rights reserved.
216
S. Verdelocco et al.
2. Geology of the area The geology of the area is characterised by outcrops belonging to the Ambin Massif (northwest zone) and to the Piemontese Area (central-east zone). The sequences, both of the Ambin Massif and of the Piemontese Area, are comprised of metamorphic rocks such as: quartzite, micaceous quartzite, dolomitic and calcitic marble, gneiss, mica schist, carbonatic schist, and calcareous schist. The superficial coverings in the area are drift deposits, fluvial deposits, recent alluvial deposits, colluvium, and detrital-colluvial covering. The structural setting is characterised by folds (Alpine compressional phase) normal faults and discontinuities (regional stretching stress).
3. Materials and methods Both active and passive methods were used for radon measurements. The Genitron Alphaguard monitor was used for active measurements. Regular checks on the air radon concentration, at the tunnel work-faces, were performed during working periods. The passive method, Karlsruhe diffusion chambers sealed in polyethylene, was used to measure the longterm radon concentration including non-working periods. In addition gamma spectrometry was performed on various rock samples. 3.1. Working methods and problems related to the conditions in the tunnels The Alphaguard instrument was chosen for real time radon measurements as its characteristics best met the frequently severe conditions in the tunnels. Notwithstanding the fact that it uses an ionisation chamber, hence normally considered delicate, it remains unaffected by high humidity. It is also capable of measuring the very high concentrations that may be found in these areas. In all tunnels the measurements were performed as close as possible to the work face (being the area where the workers pass most of there time) depending on the state of work. For the tunnels excavated using the TBM (tunnel boring machine) the monitor was placed on the machine head as close as possible to the cutting tool. On a number of occasions the measurements were performed with the machine boring with subsequent very high vibrations. On other occasions they were performed in the presence of gas and dust due to drilling and coating operations. The measurements were always performed for at least half an hour for each measurement site. The monitor was always covered by its protective bag and frequently with an umbrella. The instrument case was always used as a stand. The use of dosimeters for long term radon and ambient measurements presented problems other than those linked to humidity and dust but rather problems related to the continuous advancement of the work face. Initially the dosimeters were attached to the walls or placed in the wall supports. The area was highlighted with red paint or brought to the attention of the workers. Frequently after one or two months the dosimeters were lost, either cemented into the wall covering or buried under the floor covering, or simply damaged. Because of this highly visible steel plates were manufactured with supports for the dosimeters. These plates were fixed to the tunnel walls. In this way the visibility of the plates was guaranteed and they were made easy for the workers to move and reposition safely. For the tunnels excavated using a TBM the situation was made
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy
217
easier by positioning the dosimeters in the TBM control room. In this way the measurements could continue while the work face advanced. A further problem, related to the presence of water, mud and dust, was resolved by placing the dosimeters in polythene bags impermeable to water but permeable to radon. These dosimeters were changed at each visit and therefore remained exposed for a period of at least one month.
4. Results Measurements were made with the active monitor, with the ventilation in and out of service. In general with the ventilation in service the radon concentration in air was below 100 Bq m−3 , whilst with the ventilation out of service radon concentrations of over 10 000 Bq m−3 were measured. Figure 1 shows the instantaneous measurement results of all the tunnels monitored. Measurements performed with passive detectors showed the same results: radon concentration below 1000 Bq m−3 during normal periods and over 10 000 Bq m−3 during periods in which the construction site was closed. Figure 2 shows some of the long-term measurement results. The radon concentration measured with dosimeters reflects the mean concentration for the complete exposure period. These measurements should be considered excessive from a precautionary point of view because they include periods when work was suspended and the site was closed (i.e., during Christmas holidays). In this case the absence of recirculation of the air present in the tunnels aggravates the accumulation of radon. It should be noticed that the high concentration measured in tunnel 5 were due to the fact that the drilling (and the ventilation) was stopped for a period because of technical problems. Figure 3 shows the difference on radon concentrations for two tunnels measured in the same day with working and non-working ventilation.
Fig. 1. Instantaneous measurement results.
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S. Verdelocco et al.
Fig. 2. Long term radon results.
Fig. 3. Radon concentrations in two different tunnels with working and non-working ventilation.
It was verified that radionuclides of the uranium and thorium series contained in the rock samples analysed were in equilibrium. Measurements on rock samples showed that the concentrations of uranium-238 and thorium-232 were lower than 1 Bq g−1 . According to the legislative decree which was in force at the time the measurements were performed (d.lgs. 230/1995 point 1.3 attachment I) these are the Italian limits above which the materials must
Radon concentration in the tunnels of a hydroelectric power station under construction in Italy
219
Fig. 4. Results of gamma spectrometry measurements performed on rock samples taken in 1998.
Fig. 5. Results of gamma spectrometry measurements performed on rock samples taken in 1999.
be considered radioactive [1]. Figures 4 and 5 show results of gamma spectrometry measurements performed on rock samples taken respectively in 1998 and 1999. 5. Conclusion The results showed how radon concentration in tunnels is influenced by the ventilation rate both for active and for passive measurements. It was clear that in this type of environment
220
S. Verdelocco et al.
the only solution to the problem of reducing the radon concentration in air was to use a good ventilation system that should always be in service during periods of work. The radon source in the tunnels was not attributed to the rock type (metamorphic rocks), as was verified with the spectrometry analysis. It is probable that the large volumes of water in the tunnels acted as a conduit for the radon, since the highest radon concentrations were measured in tunnels with a large volume of water flowing through faults and fractures. With regard to the technical aspects of the monitoring campaign, it is difficult to define a standard measurement routine, because it will depend on the work programme for the site and on the decisions of the local authorities. With regard to the progression of the measurements, the number performed depends in first instance on the length of the tunnels and the excavation rate. The tunnels in which the most measurements were performed were those with extended excavation times due to problems encountered during excavation (meeting a fault plane, water sources, collapse of rock wedges, etc.), or excessive length. Meanwhile those tunnels with the smallest number of measurements were the shortest or those that encountered the least problems. The occasional gaps in the results obtained may be alleviated by the following conditions: information provided to the workers on measurements in progress, positioning of dosimeters in an area that guarantees their safety (often difficult to identify). Dosimeters may be damaged by the progression of the excavation and the difficult conditions such as dust, mud, water, cement and excavation using explosives. With regard to the measurement methods it was found to be essential for the accuracy of the measurements to use both active and passive methods. With the active method it is possible to have a real time measurement of the radon concentration and evaluate the efficiency of protective measures during actual periods of work (ventilation strength). Alternatively the passive method being an integrated measurement provides the mean radon concentration during a given time period, including periods when work was suspended and radon was allowed to accumulate.
References [1] Decreto legislativo 17 marzo 1995, n. 230, supplemento ordinario alla GURI 136 del 13 giugno 1995.
Further reading [1] T. Domanski, W. Chruscielewski, M. Hofman, Monitoring the exposure to radon decay products in mine air using passive track detectors, Health Phys. 40 (1980) 211. [2] 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. [3] Genitron Instruments, Report of the radon monitor AlphaGUARD in mines. [4] S.E.A. Impianto Idroelettrico Pont Ventoux-Susa, Nota Geologica Informativa, Commento Alle Tavole Geologiche Doc. SV9714RGD00. [5] http://members.tripod.it/PontVentoux/. [6] S. Verdelocco, D. Walker, P. Turkowsky, C. Osimani, Misure di radon-222 e radioattività ambientale nell’impianto idroelettrico di Pont Ventoux-Susa, Piemonte, Rapporto della Commissione Europea EUR 19656 IT, 2000.