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Radioenvironmental survey of the Megalopolis power plants fly ash deposits
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Radioenvironmental survey of the Megalopolis power plants fly ash deposits D.J. Karangelos, P.K. Rouni, N.P. Petropoulos, M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos Nuclear Engineering Section, Mechanical Engineering Department, National Technical University of Athens, 15780 Athens, Greece
The Megalopolis lignite field basin is located in the centre of the Peloponnesus peninsula in southern Greece. The lignite fired power plants in operation in this region have a total capacity of 850 MW and produce over 2 million metric tons of fly ash annually; this is primarily disposed of in deposits located mainly at exhausted lignite mines. The disposed fly ash has a 226 Ra content of about 1000 Bq kg−1 . An extensive research project for the determination of the natural radioactivity of lignite and ash from the Megalopolis power plants started in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) in 1983. The project has evolved to an integrated radioenvironmental survey of the Megalopolis lignite field basin area. The present work aims at the presentation of the radioenvironmental survey of the fly ash deposits in the vicinity of the power plants. Results regarding (a) γ-dose rate, (b) radon concentration in the ambient air, (c) soil radon exhalation rate and (d) soil gas radon concentration are reported from systematic field measurements at three deposit sites. In addition, soil sampling of the surface and 0–80 cm layer has been conducted at the deposits to allow for the determination of the activity of natural radionuclides, namely 226 Ra, 232 Th, 40 K, 234 Th and 210 Pb. The radiological characterisation of the fly-ash deposits obtained through these results is compared to the natural radioactivity background in the wider Megalopolis area, using the additional results of a previous similar survey. It can be concluded that, from the radiological point of view, the Megalopolis fly ash deposits do not differ significantly from the rest of the Megalopolis lignite field basin, despite the fact that most of the underground soil layers consist of fly ash with high 226 Ra content. 1. Introduction The increasing demand for electricity from coal-fired or lignite-fired power plants is associated with the production of large amounts of fly ash. The main fly ash disposal methods used are: RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07126-8
© 2005 Elsevier Ltd. All rights reserved.
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D.J. Karangelos et al.
(a) Fly ash water slurry transportation to an open disposal site using open water cycle systems. (b) Fly ash water slurry transportation to an open disposal site using closed water cycle systems. (c) Disposal of fly ash in sequestered landfills. (d) Disposal of fly ash in exhausted strip mine fields. Method (a) reportedly used in Velenje, Slovenia [1] strongly affects the natural radiation environment of the area in several ways, since the deposited ash has direct contact with lake water. Leaching of radionuclides from fly ash into lake and rainwater and pile drainage water are the main sources of radioactive contamination. Concentrations of 226 Ra in lake waters are reported to be at least one order of magnitude higher than in natural water bodies. In addition, concentrations of 226 Ra in the lake sediment and in the sediment of the outflow water are increased over levels found in natural streams. Furthermore, aerial transportation of fly ash from the surface of the deposit to the surrounding environment due to wind erosion may represent another potential contamination pathway. Method (b) is an improvement on method (a), inasmuch it reutilises the water of the circuit, minimising dumping of wet pollutants at the deposit, or in water ponds or even directly into the lakes and the water system of the area. Nevertheless, fly ash deposits of this kind need to be additionally covered with soil or other material to deal with pile drainage water or wind erosion. It is reported in [2] that method (c) is being used in Lodz, Poland. Landfill sites of capacity in the range 1 to 2 million metric tons are being exploited for this purpose, since no other suitable deposition site (e.g., exhausted coal strip mine field on the soil surface) is available in that area. The landfill sites, if constructed and managed poorly, can pose a radiological threat as a result of radionuclide migration to underground water, air and adjacent soil. The Lodz landfills examined in that study have been covered with soil and offered for recultivation. The concentrations of the natural radionuclides in and outside three such fly ash disposal sites are close to the typical values for that part of Poland. The Public Power Corporation of Greece (PPC) is typically using method (d), for the past 45 years at both the main Greek lignite field basins: Ptolemais in northern Greece and Megalopolis in southern Greece. The fly ash deposits are in the form of ash layers several meters thick, which are then covered by a layer of soil. The whole deposition process is a part of the exhausted lignite mines restoration project. The landfill like deposition at the exhausted mine is being managed continuously as part of the fuel cycle. To this end belt transporters and high capacity tracks are being used. Problems that might rise due to poor organisation and management of the whole process are minimised due to the inherent environmental precautions taken when excavating the lignite mine. The Megalopolis lignite field basin is located in the centre of the Peloponnesus peninsula in southern Greece. Two lignite-fired power plants are in operation in this region: MegalopolisA (550 MW – 3 units) since the early 1970s and Megalopolis-B (300 MW – 1 unit) since the early 1990s. The power plants produce over 2 million metric tons of fly ash annually. The disposed fly ash has a 226 Ra content which sometimes exceeds 1 kBq kg−1 , which is very high compared to the mean 226 Ra radioactivity of 25 Bq kg−1 in surface soils in Greece [3]. An extensive research project for the determination of the natural radioactivity of lignite and ash from the Megalopolis power plants started in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) in 1983. The project has evolved to an
Radioenvironmental survey of the Megalopolis power plants fly ash deposits
1027
integrated radioenvironmental survey of the Megalopolis lignite field basin area. The present work focuses on the presentation of the radioenvironmental survey of the fly ash deposits in the vicinity of the power plants. Results regarding (a) γ-dose rate, (b) radon concentration in the ambient air, (c) soil radon exhalation rate and (d) soil gas radon concentration are reported from systematic field measurements at three deposit sites. In addition, soil sampling has been conducted at the deposits to allow for the determination of the activity of natural radionuclides, namely 226 Ra, 232 Th and 40 K. The radiological characterisation of the fly ash deposits obtained through these results is compared to the natural radioactivity background in the wider Megalopolis area, using the results of previous surveys on undisturbed natural soils [4].
2. Methods Measurements were carried out at three deposit sites “A”, “B” and “C” as presented in Fig. 1. Site “A” is a deposit under development, site “B” – created in the 1990s – is a deposit used for livestock cultivation and site “C” – created in the 1980s – is now a well-organised walnut tree plantation. Measurements were also carried out at sites “D”, “E” and “F”. Sites “D” and “E” are located on the borders between the deposit soil and natural undisturbed soil. Site “F” is located in a potential strip lignite mine; mining at this site has not commenced yet. All sites “A” to “F” are located within a 5 km radius of the power plants. In situ γ-dose rates at the investigated sites were measured 1 m above the ground using a portable NaI dose rate meter. Radon concentration in the ambient air were measured using various types of active instrumentation. Surface soil radon exhalation rate measurements and soil gas radon concentration measurements were conducted using appropriate instrumentation. Samples from the surface soil and from the 0–80 cm soil layer profile were analysed for natural radioactivity, in the laboratory, using high-resolution gamma spectroscopy. The soil sampling techniques and the gamma spectroscopic methods employed have been extensively reviewed elsewhere [6].
Fig. 1. Investigated sites at and nearby fly ash deposits at the Megalopolis lignite field basin (A to F).
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3. Results and discussion Table 1 summarises the γ-dose rate results and the surface soil natural radioactivity at the surveyed sites. The site referred to as “Area range” represents values measured in undisturbed soils within a 5 km radius from the power plants [4]. The γ-dose rate is elevated at site “A” – as expected in a deposit under development – reaching a local maximum of about 500 nSv h−1 . However, the situation at sites “B”, “C”, “D” and “E” is better and indeed similar to that of the neighbouring area, as reported in [4]. The undisturbed site “F” presents an in situ rate close to that of site “A”. This may be attributed to large quantities of naturally radioactive lignite lying very close to the surface soil. The gamma spectroscopic analysis shows that the surface soil at sites “B” and “C” has a 226 Ra content ranging from 110 to 320 Bq kg−1 , which is well in agreement with the surface soil survey results reported in [4]. According to the measurements reported in [5], the disposed fly ash has a 226 Ra content, which sometimes exceeds 1 kBq kg−1 . This, in combination with the results in Table 1, leads to the conclusion that all deposits (sites “B” and “C”) are well covered with soil. There is no point in such discussion for site “A”, where deposition works are currently in progress and the fly ash layer is not yet covered by soil. The natural radionuclide content of the 0–80 cm soil layer was also studied. Although it is expected that the fly ash layer underneath the surface should dominate the natural radioactivity vertical profile characteristics, the 226 Ra content at 60–80 cm depth does not reach the anticipated 1 kBq kg−1 . It ranges from 80 to 450 Bq kg−1 , which leads to the conclusion that the surface soil layer is adequately thick and well mixed with the top layer of the disposed fly ash. Cultivation, watering and agriculture might have contributed to the mixing. Table 2 summarises the radon-related survey results. No result at any deposit site (“A” to “E”) seems to exceed the area range values measured in the framework of study [4]. The same conclusion is drawn for the potential strip lignite mine site “F”. It is worth mentioning that 210 Pb radioactivity content analysis of the collected soil samples will allow useful discussion regarding radon exhalation from the surface of the deposition fields. It is expected that there should not be any significantly increased concentration of 210 Pb at the soil surface if compared to the relevant concentrations of the 0–80 cm soil layer profile. Such an increase would indicate an increased exhalation rate, which would not comply Table 1 γ-Dose rate at surveyed sites (nSv h−1 ) and surface soil natural radioactivity (Bq kg−1 ) Site
Dose rate
226 Ra
232 Th
40 K
A B C D E F
500 300 150 250 180 450
593 320 114 80∗ 80∗ 372
34 44 32 32∗ 32∗ 37
326 524 420 300∗ 300∗ 249
Area range [4]
60–330
26–372
24–41
154–477
∗ Estimated from cartographic representations.
Radioenvironmental survey of the Megalopolis power plants fly ash deposits
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Table 2 Radon survey results Site
Radon in air (Bq m−3 )
Radon in soil gas (kBq m−3 )
Radon exhalation rate (Bq m−2 s−1 )
A B C D E F
20 40 30–80 90 20 50
16 39 42 70–250 87 500
0.01 0.03 0.07 0.11 0.08 0.21
Area range [4]
ND∗ –850
4–500
ND–2
∗ ND – not detectable.
with the recorded results of Table 2. This analysis will be progressively completed in the near future.
4. Conclusion It is concluded that, from the natural radioactivity point of view, the exhausted lignite mines at Megalopolis, in the way that they are restored by fly ash deposition, do not significantly differ from the rest of the Megalopolis lignite field basin, despite the fact that most of the underground soil layers consist of fly ash with high 226 Ra content. A possible explanation for this might be that the radon emanation power from the fly ash layer is as low as the radon emanation power from natural soil due to a fly ash crystallisation process in the power plant furnace. This explanation is currently under experimental investigation in our laboratory. Furthermore, the comparison of results from different aged deposition fields indicates that ageing and restoration processes in the deposition fields, such as tree cultivation and agriculture, have a positive effect towards reducing radon related radiological parameters.
References [1] L. Miljac, M. Krizman, Radioactive contamination of surface waters from a fly-ash depository at Velenje (Slovenia), Environ. Int. 22 (S1) (1996) S339–S345. [2] E.M. Bem, H. Bem, P. Wieczorkowski, Studies of radionuclide concentrations in surface soil in and around fly ash disposal sites, Sci. Total Environ. 220 (1998) 215–222. [3] M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos, M.G. Angelopoulos, Natural radioactivity mapping of Greek surface soils, Environ. Int. 22 (S1) (1996) S3–S8. [4] P.K. Rouni, N.P. Petropoulos, M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos, Radioenvironmental survey of the Megalopolis lignite field basin, Sci. Total Environ. 272 (2001) 261–272. [5] S.E. Simopoulos, M.G. Angelopoulos, Natural radioactivity releases from lignite power plants in Greece, J. Environ. Radioact. 5 (1987) 379–389. [6] S.E. Simopoulos, Soil sampling and 137 Cs analysis of the Chernobyl fallout in Greece, Int. J. Radiat. Appl. Instrum. Part A 40 (1989) 607–613.
Radioenvironmental survey of the Megalopolis power plants fly ash deposits D.J. Karangelos, P.K. Rouni, N.P. Petropoulos, M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos Nuclear Engineering Section, Mechanical Engineering Department, National Technical University of Athens, 15780 Athens, Greece
The Megalopolis lignite field basin is located in the centre of the Peloponnesus peninsula in southern Greece. The lignite fired power plants in operation in this region have a total capacity of 850 MW and produce over 2 million metric tons of fly ash annually; this is primarily disposed of in deposits located mainly at exhausted lignite mines. The disposed fly ash has a 226 Ra content of about 1000 Bq kg−1 . An extensive research project for the determination of the natural radioactivity of lignite and ash from the Megalopolis power plants started in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) in 1983. The project has evolved to an integrated radioenvironmental survey of the Megalopolis lignite field basin area. The present work aims at the presentation of the radioenvironmental survey of the fly ash deposits in the vicinity of the power plants. Results regarding (a) γ-dose rate, (b) radon concentration in the ambient air, (c) soil radon exhalation rate and (d) soil gas radon concentration are reported from systematic field measurements at three deposit sites. In addition, soil sampling of the surface and 0–80 cm layer has been conducted at the deposits to allow for the determination of the activity of natural radionuclides, namely 226 Ra, 232 Th, 40 K, 234 Th and 210 Pb. The radiological characterisation of the fly-ash deposits obtained through these results is compared to the natural radioactivity background in the wider Megalopolis area, using the additional results of a previous similar survey. It can be concluded that, from the radiological point of view, the Megalopolis fly ash deposits do not differ significantly from the rest of the Megalopolis lignite field basin, despite the fact that most of the underground soil layers consist of fly ash with high 226 Ra content. 1. Introduction The increasing demand for electricity from coal-fired or lignite-fired power plants is associated with the production of large amounts of fly ash. The main fly ash disposal methods used are: RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07126-8
© 2005 Elsevier Ltd. All rights reserved.
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D.J. Karangelos et al.
(a) Fly ash water slurry transportation to an open disposal site using open water cycle systems. (b) Fly ash water slurry transportation to an open disposal site using closed water cycle systems. (c) Disposal of fly ash in sequestered landfills. (d) Disposal of fly ash in exhausted strip mine fields. Method (a) reportedly used in Velenje, Slovenia [1] strongly affects the natural radiation environment of the area in several ways, since the deposited ash has direct contact with lake water. Leaching of radionuclides from fly ash into lake and rainwater and pile drainage water are the main sources of radioactive contamination. Concentrations of 226 Ra in lake waters are reported to be at least one order of magnitude higher than in natural water bodies. In addition, concentrations of 226 Ra in the lake sediment and in the sediment of the outflow water are increased over levels found in natural streams. Furthermore, aerial transportation of fly ash from the surface of the deposit to the surrounding environment due to wind erosion may represent another potential contamination pathway. Method (b) is an improvement on method (a), inasmuch it reutilises the water of the circuit, minimising dumping of wet pollutants at the deposit, or in water ponds or even directly into the lakes and the water system of the area. Nevertheless, fly ash deposits of this kind need to be additionally covered with soil or other material to deal with pile drainage water or wind erosion. It is reported in [2] that method (c) is being used in Lodz, Poland. Landfill sites of capacity in the range 1 to 2 million metric tons are being exploited for this purpose, since no other suitable deposition site (e.g., exhausted coal strip mine field on the soil surface) is available in that area. The landfill sites, if constructed and managed poorly, can pose a radiological threat as a result of radionuclide migration to underground water, air and adjacent soil. The Lodz landfills examined in that study have been covered with soil and offered for recultivation. The concentrations of the natural radionuclides in and outside three such fly ash disposal sites are close to the typical values for that part of Poland. The Public Power Corporation of Greece (PPC) is typically using method (d), for the past 45 years at both the main Greek lignite field basins: Ptolemais in northern Greece and Megalopolis in southern Greece. The fly ash deposits are in the form of ash layers several meters thick, which are then covered by a layer of soil. The whole deposition process is a part of the exhausted lignite mines restoration project. The landfill like deposition at the exhausted mine is being managed continuously as part of the fuel cycle. To this end belt transporters and high capacity tracks are being used. Problems that might rise due to poor organisation and management of the whole process are minimised due to the inherent environmental precautions taken when excavating the lignite mine. The Megalopolis lignite field basin is located in the centre of the Peloponnesus peninsula in southern Greece. Two lignite-fired power plants are in operation in this region: MegalopolisA (550 MW – 3 units) since the early 1970s and Megalopolis-B (300 MW – 1 unit) since the early 1990s. The power plants produce over 2 million metric tons of fly ash annually. The disposed fly ash has a 226 Ra content which sometimes exceeds 1 kBq kg−1 , which is very high compared to the mean 226 Ra radioactivity of 25 Bq kg−1 in surface soils in Greece [3]. An extensive research project for the determination of the natural radioactivity of lignite and ash from the Megalopolis power plants started in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) in 1983. The project has evolved to an
Radioenvironmental survey of the Megalopolis power plants fly ash deposits
1027
integrated radioenvironmental survey of the Megalopolis lignite field basin area. The present work focuses on the presentation of the radioenvironmental survey of the fly ash deposits in the vicinity of the power plants. Results regarding (a) γ-dose rate, (b) radon concentration in the ambient air, (c) soil radon exhalation rate and (d) soil gas radon concentration are reported from systematic field measurements at three deposit sites. In addition, soil sampling has been conducted at the deposits to allow for the determination of the activity of natural radionuclides, namely 226 Ra, 232 Th and 40 K. The radiological characterisation of the fly ash deposits obtained through these results is compared to the natural radioactivity background in the wider Megalopolis area, using the results of previous surveys on undisturbed natural soils [4].
2. Methods Measurements were carried out at three deposit sites “A”, “B” and “C” as presented in Fig. 1. Site “A” is a deposit under development, site “B” – created in the 1990s – is a deposit used for livestock cultivation and site “C” – created in the 1980s – is now a well-organised walnut tree plantation. Measurements were also carried out at sites “D”, “E” and “F”. Sites “D” and “E” are located on the borders between the deposit soil and natural undisturbed soil. Site “F” is located in a potential strip lignite mine; mining at this site has not commenced yet. All sites “A” to “F” are located within a 5 km radius of the power plants. In situ γ-dose rates at the investigated sites were measured 1 m above the ground using a portable NaI dose rate meter. Radon concentration in the ambient air were measured using various types of active instrumentation. Surface soil radon exhalation rate measurements and soil gas radon concentration measurements were conducted using appropriate instrumentation. Samples from the surface soil and from the 0–80 cm soil layer profile were analysed for natural radioactivity, in the laboratory, using high-resolution gamma spectroscopy. The soil sampling techniques and the gamma spectroscopic methods employed have been extensively reviewed elsewhere [6].
Fig. 1. Investigated sites at and nearby fly ash deposits at the Megalopolis lignite field basin (A to F).
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D.J. Karangelos et al.
3. Results and discussion Table 1 summarises the γ-dose rate results and the surface soil natural radioactivity at the surveyed sites. The site referred to as “Area range” represents values measured in undisturbed soils within a 5 km radius from the power plants [4]. The γ-dose rate is elevated at site “A” – as expected in a deposit under development – reaching a local maximum of about 500 nSv h−1 . However, the situation at sites “B”, “C”, “D” and “E” is better and indeed similar to that of the neighbouring area, as reported in [4]. The undisturbed site “F” presents an in situ rate close to that of site “A”. This may be attributed to large quantities of naturally radioactive lignite lying very close to the surface soil. The gamma spectroscopic analysis shows that the surface soil at sites “B” and “C” has a 226 Ra content ranging from 110 to 320 Bq kg−1 , which is well in agreement with the surface soil survey results reported in [4]. According to the measurements reported in [5], the disposed fly ash has a 226 Ra content, which sometimes exceeds 1 kBq kg−1 . This, in combination with the results in Table 1, leads to the conclusion that all deposits (sites “B” and “C”) are well covered with soil. There is no point in such discussion for site “A”, where deposition works are currently in progress and the fly ash layer is not yet covered by soil. The natural radionuclide content of the 0–80 cm soil layer was also studied. Although it is expected that the fly ash layer underneath the surface should dominate the natural radioactivity vertical profile characteristics, the 226 Ra content at 60–80 cm depth does not reach the anticipated 1 kBq kg−1 . It ranges from 80 to 450 Bq kg−1 , which leads to the conclusion that the surface soil layer is adequately thick and well mixed with the top layer of the disposed fly ash. Cultivation, watering and agriculture might have contributed to the mixing. Table 2 summarises the radon-related survey results. No result at any deposit site (“A” to “E”) seems to exceed the area range values measured in the framework of study [4]. The same conclusion is drawn for the potential strip lignite mine site “F”. It is worth mentioning that 210 Pb radioactivity content analysis of the collected soil samples will allow useful discussion regarding radon exhalation from the surface of the deposition fields. It is expected that there should not be any significantly increased concentration of 210 Pb at the soil surface if compared to the relevant concentrations of the 0–80 cm soil layer profile. Such an increase would indicate an increased exhalation rate, which would not comply Table 1 γ-Dose rate at surveyed sites (nSv h−1 ) and surface soil natural radioactivity (Bq kg−1 ) Site
Dose rate
226 Ra
232 Th
40 K
A B C D E F
500 300 150 250 180 450
593 320 114 80∗ 80∗ 372
34 44 32 32∗ 32∗ 37
326 524 420 300∗ 300∗ 249
Area range [4]
60–330
26–372
24–41
154–477
∗ Estimated from cartographic representations.
Radioenvironmental survey of the Megalopolis power plants fly ash deposits
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Table 2 Radon survey results Site
Radon in air (Bq m−3 )
Radon in soil gas (kBq m−3 )
Radon exhalation rate (Bq m−2 s−1 )
A B C D E F
20 40 30–80 90 20 50
16 39 42 70–250 87 500
0.01 0.03 0.07 0.11 0.08 0.21
Area range [4]
ND∗ –850
4–500
ND–2
∗ ND – not detectable.
with the recorded results of Table 2. This analysis will be progressively completed in the near future.
4. Conclusion It is concluded that, from the natural radioactivity point of view, the exhausted lignite mines at Megalopolis, in the way that they are restored by fly ash deposition, do not significantly differ from the rest of the Megalopolis lignite field basin, despite the fact that most of the underground soil layers consist of fly ash with high 226 Ra content. A possible explanation for this might be that the radon emanation power from the fly ash layer is as low as the radon emanation power from natural soil due to a fly ash crystallisation process in the power plant furnace. This explanation is currently under experimental investigation in our laboratory. Furthermore, the comparison of results from different aged deposition fields indicates that ageing and restoration processes in the deposition fields, such as tree cultivation and agriculture, have a positive effect towards reducing radon related radiological parameters.
References [1] L. Miljac, M. Krizman, Radioactive contamination of surface waters from a fly-ash depository at Velenje (Slovenia), Environ. Int. 22 (S1) (1996) S339–S345. [2] E.M. Bem, H. Bem, P. Wieczorkowski, Studies of radionuclide concentrations in surface soil in and around fly ash disposal sites, Sci. Total Environ. 220 (1998) 215–222. [3] M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos, M.G. Angelopoulos, Natural radioactivity mapping of Greek surface soils, Environ. Int. 22 (S1) (1996) S3–S8. [4] P.K. Rouni, N.P. Petropoulos, M.J. Anagnostakis, E.P. Hinis, S.E. Simopoulos, Radioenvironmental survey of the Megalopolis lignite field basin, Sci. Total Environ. 272 (2001) 261–272. [5] S.E. Simopoulos, M.G. Angelopoulos, Natural radioactivity releases from lignite power plants in Greece, J. Environ. Radioact. 5 (1987) 379–389. [6] S.E. Simopoulos, Soil sampling and 137 Cs analysis of the Chernobyl fallout in Greece, Int. J. Radiat. Appl. Instrum. Part A 40 (1989) 607–613.