Environmental Gamma Dose Rate Around a Radioactive Waste Site in Central Mexico

Transcription

1 Environmental Gamma Dose Rate Around a Radioactive Waste Site in Central Mexico M.I. Gaso 1, N. Segovia 2, P.R. Gonzalez 1, F. Montes 1 1 Lab. de Vigilancia Radiológica Ambiental, ININ, Ap. Post , Mexico D.F., Mexico. migp@nuclear.inin.mx 2 IGFUNAM, Ciudad Universitaria, Mexico D.F., Mexico Abstract. The environmental gamma-dose rate has been determined at the Mexican Storage Centre for Radioactive Waste (SCRW) and surrounding communities, using thermoluminescent CaSO 4 :Dy dosemeters (TLD) to measure cumulative gamma dose rate. Soil samples were also analysed for 226 Ra, 235 U, 137 Cs and 40 K concentration activities, by low background gamma spectrometry. The outdoor gamma dose rate was higher at certain monitoring stations at the SCRW where industrial radioactive sources and uranium ore piles were stored in the past. Some nearby locations where the action of the rain on the piles promoted the transport and surface migration of 226 Ra, 235 U and 137 Cs also showed an increased gamma dose rate. The regional gamma dose rate (background) at distant communities was evaluated with data obtained along 10 years measurements. The additional dose rate due to the SCRW operation in a 200 m zone external to the site, was enhanced as compared with the regional natural background. 1. Introduction In recent years, exposure to enhanced natural radiation came into the focus of radiation protection. The measurement of environmental gamma dose rate is of great interest in radiation protection, radioecological studies and earth sciences. The most contaminating industries identified on the basis of the probability of occurrence of high levels of technologically enhanced naturally occurring radionuclide materials (TENORM) are: uranium mining and milling, metal mining and smelting and phosphate industry [1]. Uranium and thorium are present in the Earth s crust at average concentrations of 4.2 and 12.5 mg/kg, respectively and natural uranium is present in the environment, with typical specific activities in the range from 10 to 30 Bq/kg. Natural world average concentration of 40 K, 226 Ra, 232 Th, and 238 U in the soil are 400, 40, 25 and 25 Bq/kg respectively. 226 Ra together with 40 K are the main naturally occurring radionuclides which enter vegetable foods, thereby becoming a source of internal radiation to man and animals [2, 3]. Activity limits by which States with TENORM regulations have exempted solid materials include 185 Bq/kg for 226 Ra [4]. However, Kraus [5] indicate that a primary level of effective dose of 1 msv/y as a consequence of uranium mining was given as an acceptable dose limit in Germany. Secondary levels for the specific activity in soil are derived with 200 Bq/kg for unrestricted use and 1000 Bq/kg for restricted use, where the radionuclide of the 238 U-series with the highest specific activity is decisive. The present study is a review of activity concentrations of radionuclides in the soil and of outdoor gamma dose rate from sources of natural and artificial radiation around the Mexican Storage Centre for Radioactive Waste (SCRW). Derived intervention level adopted by the National Regulatory Commission (CNSNS) for the SCRW is 130 Bq/kg for 226 Ra. At this site, solid and liquid radioactive waste have been stored during four decades and uranium ore tailing piles also stood for some time before being buried in trench and specific containers. A local contamination in the soil with 137 Cs also occurred at the site in the decade of 1970 as a consequence of a broken industrial source [6]. Intervention and decontamination actions, with removal of the upper soil layer, were performed at the site from 1993 to An environmental monitoring program has been systematically conducted by the Nuclear Research National Institute at the SCRW and surrounding communities. Several kinds of samples including observations of changes in concentrations of 226 Ra, 235 U, 137 Cs and 40 K, in the soil and bioindicator samples, have been collected and analysed in this period [7, 8, 9]. In the present paper 1

2 the gamma absorbed dose rates in air were determined in order to estimate the potential annual effective doses to the public and workers. The environmental gamma dose rate enhancement due to the SCRW operation was also calculated. 2. Materials and Methods 2.1. Radioactivity measurements The monitoring of the gamma dose rate was carried out with passive devices (thermoluminescent dosemeters (TLD)) and with electronic detectors (high pressure ionization chamber (HPIC) and portable window spectrometer). Dose rate in air, due to radionuclides in the soil, was calculated from the activity concentration in the soil samples, measured with a high purity germanium (HPGe) gamma spectrometry system. The sampling period considered in the present study include 1991 to Thermoluminescent dosemeters (TLD) In situ measurements of gamma exposure rate were performed using passive integrating TLD s of CaSO 4 : Dy +PTFE, developed locally by Azorin and Gutierrez [10]. These TLD s fulfill the ANSI-N- 545 code for environmental monitoring. Each dosimetric-plastic package contains two pellets, that were placed 1 m above the earth surface, and exposed for three periods of 4 months each year. TLD s were read-out in a Harshaw analyzer Model 4000 coupled to a PC; nitrogen gas was allowed to flow into the reader during read-out to avoid any spurious signals. The outdoor exposure rates measured include the contribution from cosmic and terrestrial radiation. The absorbed dose rate (ngy/h) was calculated and the coefficient of 0.8 Sv/Gy has been used to convert the absorbed dose due to gamma radiation in air to effective dose [11] High pressurized ionization chamber (HPIC) The ionization chamber used is a Reuter-Stokes type (model RSS-112). The energy response varies within 20% between 60 kev and 2 MeV and the uncertainty on the measured value is about 10%. The dose rate was measured at 1 m above the soil surface. The integration time chosen for dose rate measurements was from 1200 s to 1800 s Portable window spectrometer A SCINTREX gamma-ray integrating spectrometer, Model GIS-5 was used during 2002 at SCRW and at Zone V. The spectrometer is a NaI (Tl) detector, 82 cm 3 in volume. Calibration of the instrument was performed with a ThO 2 source supplied with the equipment. The detector was placed at 1 m above the soil surface for each sampling point (each 50 m at the SCRW surface). The accumulated counts with an acquisition time of 30 s were recorded Gamma-ray spectrometry and dose evaluation from the soil activity The soil samples were analysed for 226 Ra, 235 U, 137 Cs and 40 K specific activities, with a HPGe detector having 29.4 % relative efficiency. The soil samples were dryed at 105 C to constant weight or dry weight (d.w.), before analysing in the laboratory. The relationships between the concentrations of a radioelement in a dried and an undried soil sample is given by [12]: ( w) cd = c 1 + (1) w 2

3 where c d c w w concentration of a radioelement in a dried soil sample; concentration of a radioelement in an undried soil sample; is the fractional moisture content defined as the ratio of the mass of water to the mass of dry material; The soil samples were sieved through a mesh sieve and aliquots of g were kept in a 500 ml Marinelli beaker for a period of more than 21 days to allow radium and radon to reach the equilibrium. The measurement time was from 1000 to s and appropriate standard mixtures of γ-rays emitting isotopes were used to calibrate the detectors [13]. 235 U is measured through its very abundant gamma-ray line at 186 kev, calculating the radium contribution to the 186 kev radiation, using the observed gamma rays of the 214 chain and subtracting that contribution from the total, to obtain the part due to 235 U [14]. The gamma dose-rate per unit activity concentration of terrestrial radionuclides has been calculated using UNSCEAR [15] and Tosheva et al. [16]: where D = 0.043A A A A K Th Ra 137 Cs γ (2) D γ A i the gamma radiation dose (ngy/h); the activity concentration of the i th radionuclide (Bq/kg); The soil values of 232 Th in the present paper were estimated from the measured 226 Ra in regional background samples. Disequilibrium in the 232 Th decay series is rarely encountered in nature. 232 Th can be estimated from 226 Ra concentrations in soils, unless the soils have been chemically treated or recently disturbed [14] The site In the middle part of the Mexican Neovolcanic belt, the SCRW site (19 47' 39" N; 98 50' 04" W) is located at the State of Mexico, Mexico at altitudes between 2470 and 2490 m (FIG.1). The climate is temperate sub-humid with an average yearly temperature of 14 C, ranging from 6 to 32 C. The mean annual precipitation is mm, mainly during the rainy season, from June to October. The local vegetation has species such as Opuntia megacantha, Agave atrovirens, Schinus molle and Senecio salignus [9]. The sampling points from SCRW are: Zone I, having an area of 7372 m 2, correspond to the region where the ore was piled and the 137 Cs contamination occurred; Zone II (4500 m 2 ), at a lowest altitude than Zone I and was a private property until 1993, but the natural transport due to rain water of contaminated soil urged decommission and payment of the zone and its inclusion as part of the SCRW; Zone III (3000 m 2 ), at the bottom of the slope, was the place where rain water accumulated previous to leave the SCRW and Zone IV ( m 2 ), includes all the other sampling points inside the SCRW. Three sampling points are also located in a 200 m zone external to the site at the border line (Zone V, m 2 ), it is a private property used for local agricultural practices. Four distant communities in a 10 km radius around the site correspond to the regional background (Zone VI). A geographical information system (GIS) has been developed for the purpose of the operation of the SCRW. The place of every point is determined accurately by means of the fine global positioning system (GPS) with a Garmin GPS 12-MAP. The radiometric data are collected and stored in a data base linked with the geographical information system using the ARC VIEW software. 3

4 FIG. 1. Location of the Mexican Storage Centre for Radioactive Waste (SCRW). Zones I, II, III and IV inside the SCRW; Zone V at the border line to the SCRW. :predominant superficial streams. Count rates (counts/s) obtained in 2002 with the SCINTREX gamma-ray spectrometer. 3. Results and Discussion In Table I, the average absorbed dose rates and annual effective doses estimated at Zone VI and at SCRW, obtained with TLD from 1991 to 2002, are shown. The average background (Zone VI) absorbed dose rate was 84±5 ngy/h (range: ngy/h) and the annual effective doses to the public varied from 0.46 to 0.67 msv/y, with an average value of 0.59±0.04 msv/y. At the SCRW (Zones I, II, III and IV) the total absorbed dose rate average value was 145±8 ngy/h, (range: ngy/h), twice as compared with the regional background. Table I. Average absorbed dose rates ±error (ngy/h) and effective dose ±error (msv/y), at distant surrounding communities (Zone VI) and at SCRW (Zones I to IV), from 1991 to Site N Latitude Altitude (m) Dose rate Effective dose Average Range Average Range VI ± ± I ± ± II ± ± III ± ± IV ± ± SCRW ± ± Generally, cosmic radiation increases with altitude and magnetic latitude [17]. Previously reported data from localities belonging to the Neovolcanic Mexican belt, at different altitudes [18], indicated than the average absorbed dose rates ranged between 69 and 104 ngy/h, showing a relation with the altitude. Data obtained in the present study for Zone VI, 10 km from the SCRW, were in agreement with these results. 4

5 In Table II, the activity concentrations in the soil for 226 Ra, 137 Cs and 40 K, at distant surrounding communities (Zone VI), the outdoor absorbed dose rate from terrestrial gamma exposure (ngy/h), and the annual effective dose (msv/y) are shown. For 226 Ra, the values ranged from 12 to 57 Bq/ kg (d.w.). The total average value was Bq/kg (d.w.), equivalent to 23 Bq/kg (w.w.), showing stable values along the time. The values are slightly lower as compared with the reported world average [3]. 137 Cs average value was 3.09 (range from 1 to 10) Bq/kg (d.w.), indicating a slight decrease along the time, from long term fallout. 40 K average value was 303 (range from 187 to 435) Bq/kg (d.w.), in agreement with the world average. Table II. Average activity concentrations ± error (Bq/kg (d. w.)) and range of radionuclides in the soil; dose rate (ngy/h) and effective dose (msv/y) at Zone VI. Year N w Activity concentrations in soil Dose Dose 226 Ra 137 Cs 40 K rate 1 Average Range Average Range Average Range ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± : estimated in wet weight, by the equations (1) and (2) The total average absorbed dose rate and effective dose, were 36 ngy/h and 0.32 msv/y respectively. The results obtained confirm a normal regional outdoor gamma dose rate in air corresponding to a low level of natural radioactivity in the soil. The average absorbed dose rate from cosmic radiation was estimated by sustracting the regional dose rate due to terrestrial gamma exposure from the average background dose rate: = 42 ngy/h. This value is slightly higher than the average value (39.6 ngy/h) reported in Sichuan (1000 m altitude and 32 latitude) [17] and is higher than the value reported for the cosmic dose rate (32 ngy/h) measured over the Caspian Sea [19]. In Table III, the average activity concentrations and range values obtained for 226 Ra, 137 Cs, and 40 K, in the soil ( ) at SCRW (zones (I+II), III and IV) are indicated. Zone II, has a lower altitude than Zone I and rain water flows through the natural slopes of the terrain (FIG. 1). Due to the intervention and decontamination actions started in 1993 at the contaminated Zones I and II, 226 Ra average activity concentrations diminished almost 40 % until During 2000 and 2001, 400 m 3 of contaminated soil were removed from Zone II, confined in mobile containers and transported to another site. At zones (I+II), the average 226 Ra activity concentration diminished 75% from ( ) to 2002, while 137 Cs decreased up to 90% in the soil. At Zone III, 226 Ra and 137 Cs concentrations showed stability since 1996 with a small increase starting in This behavior indicates a recent inclusion due probably to superficial streams during the rainy season that, in 1998, were quite important due to heavy rains. In Zone IV all the additional sampling points of the site were included. 226 Ra concentration values were similar to those from the regional background, even if the extreme values were higher than this regional background. 137 Cs values at this Zone were significantly higher than the regional background ones. 5

6 Table III. Average activity concentrations ±error (Bq/kg (d. w.)) and range of radionuclides in the soil; average absorbed dose rate (ngy/h) and effective dose (msv/y) at SCRW. Year N w 226 Ra 137 Cs 40 K Dose Dose Average Range Average Range Average Range rate 1 Zone I+II ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Zone III ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Zone IV ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± : estimated in wet weight, by the equations (1) and (2) 6

7 40 K average values were similar in all the zones and constant along the time, being slightly higher at Zone (I+II) than at Zones III and IV, indicating probably the use, before 1993, of fertilizers in the soil at Zone II. At Zone (I+II), the average effective dose decreased from msv/y in ( ) to 2.86 msv/y in 2002 as a consequence of decontamination actions (Table III). At Zone IV, the average effective dose value was similar to those from Zone VI. The isotopic abundance ratio (by weight) of natural uranium is: 235 U/ 238 U = 0.72%. If the ratio of activities is considered, the value is: 235 U/ 238 U = 4.66%. The presence of depleted uranium (DU) can be assessed from the uranium isotopic ratio 235 U/ 238 U. For DU, the typical value of that ratio by weight is 0.20%, while the corresponding activities ratio is 1.29% [2]. If secular equilibrium conditions are supposed ( 238 U 226 Ra), the 235 U/ 226 Ra isotopic ratio could be used to estimate the possible presence of depleted uranium at SCRW. At Zone (I+II), this isotopic ratio ranged between (1.2%) and (4.3%) from 1991 to 2002, with an average value of 2.2%. At Zone II, the lowest isotopic ratio value (1.2%) measured in 1996, was similar to the reported by Magnoni et al. [2] as the typical value for depleted uranium (1.29%). Determinations of the average activity concentrations of 226 Ra, 137 Cs and 40 K in the surface soil of Zone V (200 m zone external to the site) are shown in Table IV. The results indicate that the soils have a significant content of 226 Ra and 137 Cs. These results sustain the assumption that the radioactive substances were transported into the soil with waste residues (tailings) and radiocesium industrial source and their migration in horizontal directions was caused mainly in dissolved form of the solid components. Table IV. Average activity concentrations ±error (Bq/kg (d. w.)) and ranges of radionuclides in the soil; average absorbed dose rate (ngy/h), and effective dose (msv/y) at Zone V. Year N w Activity concentrations in soil Dose Dose 226 Ra 137 Cs 40 K rate 1 Av. Range Av. Range Av. Range ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± : estimated in wet weight, by the equations (1) and (2) This contamination probably occurred when the ore tailings and the broken industrial source were deposited in the decade of The origin of the additional 226 Ra and 137 Cs concentrations could also be due to small punctual contaminations in the past from the container trench, located nearby the sampling points of this zone (FIG. 1). The concentration values have been stable along the years due essentially to the fact that no decontamination activities have been performed at this zone. During , the average 226 Ra concentration in the soil of the Zone V, was higher than the derived intervention level (130 Bq/kg) fixed for the studied facility. The maximum value (237 Bq/kg) obtained in the period , was higher than the limits adopted internationally to exempt solid materials or as screening level for the unrestricted use (185 and 200 Bq/kg respectively). 7

8 The correlation of the absorbed dose-rate (ngy/h) measurements performed with TLD s from (once subtracted 42 ngy/h from cosmic radiation), with the same number of data (N=78) for 226 Ra activity concentration in the soil samples (Bq/kg (w.w.)), is depicted in FIG. 2. The total absorbed dose rate in air ranged from 18 to 475 ngy/h. The highest values were obtained in 1991 from some hot spots in the soil at Zone I. The results show that the absorbed dose-rate (D γ ) values were strongly correlated with the measured activities of 226 Ra (r=0.95) in the soil. The regression equation can be expressed as: D ( / ) ( ) ( ) 226 γ ngy h = ± + ± Ra ( Bq / kg) ( w. w.) (3) Gamma dose rate in air (ngy/h) r = Ra activity concentrations in soil (Bq/kg) (w.w.) FIG. 2. Linear model to describe the relationship between 226 Ra in the soil and gamma dose rate from terrestrial radionuclides. It is worth mentioning that the largest value of the total dose rate, measured during 1991 at Zone I, with a Reuter-Stokes pressurized ionization chamber (HPIC), was: 773 ± 43.5 ngy/h (equivalent to 731± 40 ngy/h when cosmic radiation was subtracted). For TLD the maximum value obtained at the same zone (517 ngy/h = ngy/h) was lower, by a factor of 0.67, than the value measured with the HPIC. That difference could be due to the different energy responses, which, for the HPIC, is larger than that for the TLD dosemeters in the energy range from 60 to 300 kev. The results are in agreement with those reported by Losana et al.[20], who obtained a factor of 0.77 for the dose rate measurements performed in Turin with TLD s GR-200 of LiF: Mg,Cu,P and with a Reuter-Stokes HPIC. At Zone V, when the average value from gamma dose rate (67 ngy/h) obtained with the equation (2) is added to that of cosmic rays (42 ngy/h), the dose rate becomes 109 ngy/h. This last value was similar to the average value measured during 2001 and 2002 with the HPIC (103 ngy/h). If we consider the 226 Ra average concentration value in the soil (118 Bq/kg (d.w.)), equivalent to 103 Bq/kg (w.w.) (Table IV) and using equation (3), the gamma dose rate in air will be ngy/h (110 ngy/h when cosmic radiation is added). The average additional annual effective dose estimated at Zone V, due to the SCRW operation from , was 0.27±29% msv/y. The count rate values (N=90), obtained with the SCINTREX gamma-ray spectrometer inside and at the border line to the SCRW, ranged between 17 counts/s at Zone IV and 613 counts/s at Zone II. 8

9 The results are reported in the FIG. 1, and give an illustrative picture of the actual distribution of gamma-ray count rates at SCRW (Zones I to IV) and at Zone V. At Zone IV, the average count rate (N=62) was 26 counts/s and the relation between the average count rate and the equivalent concentration of 226 Ra in the soil was: 1 count per second = Ra Bq/kg (w.w.). 4. Conclusion Results showed that distant communities (Zone VI), and some sampling points located at Zones IV (inside the SCRW), can be considered as background zones with normal natural radionuclide concentrations. At Zone V, the maximum 226 Ra concentration value (237 Bq/kg) was higher than the limit adopted internationally to exempt solid materials (185 Bq/kg), and the average 226 Ra concentration values obtained during were higher than the derived intervention level (130 Bq/kg) established by the National Regulatory Commission (CNSNS) for the SCRW. Environmental absorbed dose rate were estimated by site-specific modelling and the results obtained with the different techniques are in good agreement. The new approach of evaluation of dose based on the regression equation obtained is particularly useful for Zone V, since it is a private property closed to systematic monitoring. The gamma dose-rate values did not change significantly in time and the contamination of the soil at Zone V represents a long-term problem even after intervention and decontamination actions inside the SCRW. The average annual external dose from terrestrial gamma exposure at Zone V (0.59 msv/y), is about two times higher than that from background. The average additional external dose (0.27 msv/y), obtained after deduction of the background, is about 30% of the dose limit (1 msv/y), applied to a former uranium mining and milling site for deciding on the necessity of restoration. Agricultural use of surrounding areas could cause additional contributions of radiation exposure by ingestion to the local population. Also the contribution from radon inhalation and its progeny should be considered. Although the estimates presented in this study indicate that there is little health risk from the TENORM that may be encountered by the population, the authors suggest that the CNSNS continue to alert about health and safety of the site workers employed in uranium ore tailing and radioactive waste removing. Acknowledgements The authors acknowledge E. Quintero, I.A. Farraz, L. Cervantes, G. Valentin, R. Benitez and V. Rojas for technical assistance. References 1. Vandenhove, H., European sites contaminated by residues from the ore extracting and processing industries, in High Levels of Natural Radiation and Radon Areas: Radiation Dose and Health Effects, edited by W. Burkart, M. Sohrabi, A. Bayer, Elsevier, Amsterdam, (2002) 2. Magnoni, M., Bertino, S., Belloto, B., Campi, M., Variations of the isotopic ratios of uranium in environmental samples containing traces of depleted uranium: theoretical and experimental aspects. Radiat. Prot. Dosim., 97: , (2001). 3. Van der Stricht, E., Kirchmann, R., Radioecology, Radioactivity & Ecosystems, FORTEMPS, Liège (2001). 4. Fisher, R.B., Easty, D.B., Results of surveys at United States pulp and paper mills for the presence of scales and precipitates containing naturally occurring radioactive material (NORM). Health Phys., 84: , (2003). 5. Kraus, W., Protection against enhanced levels of natural radiation: concepts and regulatory approaches, in High Levels of Natural Radiation and Radon Areas: Radiation Dose and Health Effects, edited by W. Burkart, M. Sohrabi, A. Bayer, Elsevier, Amsterdam, (2002) 9

10 6. Gaso, M.I., Segovia, N., Cervantes, L., Salazar, S., Land snails as bioindicators of soil 137 Cs and 226 Ra contamination, in Environmental Radiochemical Analysis, edited by G.W.A. Newton, The Royal Society of Chemistry, Cambridge (1999) 7. Gaso, M.I., Segovia, N., Morton, O., Quintero, E., in Proceedings of the International Symposium on In situ Nuclear Metrology as a Tool for Radioecology INSINUME, Fleurus, Belgica, 2002, edited by the Institut National des Radioéléments (IRE) and the International Atomic Energy Agency, Fleurus, 2002a, p Gaso, M.I., Segovia, N., Morton, O., In situ biological monitoring of radioactivity and metal pollution in terrestrial snails Helix aspersa from a semiarid ecosystem. Radioprot. Colloques., 37: , (2002b). 9. Gaso, M.I., Segovia, N., Morton, O., Armienta, M.A., Biological monitoring of radioactivity and metal pollution in edible eggs of Liometopum apiculatum (ants) from a radioactive waste site in Central Mexico, in Environmental Radiochemical Analysis II, edited by P. Warwick, The Royal Society of Chemistry, Cambridge (2003) 10. Azorin, J., Gutierrez, A., Preparation and performance of a CaSO 4 :Dy,Tm thermoluminescent Phosphor for long-term gamma measurements. Health Phys., 56: , (1989). 11. National Council on Radiation Protection and Measurements, Recommended screening limits for contaminated surface soil and review of factors relevant to site-specific studies, Publication No. 129, NCRP, Bethesda, MD (1999). 12. International Atomic Energy Agency, Construction and use of calibration facilities for radiometric field equipment, Technical Report Series No. 309, IAEA, Vienna (1989). 13. Quintero, E., Lopez, H., Cervantes, M.L., Gamma and beta background counting of I-131 in milk in the vicinity of the Nuclear Centre, Mexico. J. Radioanal. Nucl. Chem. Lett., 214: , (1996). 14. Harbottle, G., Evans, C.V., Gamma-ray methods for determining natural and anthropogenic radionuclides in environmental and soil science. Radioac. & Radiochem., 8: 38-46, (1997). 15. United Nation Scientific Committee on the Effects of Atomic Radiation. Report to the General Assembly, United Nations Annex B; 1982: 95. See also report, Annex A, UNSCEAR, New York (1993). 16. Tosheva, Z., Nikolchev, I., Shishenkov, M., in Proceedings of the International Symposium on In situ Nuclear Metrology as a Tool for Radioecology INSINUME, Fleurus, Belgica, 2002, edited by the Institut National des Radioéléments (IRE) and the International Atomic Energy Agency, Fleurus, 2002, p Wang, Z., Natural radiation environment in China, in High Levels of Natural Radiation and Radon Areas: Radiation Dose and Health Effects, edited by W. Burkart, M. Sohrabi, A. Bayer, Elsevier, Amsterdam, (2002) 18. Gonzalez, P.R., Azorin, J., Martinez, T., in Proceedings of the XII Annual Symposium of Mexican Society of Radiological Safety, Zacatecas, 2001, edited by the Mexican Society of Radiological Safety (SMR), 2001, p Sohrabi, M., Esmaili, A.R., New public dose assessment of elevated natural radiation areas of Ramsar (Iran) for epidemiological studies, in High Levels of Natural Radiation and Radon Areas: Radiation Dose and Health Effects, edited by W. Burkart, M. Sohrabi, A. Bayer, Elsevier, Amsterdam, (2002) 20. Losana, M.C., Magnoni, M., Righino, F., Comparison of different methods for the assessment of the environmental gamma dose. Radiat. Prot. Dosim., 97: , (2001). 10