Natural radioactivity measurements and dose calculations to the public: Case of the uranium-bearing region of Poli in Cameroon

Radiation Measurements 46 (2011) 254e260

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Natural radioactivity measurements and dose calculations to the public: Case of the uranium-bearing region of Poli in Cameroon Saïdou a, b, c, *, François O. Bochud a, Sébastien Baechler a, Kwato Njock Moïse d, Ngachin Merlin d, Pascal Froidevaux a a

University Institute for Radiation Physics, University Hospital and University of Lausanne, Grand-Pré 1, 1007 Lausanne, Switzerland Faculty of Science, University of Yaounde I, P.O. Box 812 Yaounde, Cameroon Nuclear Technology Section, Institute for Geological and Mining Research, P.O. Box 4110 Yaounde, Cameroon d Centre for Atomic Molecular Physics and Quantum Optics, University of Douala, P.O. Box 8580 Douala, Cameroon b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 November 2009 Received in revised form 21 August 2010 Accepted 3 November 2010

The objective of this work is to carry out a baseline study of the uranium-bearing region of Poli in which lies the uranium deposit of Kitongo, prior to its impending exploitation. This study required sampling soil, water and foodstuffs representative of the radioactivity exposure and food consumption patterns of the population of Poli. After sampling and radioactivity measurements were taken, our results indicated that the activities of natural series in soil and water samples are low. However, high levels of 210Po and 210 Pb in foodstuffs (vegetables) were discovered and elevated activities of 40K were observed in some soil samples. All components of the total dose were assessed and lead to an average value of 5.2 mSv/year, slightly higher than the average worldwide value of 2.4 mSv/year. Most of this dose is attributable to the ingestion dose caused by the high levels of 210Po and 210Pb contained in vegetables, food items which constitute an important part of the diet in Northern Cameroon. Consequently, bringing uranium ore from underground to the surface might lead to an increased dose for the population of Poli through a higher deposition of 222Rn decay products on leafy vegetables. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Baseline study Uranium mining Environmental radioactivity Alpha spectrometry Gamma spectrometry Radon Ingestion dose Inhalation dose External radiation dose

1. Introduction Naturally occurring radionuclides of terrestrial origin are present in various degrees in all media in the environment, including the human body itself. Geological and hydrogeological conditions can sometimes lead to their enrichment in the environment, creating uranium or thorium deposits over a geological time scale. All minerals and raw materials contain radionuclides of natural origin, of which the most important for the purposes of radiation protection are the radionuclides in the 238U and 232Th decay series and 40K. For most human activities involving minerals and raw materials, the levels of exposure to these radionuclides are not significantly greater than normal background levels. Such exposures, while having been the subject of much research, are not

* Corresponding author. Nuclear Technology Section, Institute for Geological and Mining Research, P.O. Box 4110 Yaounde, Cameroon. Tel.: þ237 74 17 44 73; fax: þ237 22 22 24 31. E-mail address: [email protected] ( Saïdou). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.11.009

of concern for radiation protection. However, certain activities can give rise to significantly enhanced exposures that may need to be controlled by regulation. It is for instance the case of the activities related to uranium mining. Material giving rise to these enhanced exposures has become known as naturally occurring radioactive material (NORM) (IAEA, 2008). Uranium deposits are exploited in many regions of the world and used as nuclear fuel after 235U enrichment. Their exploitation raises concerns related to waste management and environmental contamination by NORM. Site remediation after uranium mining and milling proves to be a major issue of radiological protection with a risk essentially associated with the daughter products of uranium. Many studies reveal the impact of uranium mining and milling in the environment (Vandenhove et al., 2006; Martin et al., 2004; Gorjanacz et al., 2006; Uzunov et al., 1992; Vaupotic and Kobal, 1999; Veska and Eaton., 1991; Carvalho et al., 2007; Winkelmann et al., 2001). However, this impact cannot be well established without performing natural radioactivity measurements onsite and in the vicinity of the site prior to mining operations.

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Fig. 1. Sampling strategy for the uranium-bearing region of Poli including the town of Poli and the Kitongo deposit. The points A, B, C and D circumscribe area (6,710 km2) where the German Federal Institute for Geosciences and Natural Resources (BGR) undertook an aerial survey [Gehnes and Thoste, 1981; Oesterlen, 1985; Thoste, 1985].

Since 1950 many geological studies for the prospecting and assessment of the uranium potential of the Kitongo deposit, situated in the region of Poli in Cameroon, have been conducted (Gehnes and Thoste, 1981; Thoste, 1985; Oesterlen, 1985). Although the project of exploiting the site is about to become a reality (Meadon, 2006), no study on the radiological impact of this deposit has yet been undertaken. Therefore, the main objective of the present work is to carry out the first part of a baseline study of the uranium-bearing region of Poli. This study requires sampling soil, water and foodstuffs, which are representative of radioactivity exposure pathways and food consumption patterns of the surrounding population. Measuring the radioactivity of these samples enables us to assess the annual dose received by the population from both external sources and ingested food. In this context, a procedure for measuring U and Th isotopes using alpha spectrometry has been specially developed for this study and already published elsewhere. Furthermore, knowing the high radiotoxicity of 210Pb and 210Po, two methods for measuring 210Po using alpha spectrometry and 210Pb using gamma spectrometry were developed and also published elsewhere (Saïdou et al., 2007). Finally, the indoor radon concentration was measured in a few dwellings in order to estimate the overall annual dose received by the public.

2. Materials and methods 2.1. Sampling Poli is situated at 490 m above sea level near the boundary of the high central plateau and the plain of northern Cameroon. The plains around Poli are typical savannah grassland with occasional trees. The Kitongo deposit is located in the northern part of Cameroon, some 15 km southeast of Poli. The Kitongo deposit could contain more than 13,000 t U3O8 at an average grade of 0.1% U3O8 (Oesterlen, 1985; Meadon, 2006). To carry out precise and reliable radioactivity measurements, sampling must be carefully performed in order to be representative of the entire site. The uranium-bearing area was zoned during a uranium prospecting run and corresponded to a surface of 6,710 km2 as illustrated in Fig. 1 (Gehnes and Thoste, 1981; Oesterlen, 1985; Thoste, 1985). In the present work, a surface of 144 km2 was sampled according to a square grid where soil samples were collected every 4 km. Each sample was collected from the top 5 cm of a 1 m2 area, and provided a dry mass of around 500 g. On one given location (Gata), a soil profile was sampled (0e5 cm, 5e10 cm, 10e15 cm, 15e20 cm and 20e25 cm of depth) to study the vertical distribution of the radioactivity. In total, 20 soil

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samples were collected. In addition, 10 water samples and 10 foodstuffs samples (vegetables, maize, groundnuts, beef, etc.) were collected in the small town of Poli and its surrounding area. The water samples were collected in rivers, wells and drinking fountains. All the soil and foodstuff samples gathered were dried, sieved (2 mm) and homogenized. 2.2. Radioactivity measurements 2.2.1. aspectrometry 2.2.1.1. Uranium, thorium isotopes and 210Po in environmental solid samples. A procedure for measuring U and Th isotopes was developed within the framework of this study and has been fully described previously (Saïdou et al., 2008). The procedure used for measuring 210Po based on microwave digestion under pressure for polonium solubilisation developed within the framework of this study has also been fully documented in a previous work (Saïdou et al., 2007). 2.2.1.2. Uranium isotopes, 226Ra and 210Po in water samples. Uranium measurements were based on thin films containing diphosphonate and sulfonate groups that have been shown to have the required selectivity for uranium (Surbeck, 2000). After 20 h of exposure inside an acidified and stirred 100 ml water sample, we obtained an overall yield of 80%. After drying at ambient temperatures, the exposed thin film was measured using a-spectrometry. To determine 226Ra, we exposed thin film containing MnO2 in a 100 ml water sample. After 6 h of exposure, 80% of the 226Ra was extracted (Surbeck, 2000). The thin film was dried at ambient temperatures and measured using a-spectrometry. To measure 210Po, we introduced a silver disk coated on one side with plastic rubber into a water sample weakly acidified and containing 50 mBq of the 209Po tracer, under magnetic stirring. 210Po and 209Po were spontaneously deposited on the disk. After 4 h of exposure at 60e80  C, we obtained an average yield greater than 80%. The silver disk, dried at ambient temperatures, was measured using a-spectrometry. The sources prepared through electrodeposition and spontaneous deposition were measured using a Passivated Implanted Planar Silicon (PIPS) detector with an active area of 450 mm2 in a Canberra Alpha Analyst Spectrometer. The source-detector distance was 5 mm for all measurements, giving an efficiency of 25%. Standard sources of 241Am and 239Pu were used for the energy and efficiency calibration of the detectors. 2.2.2. g-Spectrometry Gamma measurements were performed with a Canberra p-type HPGe well detector (GCW4523) with a total active volume of 206 cm3, a relative photopeak efficiency of 45%, and a resolution at 122 and 1332 keV of 1.24 and 1.93 keV, respectively. The associated electronics consisted of a Canberra preamplifier (model 2002 CSL) and AccuspecÒ acquisition device. Treatment of the data was carried out using GENIE 2000 software. The spectrometer was calibrated using a liquid solution of 241Am, 109Cd, 57Co, 139Ce, 137Cs, 88 Y and 60Co traceable to international standards and emitting g-rays in the energy range of 59e1836 keV. Coincidence-summing corrections for 88Y and 60Co were determined using Monte Carlo calculations (Décombaz et al., 1992). The self-absorption correction factors were also calculated using Monte Carlo simulation (Bochud et al., 2006). To measure 210Pb, we calibrated the HPGe detector efficiency at 46.5 keV, according to the sample mass for a cylindrical geometry. We then estimated the self-absorption correction experimentally, analytically and using Monte Carlo methods. The procedure is fully described in a previous publication (Saïdou et al., 2007).

2.2.3. Radon in dwellings The indoor radon concentration was measured in five dwellings located in the uranium-bearing region of Poli using E-PERM electret chamber detectors (Kotrappa et al., 1992). These detectors were exposed in dwellings for a three-month period. In dose calculations, the office and the classroom were considered as workplaces. 2.3. Dose to the public 2.3.1. External radiation dose The radiation exposure from external sources results from natural and artificial ground radiation as well as from cosmic background. For each soil sample point (Fig. 1), the corresponding ground radiation dose was calculated by multiplying the activity of each radionuclide present in the soil by its specific dose conversion coefficient (Eckerman and Leggett, 1996). Then, outdoor ground dose is calculated by averaging these obtained values. In order to combine indoor and outdoor dose rates to calculate total external dose, we used an indoor occupancy factor of 0.8, which implies that people spend 20% of the time outdoors. However, since the materials used in the construction of most of these buildings also contain radionuclides, we applied an average factor of 1.4 to take into account their contribution and estimate indoor dose rate (UNSCEAR, 2000; CPR, 2005). Exposure to cosmic rays is strongly dependent on altitude and weakly dependent on latitude (UNSCEAR, 2000). In the present work for the region of Poli, we used results of dose rate measurements carried out in Switzerland (Murith et al., 1986; CPR, 2005). The dose rate from photons and ionising components vary with latitude, but the variation is small. It is about 10% lower at the geomagnetic equator than at high latitudes (UNSCEAR, 2000). In order to combine indoor and outdoor dose rates to compute cosmic ray dose for the region of Poli, an indoor occupancy factor of 0.8 was assumed. Cosmic rays are also attenuated by 20% indoors (UNSCEAR, 2000). Estimates of cosmic ray dose rates at 0e2000 m use results published in (Murith et al., 1986; CPR, 2005; Schraube et al., 1999; Birattari et al., 1996).

EðzÞ ¼ Ec ð0Þe0:38z þ En ð0Þe0:78z

(1)

where z is the altitude in km, Ec(0) and En(0) are, respectively, the dose rates from the photon and ionising component and dose rate from the neutron component. Ec(0) and En(0) are the dose rates at sea level and are, respectively, 0.24 mSv/year and 0.066 mSv/year. 2.3.2. Inhalation dose The intake of radionuclides into the human body by inhalation is well described by the ICRP Human Respiratory Tract Model (ICRP Publication 66, 1994). Inhalation of radon daughter products is the main pathway for public exposure to natural radioactivity and is the only pathway that has been considered in this study. The inhalation dose conversion coefficient used in the present work is given in (ICRP Publication 65, 1993). Therefore, the inhalation dose for radon exposure was derived by multiplying the average concentration of radon by the dose conversion coefficient of 2.44  106 mSv/(Bqh/m3). 2.3.3. Ingestion dose The intake of radionuclides by ingestion is well described by the ICRP Human Alimentary Tract Model (ICRP Publication 30, 1979; ICRP Publication 100, 2006). All ingestion dose conversion coefficients used in the present work are given in ICRP Publications (ICRP Publication 67, 1994; ICRP Publication 69, 1995). In order to assess the ingestion dose of the uranium-bearing region of Poli, it was necessary to determine the daily activity

Saïdou et al. / Radiation Measurements 46 (2011) 254e260

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Table 1 Dietary pattern of the region of Poli. Meal

Daily consumption

Breakfast

50 g of flour (maize) 25 g groundnut paste 0.5 l of cooking water 800 g of flour (maize) 2 l of cooking water 100 g of beef 30 g of dry leaf powder (baobab or « lalo ») or dry gombo 20 g of white bean 2 l of drinking water

Lunch and dinner

intake per inhabitant for each ingested radionuclide. Thus knowing the alimentary habits of the population of this region, we elaborated the alimentary model illustrated in Table 1. To derive ingested dose, the ingested annual activity of each radionuclide was multiplied by its dose conversion coefficient. Finally, the total ingestion dose was obtained by summing the contributions of all radionuclides measured in the present work.

3. Results and discussion 3.1. Radioactivity in the soil Radioactivity was determined using a- and g-spectrometry in 20 soils sampled near the region of Poli, as illustrated in Fig. 1. All results are summarized in Fig. 2 and it is clear that the activity of 210 Po and 210Pb are higher compared to other members of the 238U series. This is the main reason which led us to verify their vertical distribution in five samples from the Gata sampling area (Fig. 1). The results show an exponential decrease of activity depending on sampling depth. Median activities of 210Po and 210Pb decrease up to their equilibrium value at around 20e25 cm deep. The enrichment of the soil surface (0e5 cm) in 210Po and 210Pb is explained by the deposition of radon daughter products coming from the disintegration of radon in the atmosphere after emanation. Maximal values of 221.6  9.6 Bq/kg and 250.2  46.6 Bq/kg were observed for 210Po and 210Pb, respectively, at the Badongo sampling area. A similar result (200 Bq/kg) was observed for 210Po and 210Pb around closed uranium mines in Portugal (Carvalho et al., 2007). 208Tl activity is lower than the equilibrium value due to the branching ratio (36%) of its mother product, 212Bi. Average activity observed in the world is 420 Bq/kg for 40K, 38 Bq/kg for 238U and 45 Bq/kg for 232Th (UNSCEAR, 2000). Activities of 40K measured in 20 soil samples, as illustrated in Fig. 2, lead to a median activity of 552  10 Bq/kg for a mean activity of 506  3 Bq/ kg and for a maximal value of 1124  27 Bq/kg at the Mont Tchegui sampling area. Measurements carried out by the German Federal Institute for Geosciences and Natural Resources (BGR) (Thoste,

Fig. 3. Box plot of the activity (Bq/kg) distribution in foodstuffs frequently consumed in the region of Poli.

1985) during an aerial survey of the area documented a high level of radioactivity in uranium, thorium and potassium at the eastern part of Mont Tchegui. Although the sampling conducted for the present work was not statistically representative for the area, our result confirms BGR’s conclusion for 40K. Moderately elevated activities of 40K could be explained by the fact that materials were sampled at granite intrusions which are naturally high in potassium. A similar study on the distribution of 226Ra, 232Th and 40K in the region of Rio Grande do Norte (Brazil) was conducted by Malanca (1996) and led to an average activity of 704 Bq/kg for 40K. An aerial survey of closed uranium mines in the former Eastern Germany (Winkelmann et al., 2001) drew out an average activity of 620 Bq/kg for 40K at the Ronneburg site, 740 Bq/kg at the Crossen site and 860 Bq/kg at the Seelingstädt site. Measurements of 40K around a Mexican storage center for radioactive waste (Gaso et al., 2005) documented an activity of 297 Bq/kg for 40K. Most of the results reported in the literature are higher than the average activity observed in the world. We can conclude that the above results do not provide enough information about environmental contamination after uranium mining because there is no correlation between uranium and potassium distributions in the environment. Quite simply, measurements of 40K are not a reliable indicator for assessing the environmental impact of uranium mining and milling. Fig. 2 illustrates median activities of 25  2 Bq/kg for 238U, 23.3  6.3 Bq/kg for 226Ra and 30.5  1.7 Bq/kg for 232Th. Measurements conducted in the region of Rio Grande do Norte (Malanca, 1996) reported an average activity of 29.2 Bq/kg for 226Ra and 47.8 Bq/kg for 232Th. An aerial survey undertaken at the uranium mines of former Eastern Germany (Winkelmann et al., 2001) recorded activities of 226Ra and 228Th, respectively, equal to 370 Bq/kg and 45 Bq/kg at the Ronneburg site; 1200 Bq/kg and 40 Bq/kg at the Crossen site and 470 Bq/kg and 57 Bq/kg at the Seelingstädt site. Measurements taken at the former uranium mines in Portugal (Carvalho et al., 2007) gave an average activity of 200 Bq/kg for 238U, 200 Bq/kg for 226Ra and 91 Bq/kg for 232Th. The above results taken from the literature concerning measurements conducted at abandoned uranium mines show that the radioactivity level of the region of Poli is low; this is consistent since the uranium mining and milling processes of the Kitongo deposits have not yet started. Finally, the fact that radioactivity is low in the Poli region does not infer the absence of a uranium deposit at Kitongo. The uranium ores, as evidenced by different aeroradiometric studies (Gehnes and Thoste, 1981; Thoste, 1985; Oesterlen, 1985), are probably deeply situated. 3.2. Radioactivity in foodstuffs

Fig. 2. Box plot of the activity (Bq/kg) distribution of 40K and natural series and 232Th in 16 soil samples of the uranium-bearing region of Poli.

235

U,

238

U

Natural radionuclides were determined using a- and g-spectrometry in foodstuffs, as illustrated in Fig. 3. Note that activities of

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Saïdou et al. / Radiation Measurements 46 (2011) 254e260 Table 2 Concentration of 222Rn in 5 dwellings located in the uranium-bearing region of Poli where E-perm dosimeters were exposed for 3 months period for 222Rn measurements. Exposure area

Concentration (Bq/m3)

Inn Room Classroom Living room Office

115 113 106 137 2000

    

19 18 19 18 100

40

K are high compared with the values observed for the other radionuclides. This is not of great concern from a radiation protection standpoint because of the homeostatic regulation of 40K in the human body. Thus the ICRP Reference Man is known to have 4,000 Bq of 40K in its body permanently. Also note that the activities of 210Po and 210Pb were higher in vegetables (especially in “lalo” and baobab leaves) due to the deposition of radon daughter products. The activities measured in beef are 5  0.5 Bq/kg for 210Po and 5  1 Bq/kg for 210Pb and could also be explained by the deposition of radon daughter products in pastureland as well as soil ingestion by cattle. Other studies on 210Po measurements in foodstuffs have been documented, especially in marine foods (Harrison and Phipps, 2002) and caribou meat (Macdonald et al., 1996). Activities of 210Po in the latter are within the range of 9e40 Bq/kg while 210Po activities in kidneys and liver range between 90 and 620 Bq/kg (Thomas et al., 2001; Macdonald et al., 1996). These studies illustrate how the activity of 210Po can be high in foodstuffs. Therefore, activities of 210Po in foodstuffs sampled at Poli are not exceptionally elevated compared with other reported values and are primarily caused by the atmospheric deposition of radon daughter products on leaves. 3.3. Radioactivity in water Activities of 238U, 234U, 226Ra and 210Po were measured. All measurements of 226Ra fell below the detection limits of a-spectrometry (10 mBq/l). For the ingestion dose assessment, we considered the activity of 226Ra equal to the detection limit of the spectrometer. Thorium isotopes were not determined because of their low solubility in water, which necessarily excludes a significant dose of this radionuclide from water consumption (Ivanovich, 1994; Plater, 1992). The literature already contains many studies conducted to determine radioactivity measurements in water. Measurements of 226 Ra and 210Pb average activities in drinking water in Brazil led, respectively, to 63 mBq/l and 18 mBq/l. Jia and Torri (2007) conducted activity measurements of uranium and radium isotopes in drinking water in Italy that reported average values of 22.6 mBq/l for 238U, 27.6 mBq/l for 234U, 8.5 mBq/l for 226Ra and 3 mBq/l for 210 Po. Activity measurements in underground water samples taken in Slovenia gave values ranging between 24 and 37 mBq/l (Popit et al., 2004). In Spain, activities of 226Ra reached up to 20 mBq/l in river water and 600 mBq/l in drinking water (Martinez-Aguire et al., 1991; Dueñas et al., 1999). In Finland, the following activities in well water were recorded: 50 mBq/l for 226Ra, 350 mBq/l for 234 U, 260 mBq/l for 238U, 40 mBq/l for 210Pb and 50 mBq/l for 210Po (Vesterbacka and Ikäheimonen, 2005). Measurements of 226Ra and 210 Po activities in water were taken around the abandoned uranium mine of Rayrock in Canada (Veska and Eaton, 1991). In underground water, activities reached up to 20 Bq/l for 226Ra and 1.1 Bq/l for 210 Po. In surface water, activities of 226Ra reaching up to 14 Bq/l were recorded. The above results show that the radioactivity in water of the region of Poli is low compared with some regions of the world.

Table 3 Components of total dose to the public; for the external irradiation dose, the contribution of the 238U and 232Th series is 0.31 mSv/year. For the ingestion dose, the contribution of 40K is taken into account. For the inhalation dose, a measurement of 222 Rn undertaken in a room used as office is not considered in the calculation. The cosmic ray dose was calculated for the small town of Poli, situated at 490 m above sea level. Type of exposure

Exposure area

Effective dose (mSv/year)

External irradiation Ground

Poli Switzerland [CPR, 2005] World [UNSCEAR, 2000] Poli Switzerland [CPR, 2005] World [UNSCEAR, 2000] Poli Switzerland [CPR, 2005] World [UNSCEAR, 2000] Poli Switzerland [CPR, 2005] World [UNSCEAR, 2000]

0.31 (238U þ 232Th)þ0.32 (40K) 0.35 0.5 (0.3e0.6) 0.37 0.38 0.4 (0.3e1) 2 1.64 1.2 (0.2e10) 2.2 0.34 0.3 (0.2e0.8)

External irradiation Cosmic ray Inhalation

Ingestion

3.4. Radon in dwellings Table 2 presents the results of 222Rn measurements taken from 5 dwellings in the region of Poli. The average indoor concentration without the value obtained for a room used as an office is 120  21 Bq/m3 while the latter measurement leads to 2000  100 Bq/m3, which is quite high compared to the world average value of 46 Bq/m3 (UNSCEAR, 2000). There have also been many studies which assess radon levels in dwellings. For instance, an average concentration of 75 Bq/m3 was determined in Switzerland (CPR, 2005). In Rio de Janeiro, Brazil (Magalhães et al., 2003), measurements in dwellings documented an average concentration of 50 Bq/m3. Average and maximal concentrations, respectively, equal to 200 Bq/m3 and 3000 Bq/m3 were recorded in Portugese dwellings (Faisca et al., 1992). A similar study was conducted in a uranium-bearing region of Brazil (Binns et al., 1998) whose average concentration was recorded at 82 Bq/ m3. A study conducted in dwellings near the closed uranium mine of Zirovski in Slovenia reported concentrations between 10 and 180 Bq/m3. An average concentration of 483 Bq/m3 was documented in dwellings around a former uranium mine in Hungary (Somlai et al., 2006; Gorjanacz et al., 2006). These results indicate a large variation in the radon level in houses. Most studies also recognize that in most houses with high radon levels, the main source is not the building material but the convective radon influx from the soil. Radon levels also depend on geological and meteorological factors, ventilation conditions and building habits. Results on radon concentrations at former uranium mines indicate the environmental impact of uranium processing and describe fluctuations of radioactivity that could happen every time a uranium deposit is exploited. In the recent ICRP publication (ICRP Publication 103, 2007) the Commission recommends applying the source-related principles of radiological protection for controlling radon exposure. This means that national authorities need to set national reference levels to aid the optimization of protection. Thus, the upper values for the reference level expressed in activity concentrations remain at 1500 Bq/m3 for workplaces and 600 Bq/m3 for homes. In view of the latest scientific data, WHO proposed a reference level of 100 Bq/m3 to minimize health hazards due to indoor radon exposure (WHO, 2009). However, if this level cannot be reached under the prevailing country-specific conditions, chosen reference level should not exceed 300 Bq/m3 which represents approximately 10 mSv/year. Although action levels are not yet defined in Cameroon, the room used as an office having a radon concentration of 2000  100 Bq/m3 should be investigated and, in case this high value is confirmed, the room must be refurbished.

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3.5. Dose calculation to the public The external radiation dose from the ground of the uraniumbearing region of Poli is presented in Table 3 and is 0.63 mSv/year higher than the world average value, 0.5 mSv/year. This dose increase stems from the 40K spikes in some soil samples. We ignored the contribution of the 235U series because of its low ratio compared to the 238U series in the environment. This ratio is around 0.046 in NORM containing materials (De Corte et al., 2005). To determine the external radiation dose from cosmic rays, we used the average altitude of the region of Poli (490 m) in eq. (1) and we obtained an annual dose of 0.37 mSv (Table 3). The inhalation dose of the uranium-bearing region of Poli is presented in Table 3. Note that its value of 2 mSv/year is higher than the world average value of 1.2 mSv/year. Also keep in mind that the average concentration used to derive the inhalation dose of the uranium-bearing region of Poli does not take into account any measurement carried out in an office having a radon concentration of 2000 Bq/m3. A permanent occupant of this room would receive an additional 12.6 mSv/year, which is considerable and would necessitate the remediation of the building. On the other hand, distribution of radon in houses depends on many factors (ventilation rate, buildings habits, geology and meteorology, etc.) (ICRP Publication 65, 1993) and the number of dwellings (n ¼ 5) where radon exposure was quantified is not statistically representative at the present time. Thus our conclusion must be considered with caution. Table 3 shows that the ingestion dose of the uranium-bearing region of Poli is around six times higher than the corresponding values determined for the rest of the world (UNSCEAR, 2000). The contribution of 40K is 0.19 mSv/year and is the same for each adult person in the world due to the homeostatic regulation of potassium in the body. The contribution of 238U and the 232Th series is around 2 mSv/year. Primarily, this is attributable to 210Po and to 210Pb at the rate of 62% and 36%, respectively. This high dose stems from the regular consumption of vegetables rich in 210Po and in 210Pb coming from the atmospheric deposition. The contribution of radioactivity in drinking water is almost insignificant. Taking into account external radiation (ground and cosmic rays), inhalation and ingestion, the average value for the uraniumbearing region of Poli is 5.2 mSv/year while the worldwide value is 2.4 mSv/year (UNSCEAR, 2000), 2.6 mSv/year for Switzerland (CPR, 2005), 3 mSv/year for United States of America (NCRP, 1987), 2.2 mSv/year for United Kingdom (Watson et al., 2005) and 2.5 mSv/year for France (Billon et al., 2005). The difference observed between the total dose values of the region of Poli and the rest of the world is significant. The main contribution to the total dose is attributable to the ingestion dose caused by the regular consumption of vegetables rich in 210Po and in 210Pb. In term of radionuclides, the contribution of inhaled 222Rn is a noteworthy factor, but our results are statistically weak. 4. Conclusion The project was to conduct an environmental baseline study of the uranium-bearing region of Poli prior to an impending exploitation. Through this study, we discovered that soil, drinking water and food do not contain high levels of radioactivity. At the same time, vegetable leaves do contain elevated levels of 210Pb and 210Po. The associated dose through ingestion is thus increased twofold compared to the worldwide average ingestion dose because leaves and vegetables constitute an important part of dietary habits in Northern Cameroon. Using the results of the present study we can expect that any changes in environmental radioactivity distribution caused by mining activities will be readily evidenced by an adequate

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environmental monitoring program. Moreover, we can also anticipate that once the uranium ore is brought to the surface, 222Rn emanation and 210Pb and 210Po deposition might be a real problem leading to an increase of the dose to the population of the Poli area, again as a result of dietary habits. Thus, an adequate monitoring program targeting radon daughter products must be required during mining activities. Additionally, we think that well water will be an interesting compartment to assess any environmental damage from mining based on its currently very low level of uranium and radium activities. Another project to extend radon measurements in dwellings of the same uranium-bearing area, more statistically representative is submitted for funding. Acknowledgements We would like to thank the Swiss Federal Commission for Scholarships for Foreign Students and the Société Académique Vaudoise (SAV) for a grant, Dr. Philipp Steinmann for fruitful discussions, Ms. Michèle Bailat-Jones for English correction, M. Ngoko Djiokap for sampling. ICTP (through the OEA-AC-71) is acknowledged for supporting one of the co-authors (MKN). References Billon, S., Morin, A., Caër, S., Baysson, H., Gambard, J.P., Backe, J.C., Rannou, A., Tirmarche, M., Laurier, D., 2005. French population exposure to radon, terrestrial gamma and cosmic rays. Radiat. Prot. Dosim 113, 314e320. Binns, D.A.C., Figueiredo, N., Melo, V.P., Gouvea, V.A., 1998. Radon-222 measurements in a uranium-prospecting area in Brazil. J. Environ. Radioact 38, 249e254. Birattari, C., Moy, B., Rancati, T., 1996. Neutron Measurements at Some Environmental Monitoring Stations. Internal Report, TIS-RP/IR/96-13. CERN, Geneva. Bochud, F., Bailat, C.J., Buchillier, T., Byrde, F., Schmid, E., Laedermann, J.-P., 2006. Simple Monte Carlo method to calibrate well-type HPGe detectors. Nucl. Instr. Meth. A 569, 790e795. CPR, 2005. Commission Fédérale de la Protection contre les Radiations et de Surveillance de la Radioactivité. Analyse des contributions à l’irradiation de la population Suisse en 2004 (in French and in German). http://www.bag.admin. ch/ksrcpr/04336/04783/04831/index.html?lang¼de&download¼M3wBPgDB/ 8ull6Du36WcnojN14in3qSbnpWYam6alE6p1rJgsYfhyt3NhqbdqIVþbaqwbKbXr Z6lhuDZz8mMps2go6fo. Carvalho, F.P., Madruga, M.J., Reis, M.C., Alves, J.G., Oliveira, J.M., Gouveia, J.S., 2007. Radioactivity in the environment around past radium and uranium mining sites of Portugal. J. Environ. Radioact 96, 39e46. De Corte, F., Umans, H., Vandenberghe, D., De Wispelaere, A., Van den Haute, P., 2005. Direct gamma-spectrometric measurement of the 226Ra 186.2 keV line for detecting 238U/226Ra disequilibrium in determining the environmental dose rate for the luminescence dating of sediments. Appl. Radiat. Isot 63, 589e598. Décombaz, M., Gostely, J.J., Laedermann, J.-P., 1992. Coincidence-summing corrections for extended sources in gamma-ray spectrometry using Monte Carlo simulation. Nucl. Instr. Meth. A 312, 152e159. Dueñas, C., Fernandez, M.C., Carretero, J., Liger, E., Canete, S., 1999. 226Ra and 222Rn concentrations and doses in bottled waters in Spain. J. Environ. Radioact 45, 283. Eckerman, K.F., Leggett, R.W., 1996. DCFPAK: Dose Coefficient Data File Package for Sandia National Laboratory, ORNL/TM-13347. Oak Ridge National Laboratory, Oak Ridge, Tennesee. Faisca, M.C., Teixeira, M.M.G.R., Bettencourt, A.O., 1992. Indoor radon concentrations in Portugal: a national survey. Radiat. Prot. Dosimetry 45, 465e467. Gaso, M.I., Segovia, N., Morton, O., 2005. Environmental impact assessment of uranium ore mining and radioactive waste around a storage centre from Mexico. Radioprotection 40, S739eS745. Gehnes, P., Thoste, V., 1981. Rapport sur la mission d’expertise du projet eProspection d’uranium au Nord-Cameroun-, 16 Oct- 19 Déc 1980, rapport non diffusé, Hanovre. Gorjanacz, Z., Varhegyi, A., Kovacs, T., Somlai, J., 2006. Population dose in the vicinity of closed hungarian uranum mine. Radiat. Prot. Dosim 118, 448e452. Harrison, J.D., Phipps, A.W., 2002. Comparing Man-made and Natural Sources of Radionuclide Exposure. Consultative Exercise on Radiation Risks of Internal Emitters (CERRIE). National Radiological Protection Board, Chilton, Didcot, Oxon. OX11 0RQ. IAEA, International Atomic Energy Agency, 2008. Naturally Occurring Radioactive Material (NORM V). Proceedings of an International Symposium, Seville, Spain, 19e22 March 2007. ICRP, 1979. International Commission on Radiological Protection. Limits for the intake of radionuclides by workers, part 1. Pergamon Press, Oxford, England. ICRP, 1993. International Commission on Radiological Protection. Protection against radon-222 at home and at work. Pergamon Press, Oxford, England.

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