Radiation Dose Assessment in the Vicinity of an Old Uranium Mine : Case of the site of Vatovory, Vinaninkarena Antsirabe, MADAGASCAR

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1 Radiation Dose Assessment in the Vicinity of an Old Uranium Mine : Case of the site of Vatovory, Vinaninkarena Antsirabe, MADAGASCAR Raoelina Andriambololona, H. F. Randriantseheno, J.F. Ratovonjanahary, J.L.R. Zafimanjato, J.B. Ratovoson, H.N. Rabenandrasana Association Nationale de Radioprotection (ANARAP-Madagascar) B.P Antananarivo 101, Madagascar. instn@dts.mg Abstract. All form of life is unavoidably exposed to radiation from natural sources particularly, in the old mine of uranium. Workers and public living around the mine, can receive both external and internal exposure due to ionising radiation. The objective of the work is to assess the scope of the problem and to review data reported on exposure which may occur around uranium mine site due to natural sources : gamma radiation, radon, thoron. In Madagascar, radiation protection data measurement regarding abandoned uranium deposits exploitation are not available. However, external exposure comes from beta and gamma radiation emitted from soil and internal exposure arises mainly from the inhalation of radon gas. Decay products and long-lived alpha emitters in ore dust are released in the atmosphere. To assess exposure from mine, measurements have been carried out around the site, particularly where the population density is higher. For this purpose, different equipment are used : - Radiation detector, contamination meter, for beta and gamma measurement, - Alpha Prism for radon and thoron measurement - Gamma spectrometry for soil measurement - GPS, barometer, altimeter A software GIS ArcView has been used for establishment of radiometric map. As results, the Total Exposure Rate calculation allow us to assess the public exposure, and to make a comparison with international standards. The exposure in the site and in his vicinity is 13 times higher than normal exposure. The risk assessment of lung cancer due to inhalation of radon gas is also carried out through epidemiological approach. The results of high exposure around the old mine above mentioned justify the necessity to rehabilitate the site, in order to reduce the public exposure. For example, by covering it with a layer of soil or to delimit a protection area with barbed wire. I- INTRODUCTION In Madagascar, radiation protection data regarding abandoned uranium deposits exploitation are not available. However, external exposure comes from beta and gamma radiation emitted from soil and internal exposure arises mainly from the inhalation of radon gas. Decay products and long-lived alpha emitters in ore dust are released in the atmosphere. The Radon, which is a radioactive gas released from earth material, very abundant in uranium-bearing area, increase the lung cancer risk. To assess the radiological impacts of abandoned mines on the human health and in order to make adequate decisions to decrease this risk, a study of exposures around this site is necessary. The study has been made on a site exploited between 1937 and 1954 by French Company (Commissariat à l Energie Atomique, CEA) for uranium ores extraction (Figure 1). The extraction was lead in opencast [6]. To assess exposure from the mine, measurements have been made around the site, particularly where the population density is higher. For this purpose, different equipment are used : - Radiation detector, contamination meter, for beta and gamma measurement, - Alpha Prism for radon and thoron measurement - Gamma spectrometry for soil measurement - GPS, barometer, altimeter 1

2 A software GIS ArcView has been used for the establishment of radiometric map. This work intends to assess the scope of the problem and to review data reported on exposure which may occur around uranium mine site due to natural sources : gamma irradiation, radon, thoron. FIG. 1. An Old Uranium-bearing Site of Vatovory Vinaninkarena Antsirabe Madagascar II- EQUIPEMENTS, METHODS AND RESULTS The choice of site depend on the radiation level and the population density. The localisation is made with GPS. Radiometric map has been established with GIS ArcView software with Spatial Analyst extension. II.1. Gamma measurement Gamma radiation measurement has been performed with detector type GREATZ 5XDE, which is a portable dosimeter, type Geiger Müller with audible alarm. Note that before this work, measurements of ambient radioactivity [9] in Madagascar have been performed by the Laboratoire de Physique Nucléaire et de Physique Appliquée (LPNPA), continuated by Madagascar - Institut National des Sciences et Techniques Nucléaires (Madagascar- INSTN). 2

3 Table I: External exposure due to gamma radiation (in ngy h -1 ) and corresponding annual dose equivalent in the vicinity of uranium-bearing mine of Vatovory Locality Geographical Average gamma Annual equivalent Co-ordinate dose rate (ngy h -1 ) dose rate (msv y -1 ) Uranium-bearing site S E At 65 m from the site S E Ambanimaso S E Mangamasoandro S E Village de jeunesse S E Galerie S E FIG. 2. Mapping of ambient absorbed dose rate 3

4 II.2. Telluric diffusion of radon and atmospheric dispersion Radon concentration in the vicinity of the site has been calculated from the concentration of radium for the earth material using the formula [13]: J D = C Ra λ Rn f ρ D e λ Rn ε where C Ra is the activity concentration in earth material (in Bq kg -1 ) λ Rn, Rn-222 decay constant (λ Rn = s -1 ) f : emanation fraction for earth material (f = 0.2) ρ : density of earth material ( ρ = 1600 kg m -3 ) ε : porosity of earth material (ε = 0.25) D e : effective diffusion coefficient for earth material (D e = m 2 s -1 ) After emanation in free air, the radon diluted in the air follows atmospheric dispersion law. The mean concentration of pollutant in co-ordinate point (x, y, z) at the time t is given by Q E 1 y 2 (z h) 2 C (x, y,z, t) = exp + 2πU σ 2 y σ z 2 σ 2 [8] y σ z with the following assumptions: 1) transversal average velocity U y and U z = 0 2) regular flow (U x = constant ) 3) homogeneous turbulence and where - Q is the mass or activity rate - E: the factor which express the contribution or attenuation mechanism that reduces pollutants concentration, by various phenomena (radioactive decay, deposit, suspension ) - σ x, σ y and σ z are the longitudinal, lateral and vertical standard deviation (following the wind direction) - x : co-ordinate of the air polluted - U: average velocity of the wind - t : duration since emission. Data - Source co-ordinate: Longitude : 19 58' 09" South Latitude : 47 03' 41" East - Wind direction : 46,25 North-East - Wind velocity : 2.38 m s -1 - Diffusion coefficient [3] : k x = k y = Diffusion parameters [3] : A h = A z = 20 - Standard deviation : σ h = σ z = Samples analysis of the earth material has been made with gamma spectrometry system. Atmospheric Transfer Coefficient has been used in order to assess the pollutants concentration in each locality from the concentration of pollutants in the site. This coefficient depend on the meteorological conditions, particularly the velocity and direction of the wind, precipitation, altitude in comparison with the point source. 4

5 FIG. 3.Mapping of the variation of Atmospheric Transfer Coefficient (s m -3 ) around the uranium site. Other phenomena may complicate the transfer processes in the atmosphere and must be taken into account in the calculation in order to keep a certain realism in the results. It is a matter of - dry deposit : the particles of pollutants, under turbulence effect, - wet deposit : the wet deposit appears when the drop of rains cross the cloud of pollutant, wash this cloud and remove one part of his content in the earth. Taking into account the radon concentration released from the earth material, the atmospheric transfer phenomena, and other phenomena (dry deposit, wet deposit, impoverishment, wind direction and wind speed, pluviometry, etc.) and we infer the following results: - Diffusive entry rate of radon in atmosphere per unit area : Bq s -1 - Meteorological conditions: wind velocity : 2.38 m s -1 dry weather fraction : 0.94 wet weather fraction : 0.05 The average annual concentration due to the atmospheric dispersion of radon in each locality is given in table 2. 5

6 Table II. Average annual concentration of radon Locality Geographical ATC (x, y, z) Mean annual ATC Mean annual Co-ordinate du to concentration impoverishment (s m -3 ) (Bq m -3 ) At 65 m from the site S 4.1 E E E Ambanimaso S 1.2 E E E Mangamasoandro S 9.1 E E E Galerie S 7.7 E E E Village de jeunesse S 5.9 E E E FIG. 4. Mapping of the radon concentration Radon, thoron concentrations and their progeny in air have been measured with the ALPHA PRISM Dosimeter. This detector can perform an integrated measurements, and it provides a software PRISM II specially designed for simultaneous treatment of radon and thoron data. The assessment of absorbed dose per exposure unit in critical cell of the respiratory system may be calculated with the available information concerning the activity mean diameter (AMD) of the 6

7 particles, the unattached fraction of the decay product mixture (f p ), the breathing rate (T), the localisation and target cells. In accordance with the ICRP, for AMD = 0.2 µm, f p = 0.025, w T = 0.12 (tissue weighting factor) for lung and w R = 20 (radiation weighting factor) for alpha particles [13]. Estimated conversion coefficients for radon exposures in the atmosphere with ambient absorbed dose by cells of the respiratory track are: - for radon: 6 nsv (Bq h m -3 ) -1 in outdoor and indoor - for thoron : 10 nsv (Bq h m -3 ) -1 in outdoor and 32 nsv (Bq h m -3 ) -1 indoor [13]. Table III. Annual equivalent dose due to radon and thoron progeny Annual equivalent dose (msv y -1 ) Radon Thoron Radon + Thoron Localisation Indoor Outdoor Indoor Outdoor Total Uranium-bearing site At 65 m from the site Ambanimaso Mangamasoandro II.3 Risk of lung cancer due to radon progeny (Table IV) The estimation of the risk of lung cancer due to inhalation of radon gas is also carried out through an epidemiological approach. With a risk factor equal to per working level month, risk of lethal cancer due to radon progeny is given in the following table: Table 4: Risk of lung cancer due to radon progeny Exposure (WLM) Risk of cancer by epidemiological approach Uranium-bearing site E-06 At 65 m from the site E-06 Ambanimaso E-07 Mangamasoandro E-08 7

8 FIG. 5. Mapping of the probability of cancer risk in the vicinity of abandoned uranium mine. III. CONCLUSIONS If the normal world-wide absorbed dose rate is 57 ngy h -1, (69 in Africa, 48 in America, 54 in Europe, and 69 in Asia) [13], In certain areas, the amount of the radioactivity is higher than normal activity. In Madagascar, recognised as uranium-bearing areas, around the uranium-bearing site of Vatovory Vinaninkarena, the local of our study, the ambient gamma dose rate vary between 250 to 1466 ngy h -1. In the locality called Village de Jeunesse (before, the treatment s locality of uranium ores before its exportation), the gamma dose rate is very high (up to 1200 ngy h -1 ). For radon, the global average values unregistered of indoor exposure are, 30 Bq m -3 in Africa, 28 Bq m -3 in Asia and 41 Bq m -3 in Europe [13]. In our studies, it may reach up to 95 Bq m -3 in abandoned uranium mine, 87 Bq m -3 at 65 meter from the site, thus tree times the world-wide values. For thoron exposure, if we have respectively 0.4 Bq m -3 and 0.3 Bq m -3 in Africa and in Europe [13], thoron concentration in the site and in its vicinity is very low (0.003 Bq m -3 ). However, it may reach 2.5 Bq m -3 at Ambanimaso and Mangamasoandro, thus, six times the average world-wide values. The Total Exposure Rate calculation allow us to assess the public exposure, and to make a comparison with international standards. The exposure in the site and in its vicinity is 13 times higher than normal exposure. The results of the high exposure around this old mine justify the necessity of rehabilitation of the site, in order to reduce the public exposure. For example, by covering with layer of soil or only to delimit the site with barbed wires. 8

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