International Symposium on Uranium Raw Material for the Nuclear Fuel Cycle: Exploration, Mining, Production, Supply and Demand, Economics and Environmental Issues (URAM-2018)

NATURAL RADIATION EXPOSURE TO THE PUBLIC IN THE URANIUM AND THORIUM BEARING REGIONS OF CAMEROON: FROM MEASUREMENTS, DOSE ASSESSMENT TO A NATIONAL RADON PLAN

28 Jun 2018, 12:40

20m

Vienna

ORAL Track 10. Health, safety, environment and social responsibility Health, Safety, Environment and Social Responsibility

Speaker

Prof. SAÏDOU (Institute of Geological and Mining Research)

Description

ABSTRACT The present paper summarizes the findings of studies carried out since 2014 in the uranium and thorium bearing regions of Poli and Lolodorf respectively located in northern and southern Cameroon. It also underlines future prospects to strengthen the radiological protection of members of the public exposed to environmental natural radiation in Cameroon. In-situ gamma spectrometry and car-borne survey were performed in the above regions to respectively determine activity concentrations of natural radionuclides in soil and air kerma rates to assess effective external dose received by members of the public. High natural radiation areas were located and selected for indoor radon, thoron and thoron progeny measurements. Raduet detectors and thoron progeny monitors were deployed in 300 dwellings to measure radon, thoron and thoron progeny indoors to assess inhalation dose received by members of the public. External effective dose ranges between 0.15-0.63 mSv.yr-1 with the average value of 0.4 mSv.yr-1 in the uranium bearing region of Poli and between 0.1-2.2 mSv.yr-1 with the average value of 0.33 mSv.yr-1 in the uranium and thorium bearing region of Lolodorf. The inhalation dose due to radon and thoron ranges respectively between 0.87-2.7 mSv y-1 and 0.08-1 mSv y-1 with the average values of 1.55 mSv y-1 and 0.4 mSv y-1 for Poli, between 0.6-3.7 mSv y-1 and 0.03-3 mSv y-1 with the average values of 1.84 mSv y-1and 0.67 mSv y-1 for Lolodorf. Contribution of thoron to the total inhalation dose ranges between 3-34% with the average value of 20.3% in the uranium region of Poli and between 1-79% with the average value of 27% in the uranium and thorium bearing region of Lolodorf. Thus thoron can not be neglected in dose assessment to avoid biased results in radio-epidemiological studies. INTRODUCTION Since one decade many environmental radiation surveys were carried out in Cameroon [1-10]. Most of these studies deal with natural radioactivity measurements and corresponding dose assessment in mining and ore bearing regions of Cameroon. They started by collecting soil, foodstuff and water samples and by deploying Electret Ionization Chambers (EIC) (commercially E-PERM) and passive integrated radon-thoron discriminative detectors (commercially RADUET) in dwellings before determining activity concentrations of natural occurring radionuclides. This determination is followed by assessing inhalation, ingestion and external radiation dose helpful to perform radiation risk assessment. The present work uses car-borne survey method to measure air absorbed dose rates and in-situ gamma spectrometry to determine activity concentrations of natural radionuclides in soil in all the uranium and thorium bearing regions of Poli and Lolodorf. The radiological mapping of these regions was established to locate the high natural radiation areas. This information was used to deploy RADUET and thoron progeny monitors in 400 houses for radon (222Rn), thoron (220Rn) and thoron progeny measurements indoors. Measurements of absorbed dose rates in air, 238U, 232Th, and 40K activity concentrations in soil, radon, thoron and its progeny indoors were followed by external and inhalation dose assessment helpful to assess radiation risk of members of the public in the above regions. The above results highlight the importance to put in place a national radon plan in Cameroon in agreement with the International Atomic Energy Agency (IAEA) Safety Standards Series No. GSR Part 3: Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards [11]. A Technical Cooperation (TC) project (CMR9009) on Establishing national radon plan for controlling public exposure due to radon indoors was initiated and is ongoing for IAEA TC cycle 2018-2019. MATERIAL AND METHODS Car-borne survey The detailed method of the car-borne survey was described in many publications [12-14], only an outline is described here. A car-borne survey which used a 3-in × 3-in NaI(Tl) scintillation spectrometer (EMF-211, EMF Japan Co., Japan) was carried out in the uranium and thorium bearing regions of Poli and Lolodorf from November 2015 to August 2016. This spectrometer was positioned inside the car and the car speed was kept around 30-40 km h-1. Measurements of the counts inside the car were carried out every thirty seconds along the route. In order to generate a dose rate distribution map, the latitude and longitude coordinates were recorded using a global positioning system (GPS) in each measurement point at the same time as the gamma-ray count rates. Since count rate is measured inside the car, it is necessary to estimate a shielding factor of the car body towards terrestrial gamma-rays in order to represent the unshielded external dose rate. The shielding factor was estimated by making measurements inside and outside the car at 150 points. Those measurements were recorded consecutively at 30-s intervals during a total recording period of 2 min. Measurements of gamma-ray pulse height distributions were also carried out 1 m above the ground surface outside the car for 15 min at 24 points along the survey route. The gamma-ray pulse height distributions were unfolded using a 22 × 22 response matrix for the estimation of absorbed dose rate in air. These dose rates were used to evaluate the dose rate conversion factor (nGy h–1 cpm–1). Radon-thoron discriminative measurements indoors To determine the concentrations of radon and thoron, RADUET detectors developed at the National Institute of Radiological Sciences (NIRS) in Japan were used in this study [15]. CR-39 was used to detect alpha particles emitted from radon and thoron as well as their progenies. To determine conversion factors of radon and thoron concentrations, these detectors were placed into the radon and thoron chambers at NIRS, respectively [16, 17]. After exposure tests, CR-39 plates were taken out of the chamber and chemically etched with a 6.25 M NaOH solution at 90°C over 6 h, and alpha tracks were counted with a track reading system. The evaluation of track in Image J and Microscope methods is well described by Bator et al [18]. Using two alpha track densities of low and high air-exchange rate chambers (NL and NH), radon and thoron concentrations were determined by solving the following equations [15]: (formula 1, 2) where XRn and XTn are the mean concentrations of radon and thoron during the exposure period in Bq m-3, CFRn1 and CFTn1 are respectively the radon and thoron conversion factors for the low air-exchange rate chamber in tracks of 2.3 cm-2 kBq-1 m3 h-1 and 0.04 cm-2 kBq-1 m3 h-1, CFRn2 and CFTn2 are respectively the radon and thoron conversion factors for the high air-exchange rate chamber in tracks of 2.1 cm-2 kBq-1 m3 h-1 and 1.9 cm-2 kBq-1 m3 h-1, T is the exposure time in hours, and B is the background alpha track density on the CR-39 detector in cm-2. The lower detection limit of the detector was practically estimated on the basis of the fact that one concentration depends on the other. The lower detection limits were 3 Bq m-3 for radon and 4 Bq m-3 for thoron. RADUET detectors were placed at a height of 1–2 m and 20 cm from the wall in 100 and 150 dwellings of the uranium and thorium bearing regions of Poli and Lolodorf respectively for 2-3 months. Spot outdoor and indoor gamma dose rate measurements were performed using a RadEye dose rate survey meter calibrated by comparison to a Gamma-RAD5 NaI(Tl) scintillation spectrometer. Measurements were conducted at 1 m height above the ground surface. Inhalation dose due to radon and thoron The inhalation dose is given by the following equation [19]: formula (3) XRn and XTn are the median radon and thoron concentration, einh,Rn is the inhalation dose conversion factor of 9 nSv/(Bq h m-3) for radon and einh,Tn is the dose conversion factor of 40 nSv/(Bq h m-3) for thoron, Focc is the occupancy factor of 0.6 for the studied areas, FRn, Tn is the equilibrium factor considered of 0.4 for radon and 0.02 for thoron, t corresponds to a year expressed in hours. The occupancy factor was derived from an in situ inquiry performed in the studied areas during field work. The equilibrium factor used is the default value given by United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) [19]. RESULTS AND DISCUSSION In the uranium bearing region of Poli, activity concentrations of 238U, 232Th, and 40K range respectively between 13-52 Bq kg-1, 10-67 Bq kg-1, and 242-777 Bq kg-1 with respective average values of 32 Bq kg-1, 31 Bq kg-1, and 510 Bq kg-1. In the uranium and thorium bearing region of Lolodorf, those activity concentrations range between 6-158 Bq kg-1, 6-450 Bq kg-1, and 98-841 Bq kg-1 with 34 Bq kg-1, 58 Bq kg-1, and 200 Bq kg-1 as respective mean values. The world average values for these radionuclides given by UNSCEAR [19] are respectively 33 Bq kg-1, 45 Bq kg-1, and 420 Bq kg-1. Air kerma rates range respectively between 25- 102 nGy h-1 and 11-357 nGy h-1 for the uranium and thorium bearing regions of Poli and Lolodorf with the mean values of 57 and 54 nGy h-1. At the worldwide level, they range between 24-160 nGy h-1 with the average value of 57 nGy h-1. The annual effective dose ranges respectively between 0.20- 0.83 mSv y-1 and 0.1-2.2 mSv y-1 with the mean value of 0.35 mSv y-1 and 0.33 mSv y-1 less than the world average value of 0.5 mSv y-1 given by UNSCEAR [19]. In the uranium region of Poli, radon and thoron concentrations indoors range respectively between 46-143 Bq m-3 and 18-238 Bq m-3 with the average values of 82 Bq m-3and 94 Bq m-3. The inhalation dose due to radon and thoron ranges respectively between 0.87-2.7 mSv y-1 and 0.08-1 mSv y-1 with the average values of 1.55 mSv y-1 and 0.4 mSv y-1. The total inhalation dose due to radon and thoron range respectively between 0.95-3.7 mSv y-1 with the average value of 1.95 mSv y-1. In the uranium and thorium bearing region of Lolodorf, radon and thoron concentrations indoors range respectively between 31-197 Bq m-3 and 6-700 Bq m-3 with the average values of 97 Bq m-3 and 159 Bq m-3. The inhalation dose due to radon and thoron ranges respectively between 0.6-3.7 mSv y-1 and 0.03-3 mSv y-1 with the average values of 1.84 mSv y-1and 0.67 mSv y-1. The total inhalation dose ranges between 0.6-6.7 mSv y-1. At the worldwide level inhalation dose due to radon ranges between 0.2-10 mSv y-1 with the mean value of 1.26 mSv y-1. The contribution of thoron to the total inhalation dose in the uranium and thorium bearing regions of Poli and Lolodorf ranges respectively between 3-34% and 1-79% with the average values of 20.3% and 27%. Thus thoron can not be neglected in dose assessment to avoid biased results in radio-epidemiological studies. CONCLUSION Natural radioactivity in most of the surveyed areas is normal. However there are high natural radiation areas found in most of the study areas. Radon and thoron exposure is reality in Cameroon. Thoron contribution to inhalation dose is higher than 20%. Thus thoron can not be neglected in dose assessment. Thoron is abundant in the uranium and thorium regions of Poli and Lolodorf. However extensive measurements of radon and thoron at nationwide scale are needed. In case high inhalation doses are confirmed, epidemiological study could be planned. A project (CMR9009) dealing with Establishing a national radon plan for controlling public exposure due to radon indoors is ongoing since the beginning of 2018. This two years project is funded within the framework of the technical cooperation between the International Atomic Energy Agency (IAEA) and Cameroon. REFERENCES 1. Saïdou, Bochud F, Baechler S, Kwato Njock M, Ngachin M, Froidevaux P. 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