Radiometric risk assessment of naturally occurring radioactive materials in the proposed radioactive waste storage area of Pilikwe, Botswana

Abstract

Determination of natural occurring radioactive material concentrations followed by the radiological risk assessment were conducted for the radioactive waste storage facility to be constructed at Pilikwe village of Central district in Botswana. Sixty soil and two borehole water samples were collected representing the area of construction site and its surroundings. The radioactivity measurements were performed by determining gamma-ray energies with high resolution provided by a high-purity germanium (HPGe) detector, inductively coupled plasma mass spectrometer (ICP-MS), in a low background configuration at the Centre for Applied Radiation Science and Technology laboratories, North-West University and alpha spectroscopy was used to identify and quantify radionuclides at the Radiation Protection Inspectorate. The high resolution purity germanium detector attained activity concentration results for the soil samples showed that the maximum value for 226Ra was found to be 36.31 Bq/kg, with a minimum value of 6.66 Bq/kg and an average value of 15.86 Bq/kg; maximum value of 238U was found to be 53.47 Bq/kg, with a minimum value of 7.75 Bq/kg and an average value of 25.55 Bq/kg; a maximum value of 234Th was found to be 46.78 Bq/kg, with a minimum value of 11.58 Bq/kg and an average value of 24.98 Bq/kg and the maximum value for 40K was found to be 468.60 Bq/kg, with a minimum value of 59.15 Bq/kg and an average value of 201.30 Bq/kg for the Pilikwe area before any construction work of the radioactive waste storage facility had been conducted. The average estimated absorbed dose rate (D) for soil samples was found to be 35.29 ± 2.26 nGy/h, which was lower than the worldwide absorbed dose rate of 57.00 nGy/h for soil. The average estimated annual effective dose equivalent (AEDE) from the soil samples was also found to be 43.28 ± 2.77 μSv/y, which was lower than the recommended worldwide value of 70.00 μSv/y for soils. The radium equivalent activity (Raeq) for soil was 76.78 ± 5.06 Bq/kg. The external hazard index (H𝑒𝑥𝑡) for soil was 0.21 ± 0.01. The mean Raeq values for soil, were all below the worldwide accepted value of 370.00 Bq/kg. The average H𝑒𝑥𝑡 value for soil, was found to be below the worldwide recommended value of 1.00. All the hazard indices showed that the samples from the Pilikwe village area and its surrounding had acceptable indices with no hazard. Thus, a radioactive storage facility should be built in the identified location provided there will be proper management and safeguarding the leaching of the stored radioactive waste into the soil, as well as having measures in place for monitoring and inspecting the NORMs activity concentrations of the area on specified time frames. There has been little development regarding analysis of polonium in environmental samples since 1960 as radiochemical analysis of polonium is quite straight forward due to easy of source preparation through auto deposition onto metal surfaces. Thus, in this study the determination of polonium-210 concentrations on the soil and water samples was given more emphasis for the radiological impact

assessment. The average activity concentrations were very low, for the soil samples analysed and for water samples there was no activity concentrations recorded as it was below the instrument detection limit. The reference material: BOT 7003 soil test sample’s determined average activity concentration level was 3.27 ± 1.40 Bq/kg for 210Po from a control area. The average activity concentration for all the 60 soil samples on preparation date was 3.10 ± 1.60 Bq/kg. The soil samples were collected from a depth of about 30 cm from the study area. The 210Po measured/ 209Po reference (tracer) ratios for soils from the study area of Pilikwe varied considerably, ranging between 0.17 and 5.98 with an average value of 0.85. This was constantly in agreement with the established view that 210Po measured/ 209Po reference ratios from soils samples could be anticipated to reflect equilibrium for these isotopes (i.e., be near to 1.00) and that ingrowth of 210Po is from 210Pb in the soil as the main source having some excess 210Po decayed away in a relatively short period following deposition. The ICP-MS attained average elementary concentration results for the soil samples for selected heavy metals were: As (0.02); Pb (0.29); Cr (0.20); Cu (0.03); Zn (0.02); Co (0.47); Th (0.05) and U (0.13) in mg.kg-1, while for Cd, it was below the instrument detection limit. This average elemental concentrations of all selected nuclides in the soil samples were lower than the permissible limits. The heavy metal concentrations in water (mg. L-1) were: Cu (0.03); Zn (0.09) and Ni (0.27) while for As, Pb, Hg, Cd, Cr and Co were below the instrument detection limit. The results showed that the average concentrations of the selected nuclides in soil and water were lower than the WHO permissible limit for all selected nuclides, indicating that it is safe to reside in the study area. The total excess cancer risk CNRS (i, p, t) for initially existent radionuclides (i) and pathways (p) and fraction of total risk at time (t) years for water dependent pathways and water independent pathways (inhalation excludes radon) analysis obtained from the average activity concentrations of the three radiometrical techniques used. Thus, demonstrated that the total cancer fraction and risk were very low and negligible to have the potential to cause any harm with respect from the initial (0 year) to the final (1000 years) year of interest from the study. Therefore, the cancer risk results from both the water dependent and independent pathways showed that the risk decreased with time from the initial year. The values were less than 1, which indicated that there was no health risk associated with NORMs to the residents of the study area.