Assessment Of Natural Radioactivity In The Soils Of The Ohorongo Cement Plant And The Town Of Otavi, Namibia

ABSTRACT

The activity concentrations of the naturally occurring radionuclides 238U, 232Th, and

40K in soil samples collected from the Ohorongo Cement plant and the town of Otavi,

Namibia have been determined using a High-Purity Germanium (HPGe) detector. In

addition, an assessment of the radiological hazards due to these natural radionuclides

in the soils of the plant and town was carried out. The plant was divided into three

geographical areas, and fifty soil samples were collected across the areas. Also, the

town of Otavi was divided into ten geographical areas, and fifty soil samples were

collected across the areas. These soil samples were processed and 500 g of each sample

was placed in a well-labelled 500 ml polythene bottle and analyzed using an HPGe

detector.

The average activity concentrations of the radionuclides 238U, 232Th, and 40K in the soil

samples from the Ohorongo Cement plant were 15.0 ± 4.7 Bq/kg, 25.1 ± 9.9 Bq/kg,

and 310.7 ± 97.2 Bq/kg respectively. Similarly, the average activity concentrations of

the radionuclides in the soil samples from Otavi were 21.6 ± 7.2 Bq/kg for 238U, 20.3

± 8.5 Bq/kg for 232Th, and 256.4 ± 113.7 Bq/kg for 40K. The average activity

concentrations of 232Th and 40K were found to be higher in the Ohorongo Cement plant

than in Otavi. In contrast, the average activity concentration of 238U was higher in

Otavi than in the Cement plant. These concentrations were used to calculate the

absorbed dose rates and the effective dose rates in air across the different geographical

areas in the plant and town. The values of (4.3 ± 1.5) × 10−2 mSv/y and (4.0 ±

1.5) × 10−2 mSv/y obtained respectively for the mean effective dose rates in the

Cement plant and town are both less than the maximum permissible effective dose rate

of 1.0 mSv/y recommended for the public by the International Commission on

Radiological Protection. In order to evaluate the associated health hazard, the

concentrations were also used to calculate the mean Radium equivalent activity (Raeq)

and the mean external hazard index (Hex) for the plant and for the town. The values

obtained for Raeq in the Cement plant and town were 74.9 ± 25.6 Bq/kg and 70.4 ±

26.7 Bq/kg, respectively. These values are below the maximum permissible limit of

370 Bq/kg. Furthermore, the values of 0.20 and 0.19 obtained respectively for Hex in

the Cement plant and town are again below the maximum permissible limit of 1.0. It

is therefore concluded that radiation hazards are negligible in the plant and town and

hence the plant and town have normal background radiation. The data generated in this

study will contribute to the baseline data on radiological hazards in Namibia.