Radiological and chemical assessment of Uro and Kurun rock phosphates

The natural radionuclide content of Uro and Kurun rock phosphates, western Sudan, has been determined using α-spectrometry and high-resolution γ-spectrometry. The data show that 238U and its decay products are the principal contributors of radioactivity in both phosphate deposits. The highest activity concentrations were measured in grey-coloured rock phosphate samples. The mean activity concentrations weighted by the average agricultural consumption of 300 kg ha-1 of untreated ground rock phosphate fertilizer resulted in an annual distribution of 120.63 Bq m-2 (226Ra), 0.22 Bq m-2 (232Th) and 1.77 Bq m-2 (40K) with Uro and 13.29, 0.21 and 4.24 Bq m-2, respectively, with Kurun ground rock fertilizer. From these values the external radiation exposure over agricultural areas was estimated. This resulted in an additional external radiation exposure for the population of 23.41×10-9 and 2.59×10-9 nGy h-1 at 1 m above ground level for Uro and Kurun rock phosphate fertilizers, respectively. These estimates confirm our earlier results and show that natural radionuclides in the Uro and Kurun rock fertilizers do not contribute to the mean terrestrial radiation exposure of the population. However, exposure rates in air 1 m above Uro and Kurun phosphate deposit areas as estimated using dose rate conversion factors reach 7108 and 402 nGy h-1, respectively, which place them among high background radiation areas. The exchangeable radium fraction in rock fertilizers constitutes 0.2% (Uro) and 0.1% (Kurun) of the total amount of radium. Estimates of the maximum emanation power (Mep) have shown that the extent of contamination that could be expected for the calcium source of the plant from Kurun rock fertilizer is negligible compared with that of unfertilized soils in Sudan.