Detection of uranium mining activities

In undisturbed natural uranium ore the 238U decay chain isotopes appear in secular decay equilibrium with activity ratios equal to one. In the course of ore processing the bulk of the uranium decay products is separated from the uranium product and concentrated in the tails. Therefore the disturbed activity ratios of short-lived daughters to long-lived parents can be indicators of ore processing. Using 234Th and 238U activities (the short-lived daughter with T1/2=24.1 days and the long- lived parent respectively) one can roughly estimate how much time has elapsed since ore processing occurred. Equilibrium is reached in about three months after processing and the 234Th and 238U activity levels are approximately equal (taking into account the error of measurements). Higher or lower 234Th activity levels, relative to 238U, indicate the material has been recently processed. Assuming the product is depleted in Th and the tails are enriched, the activity of 234Th in fresh product should be lower than 238U and higher in fresh tails. The 234Th/230Th activity ratio can also be used for age estimations (230Th is a long-lived nuclide). Five samples were taken from the Ranger Uranium Mine and Concentration Plant in Australia, and one sample was taken from the Jabiluka mine (10 km far from the Ranger Mine). The samples included non-processed ore, coarse ore from the stockpile, final crushed ore, fresh and old tails, and fresh product (U3O8). All the samples were analyzed by HRGS to measure the activities of gamma emitting nuclides. XRF and IDMS were used to measure uranium content and isotopic composition. The 238U activity was calculated from these measurement results. The 234Th activity was measured by HRGS with a planar HPGe detector and a calibrated low activity 241Am solution as an internal standard. The 234Th/230Th activity ratio was measured using the 60 keV energy region where both isotopes have gamma lines. Use of gamma lines with close energies (63.29 keV for 234Th and 67.67 keV for 230Th) allows one to reduce the large contribution of geometry and absorption effects on the measured ratio. This is a clear advantage of the method. Disadvantages of the method include the low intensity of analytical lines (long measurement time) and possible interference with the 63.9 keV line of 324Th, which can be significant for uranium ores with a high Th content. The intensity of the 63.9 keV line is about 6 times lower than the 63.29 keV line if concentrations of both thorium isotopes are equal. The measurement results are shown. The 234Th/230Th activity ratios, measured by HRGS in uranium mining samples provide the inspector with information on the type of material measured (non-processed ore, uranium product or tails) and can be used for the verification of the operator data on the date of ore processing. A portable gamma spectrometer with a planar HPGe detector can be used to measure the 234Th/230Th activity ratio in the field