DOSIMETRY USING RADIATION-INDUCED DEFECTS IN KAOLINITE FROM THE PENA BLANCA NATURAL ANALOGUE: A TIME-INTEGRATED ASSESSMENT OF THE TRANSFER OF RADIONUCLIDES

Radionuclide-free minerals often exhibit radiation-induced damages of their crystal lattice, produced by high-energy radiation emitted by short-lived radionuclides transported by fluids. These defects may be directly analyzed using solid-state spectroscopic methods, such as Electron Paramagnetic Resonance (EPR), which makes these minerals suitable for use as a dosimeter for past natural irradiation. We present a review of the spatial evolution of the radiation-induced defects, which are present in the kaolinites from the Nopal I (Chihuahua, Mexico) natural analogue. In this uranium deposit of the Peña Blanca range, the volcanic rocks have been extensively altered, with the formation of various kaolinite generations within the breccia pipe and the surrounding barren tuffs. These kaolinites present various radiation-induced defect centers, with different thermal stability, and the concentration of the defect centers varies by almost three orders of magnitude within the deposit. Using a calibration based on the experimental irradiation of natural kaolinites, it is possible to calculate the cumulative radiation dose suffered by kaolinites. Using time constraints on kaolinite formation, the paleodose may be related to the overall migration of the radionuclides over time. This methodology reveals migrations of uranium within the deposit after the kaolinite formation, with an accumulation of uranium in the mineralized breccia and an extensive leaching in the fissure network of the present barren rock. They indicate that the fissure system generated by the breccia pipe was the main transport for radioelements. As these data are integrated over time, it is interesting to compare them to the available U- and Th-series disequilibrium data in order to improve performance assessment models for geologic nuclear waste disposal.