A U-deposit located in hydrothermally altered tuff in Mexico, and two tropical weathering systems from Brazil and Cameroon were studied as natural analogues of radionuclide release and migration in the geosphere. In these alteration systems, kaolinite is the main secondary phyllosilicate associated with variable amounts of iron oxides. Defect centers are detected in natural kaolinites by means of electron paramagnetic resonance. The origin of the paramagnetic defect centers (PDC) is from radionuclide irradiation. This interpretation is supported by laboratory experiments on kaolinite using four different types of radiation: x-rays, γ-rays, and He+ and Pb2+ ion beam implantations. Several types of PDC (designated A-, A‵- and B-centers) are clearly identified and can be distinguished with respect to their relative stability. The A-center is stable over geological periods (on the order of 109 years) at surface temperatures. Moreover, it is shown that PDC’s in kaolinites reflect the cumulative radiation dose.
PDC’s in kaolinite have been used to trace radionuclide migration in the three natural alteration systems studied. The kaolinites investigated show a significant concentration of PDC’s which corresponds to background natural irradiation. The different types of PDC detected may record successive irradiations of kaolinite directly linked to geochemical conditions prevailing during and after kaolinite growth. Because of their different thermal stability, these different PDC–s may be considered to have formed at different time periods. This makes a unique tool for tracing the dynamics of the transfer of radionuclides and allows the use of kaolinite as an in-situ dosimeter. Because some of the geological settings chosen for nuclear wastes are located in kaolinite-bearing strata, PDC dosimetry could be used in a safety assessment of these wastes.