KAOLINITE AS AN IN-SITU DOSIMETER FOR PAST RADIONUCLIDE MIGRATION AT THE EARTHS SURFACE

The origin of 3 types of point defects (A-, A'- and B-centers) in kaol inite, due to natural irradiation and detected by electron paramagneti c resonance spectroscopy (EPR), has been demonstrated by artificial ir radiation. The potential use of tracing the dynamics of the transfer o f radionuclides through A-centers (i.e. the most stable centers) was q ualitatively tested on different low-temperature alteration systems, s ome associated with U-concentrations. This paper proposes a quantitati ve approach to the reconstruction of the past migration of radionuclid es by dosimetry of A-centers. With this aim in mind, the efficiency of alpha- and gamma-radiations to produce A-centers was determined by ex perimental irradiation. Parameters extracted from A-center growth curv es, together with their relationship with a parameter describing the d egree of order of kaolinite, permitted (i) a definition to be made of the dose range in which a given kaolinite could be used as a dosimeter and (ii) the quantitative derivation of U-concentration from the cumu lative dose (paleodose) of kaolinites. This was achieved by a formalis m that accounted for the contribution of natural radiosources to the p roduction of A-centers. The formalism was applied to the Nopal I U-dep osit (Chihuhua, Mexico), considered as a natural analogue of a high le vel nuclear waste repository. Irrespective of the scenario considered, in terms of kaolinite age and of degree of isotopic disequilibrium in the system, A-center dosimetry permitted the determination of past oc currences of U which were several orders of magnitude higher than the present-day measured U-concentrations. Furthermore, this approach also provided evidence for several previous episodes of U-migration. EPR s pectroscopy is thus a unique tool for the quantitative, indirect asses sment of past radionuclide migration in the geosphere and kaolinite is a reliable in-situ dosimeter. (C) 1998 Elsevier Science Ltd. All righ ts reserved.