Research in actinide geochemistry: Do we need speciation information at the molecular level?
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Research in actinide geochemistry: Do we need speciation information at the molecular level ? Horst Geckeis, Institut für Nukleare Entsorgung, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany Abstract Radionuclide sorption to rock or sediment surfaces in the vicinity of a nuclear waste repository contributes significantly to the overall safety performance of a given disposal concept. Stateof-the-art speciation techniques as laser - and X-ray spectroscopy are becoming more and more involved into the elucidation of sorption mechanisms and to the quantification of surface speciation. They are extremely valuable tools to gain fundamental understanding as a basis for the development of geochemical sorption models which in turn are needed to predict radionuclide behaviour in the environment. Within the present paper, examples are given on how molecular scale information obtained from spectroscopic techniques can assist to decrease uncertainties of geochemical model assumptions and thus to enhance the confidence in model predictions. It is also shown that the interpretation of laboratory and field scale radionuclide migration experiments requires the application of speciation methods. The combination of spectroscopic studies with wet chemical experiments on actinide interaction with mineral and colloid surfaces is discussed as an appropriate strategy to assess and to quantify radionuclide migration.
Introduction Geochemical aspects of radionuclide migration are considered in many performance assessment (PA) calculations in a rather simplified way. Radionuclide release from the nuclear waste form is determined by solubility constraints with regard to a given solid phase and radionuclide retardation is described by a Kd value, i.e. the radionuclide distribution between solid and liquid phase for a given groundwater rock/sediment system. Geochemical sorption models have been developed in order to allow the adequate description of sorption phenomena at variable geochemical conditions. They take different sorption mechanisms and the influence of surface charge into account but in many cases without unambiguous and independent evidence for the assumed reaction processes. Strong efforts have been made during the last decade to collect sorption data, to critically assess and to implement those data into quasi-thermodynamic sorption models in order to broaden their applicability in geochemical speciation codes (e.g. [1]). Whether it is useful to perform basic research forging ahead to the molecular scales of geochemical processes has been discussed controversially from time to time. The applicability of such detailed information to the safety assessment of nuclear waste disposal is partly questioned. It is feared that the enhanced complexity of radionuclide speciation and their geochemical reactions cannot be implemented into PA in an appropriate way. The alternative strategy to deal with remaining uncertainties is then to select partly unrealistic conservative data. Bradbury& Baeyens [2] recently demonstrate
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