HR: 09:00h
AN: GS22A-04    [Abstracts]
TI: Radiation-Induced Defects in Kaolinite as Tracers of Past Occurrence of Radionuclides in a Natural Analogue of High Level Nuclear Waste Repository
AU: * Allard, T
EM: thierry.allard@impmc.jussieu.fr
AF: IMPMC, UMR7590, UPMC, IPGP 140 rue de Lourmel, PARIS, 75015, France
AU: Fourdrin, C
EM: chloe.fourdrin@impmc.jussieu.fr
AF: IMPMC, UMR7590, UPMC, IPGP 140 rue de Lourmel, PARIS, 75015, France
AU: Calas, G
EM: georges.calas@impmc.jussieu.fr
AF: IMPMC, UMR7590, UPMC, IPGP 140 rue de Lourmel, PARIS, 75015, France
AB: Understanding the processes controlling migrations of radioelements at the Earth's surface is an important issue for the long-term safety assessment of high level nuclear waste repositories (HLNWR). Evidence of past occurrence and transfer of radionuclides can be found using radiation-induced defects in minerals. Clay minerals are particularly relevant because of their widespread occurrence at the Earth's surface and their finely divided nature which provides high contact area with radioactive fluids. Owing to its sensitivity to radiations, kaolinite can be used as natural, in situ dosimeter. Kaolinite is known to contain radiation-induced defects which are detected by Electron Paramagnetic Resonance. They are differentiated by their nature, their production kinetics and their thermal stability. One of these defects is stable at the scale of geological periods and provides a record of past radionuclide occurrence. Based on artificial irradiations, a methodology has been subsequently proposed to determine paleodose cumulated by kaolinite since its formation. The paleodose can be used to derive equivalent radioelement concentrations, provided that the age of kaolinite formation can be constrained. This allows quantitative reconstruction of past transfers of radioelements in natural systems. An example is given for the Nopal I U-deposit (Chihuahua, Mexico), hosted in hydrothermally altered volcanic tufs and considered as analogue of the Yucca Mountain site. The paleodoses experienced by kaolinites were determined from the concentration of defects and dosimetry parameters of experimental irradiations. Using few geochemical assumption, a equivalent U-content responsible for defects in kaolinite was calculated from the paleodose, a dose rate balance and model ages of kaolinites constrained by tectonic phases. In a former study, the ages were assumptions derived from regional tectonic events. In thepresent study, ages of mineralization events are measured from U systematics. The corresponding results reveal past accumulation of uranium in the mineralized zone and past leaching in the fissure network of the present barren rock. Geochemical implications for HLNWR will be discussed.
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
SC: Geochemical Society [GS]
MN: 2007 Joint Assembly