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Enhancing Nuclear Safety


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Coupled modeling of cement/claystone interactions and radionuclide migration

L. De Windt (1),  D. Pellegrini (2), J. van der Lee (1) - Journal of Contaminant Hydrology 68 (2004) p165 -182
(1)Ecole des Mines de Paris, Centre d’Informatique Geologique, 35 R. St-Honore , 77300, Fontainebleau, France
(2)IRSN

 

Document type > *Article de revue

Keywords > geology, radionuclides

Research Unit > IRSN/DSU/SSD, IRSN/DSU/SSD/BECS

Authors > PELLEGRINI Delphine

Publication Date > 16/03/2004

Summary

The interactions between cement and a clayey host-rock of an underground repository for intermediate-level radioactive waste are studied with the reactive transport code HYTEC for supporting performance assessment. Care is taken in using relevant time scales (100,000 years) and dimensions. Based on a literature review, three hypotheses are considered with respect to the mineralogical composition of the claystone and the neo-formed phases. In the long term, the pH is buffered for all hypotheses and important mineral transformations occur both in cement and the hostrock. The destruction of the primary minerals is localized close to the cement/claystone interface and is characterized by the precipitation of secondary phases with retention properties (illite, zeolite). However, beyond the zone of intense mineral transformations, the pore water chemistry is also disturbed over a dozen meters due to an attenuated but continuous flux of hydroxyl, potassium and calcium ions. Four interdependent mechanisms control the profile in the whole system: diffusion of the alkaline plume, mineralogical buffering, ion exchange and clogging of the pore space at the cement/claystone interface. The migration of a selected group of radionuclides (Cs, Ra, Tc and U) is explicitly integrated in the simulations of the strongly coupled system. Theoretical profiles of distribution coefficient (Kd) and solubility limit values are derived from the simulations, and their sensitivity with respect to the system evolution is estimated.

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