Modeling of Geochemical Compatibility of Near Field Materials in Terms of Radionuclide Retention Properties
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P.L. LUCILLE*, A BURNOL, Ph. OLLAR** Electricitd de France, Research and Development Division *Laboratoire National d'Hydraulique, Chatou, France Ddpartement Etudes de Matdriaux, Moret sur Loing, France
ABSTRACT The containment of radionuclides over very long periods of time is based on the interposition of a multi-barrier system between the waste and the biosphere. A performance allocation study is usually conducted for each barrier. Each barrier is then designed, in terms of geometry and composition, to cope with the performance it has been allocated. However geochemical interactions will occur between the different barriers. An alkaline plume will be generated by cement materials, a redox front will be generated by container corrosion and geochemical gradients will be generated by the dissolution of artificial barriers by natural groundwater. Radionuclide retention mechanisms are strongly pH and Eh dependent, therefore the impact of these geochemical transients on retention must be quantitatively evaluated to check the performance of each barrier for realistic in-situ situations. To assess this impact, two types of engineered barriers (clay and cement) for a spent fuel repository are simulated with a coupled hydrogeochemical model. Comparisons between hydraulic heterogeneous (fractured) and simple homogeneous systems are also carried out in terms of waste dissolution. INTRODUCTION The goal of this paper is to study the chemical influence of the main components of the near field of a nuclear waste repository on the dissolution of spent fuel and hence on the radionuclide containment ability of the spent fuel matrix. It is usually admitted that an important fraction, about 95 %,of radionuclides are released congruently to the dissolution of the U0 2 matrix, especially after the instantaneous release of the mobile fission products at the gap between the U0 2 matrix and the cladding'. In order to assess the robustness of a repository concept for spent fuel, the impact of several chemical and hydraulic perturbations on radionuclide release must be
quantitatively evaluated. Most experiments are conducted on simple homogeneous systems over limited periods of time, usually a few months and the results are not easy to extrapolate in time2 A modeling approach gives access to complex systems over the long term. However, the work presented in this paper remains highly methodological and must be seen as an illustration of the potential of advanced modeling tools as a complement to experiments rather than as a predictive study. Several hypotheses are questionable and most of all, many thermodynamical data are still missing or poorly documented and justified. The purpose of this work is to perform numerical experiments on the containment of radionuclides within spent fuel under different scenarios of near field chemical environment. CHEMTRAP (CHEMical And TRAnsport Processes), a coupled mass transport and geochemical model, developed by the Laboratoire National
1075 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Resear
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