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La Recherchev2


High performance finite volume finite element multidomain numerical method. Application to radionuclide transport through geological layers



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Titre du congrès :Clays in natural and engineered barriers for radioactive waste confinement
Ville du congrès :Lille
Date du congrès :17/09/2007

Titre de la revue : Physics and Chemistry of the Earth, Parts A/B/C Volume 33 supplément 1 Pages S216-S224


The aim of this paper is to describe numerical methods developed within the Melodie software to perform 3D saturated flow and transport calculations in the framework of assessing the performance of a deep geological repository. The diffusive terms are discretised by a standard finite-element method and the convective term is treated with a Godunov-type finite-volume approach. The simulation domain is meshed using a structured tetrahedral mesh generator, which satisfies the Delaunay and cotangent criteria. A domain-decomposition strategy is developed to deal with a high number of computational nodes and different refinement levels. To illustrate the relevance of this numerical method, this study presents IRSN calculations performed under the NF-PRO European project based on the French "spent fuel / iron canister / clay" concept designed by ANDRA in the framework of its feasibility study of a radioactive waste geological repository. Through various evolution scenarios and sensitivity analyses, these calculations aim at assessing the influence of design features, radionuclide transfer properties of repository components or waste degradation mechanisms on the behaviour of the containment system as a whole. Calculations show, firstly, that seals are effective in limiting the influence of advection with respect to diffusion in the drifts, and the dead-end design of the disposal cells ensures a diffusive transport regime in the vicinity of the canisters. A second set of calculations shows that, when UO2 dissolution is controlled by uranium solubility, the fluxes released from the host rock are almost 10 times lower than when UO2 matrix dissolution is controlled by alpha-radiolysis. The fluxes released from the host rock are therefore essentially supplied by the instantaneous release fraction representing 8 % of the initial inventory. Finally, uncertainty on the chemical speciation of 79Se and 94Nb leads to an uncertainty of several orders of magnitude on the fluxes calculated at the exits of the cells.