Realistic integration of sorption processes in transport programs for long-term safety analysis


Realistic integration of sorption processes in transport programs for long-term safety analysis

Brendler, V.; Stockmann, M.; Noseck, U.; Püschel, A.

Safety assessment of radioactive waste repositories in salt rock formations considers the overlying sedimentary rock above the salt domes as important barrier. Sorption on mineral surfaces of the sediment can retard the transport of many contaminants, namely radionuclides, considerably. Previously, the retention of radionuclides has been described in respective computer programs by temporally constant distribution coefficients.
In the present study, an existing transport program r3t [1] is extended towards a more realistic description of the radionuclide migration under changing geochemical conditions. The methodology developed here is based on a description of the sorption of radionuclides as a function of important influence factors such as pH, pCO2, ionic strength, and the mineral phases being present. Applying surface complexation models, multidimensional matrices of “smart Kd”-values can be computed a-priori. The reactive transport model r3t then can call for each time-space point Kd values adapted for the correct geochemical conditions. The biggest challenge here was finding a fast and robust algorithm for search and averaging in multidimensional matrices with non-equidistant population (see e.g. [2]). First results are presented for a test case where respective site-specific geochemical conditions in the overlying rock have been evaluated, mainly consisting of tertiary and quaternary sands and clays [3].
References:
[1] Fein, E., Software Package r3t. Model for Transport and Retention in Porous Media. Final Report. Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Report GRS-192, Braunschweig 2004.
[2] McNames, J.: A fast nearest neighbor algorithm based on a principal axis search tree. IEEE Trans. Pattern Analysis Machine Intell. 23 (2001), 964-976.
[3] Klinge, H., Köthe, A., Ludwig, R.-R., Zwirner, R., Geologie und Hydrogeologie des Deckgebirges über dem Salzstock Gorleben. Z. Angew. Geol. 2 (2002), 7-15.

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