Speaker
Description
Colloid-facilitated transport is a key process in the migration of radionuclides through the geosphere and is highly relevant for the long term safety assessment of deep geological repositories. COFRAME-2 is a new computational module for colloid-facilitated radionuclide transport in fractured–porous systems, developed for application in repository safety analyses.
The physical system is conceptualized as a fractured-porous medium, modeled as a planar, water-filled fracture of specified width and length embedded within a fully saturated porous rock matrix with groundwater flow in the fracture and matrix diffusion into stagnant pore water.
Transport and retention processes include advection–dispersion in the fracture, sorption at the fracture surface (either kinetic or equilibrium with retardation), diffusive mass exchange with a sorbing porous matrix described by linear equilibrium sorption, and radioactive decay acting on all radionuclide inventories in both fracture and matrix domains. The central feature of COFRAME-2 is the refined treatment of sorption on colloids, represented by two parallel kinetic sorption sites for each radionuclide on both mobile and filtered colloids. Each sorption path follows the same linear kinetic law but is characterized by its own distribution coefficient and rate constant, allowing a single radionuclide to exhibit fast and slow sorption components on the same colloid population and thus providing a more flexible and mechanistically plausible description of colloid facilitated transport.
Colloid-facilitated transport is represented by separate balance equations for dissolved radionuclides, radionuclides sorbed on mobile colloids, and radionuclides sorbed on filtered colloids, each coupled to the others via kinetic exchange terms. Depending on whether sorption at the fracture surface is modelled kinetically or via an equilibrium retardation concept, the resulting system comprises seven or six coupled equations per radionuclide, respectively. The equations are discretized in space using finite differences (with upwind advection and central differences for dispersion and diffusion) and in time with a Crank–Nicolson scheme, with particular emphasis on strict mass conservation at inflow boundaries and on robust handling of user-defined parameter choices that may deviate from equilibrium assumption.
COFRAME-2 is implemented as a computational module within RepoTREND [1, 2], the GRS developed program package for integrated long-term safety analyses of radioactive waste repositories, allowing its combination with other transport models along repository relevant pathways and with biosphere models.
COFRAME-2 is applied to the numerical re-analysis of colloid-facilitated radionuclide transport experiments to test the capabilities and parameter sensitivity of the two-site colloid sorption model. The contribution presents the mathematical model, its numerical implementation, and representative test cases that demonstrate the impact of two-site colloid sorption on breakthrough behavior.
| References | [1] Reiche, T. et al.: Simulation von Nahfeldprozessen in einem Endlager für radioaktive Abfälle, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, GRS-742, Braunschweig, 2025 [2] https://www.grs.de/en/research-and-assessment/disposal/repotrend-repository-safety-analysis |
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| Country | Germany |
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