Mathematical modelling of radioactive waste leaching

Abstract

To assess the safety of radioactive waste disposal, knowledge and understanding of radionuclide migration mechanisms must be improved. The long time scale leachability of cement-based waste forms is considered, comparing mathematical modelling and results of dynamic leaching tests on full sized blocks of radioactive wastes. Comparisons for 137Cs release curves show a good fit and establish changes in the cement-based matrix pore structure with time. Variation of distribution coefficients and/or variation of effective diffusivity could be used to better explain small discrepancies between observed experimental data and calculation. Further more, the progressive appearance on the surface of concrete solidified wastes of whitish neo-formed minerals (calcium carbonate) could be a complementary explanation to these deviations. The use of low solubility limitation is necessary in order to better fit experimental data and calculation results for 241Am. In this case, the migration of americium is not sorption dependent. The displacement of the dissolution front is so slow that a steady state concentration profile will have time to develop regardless of the retardation factor. The confining capacity of cement-based matrix quantified through the annually leached radionuclide fractions is calculated. Conclusions are drawn for the long time scale behavior of concrete solidified L/ILW which may influence the design of the confining system.