Geochemical sensitivity analysis for performance assessment of HLW repositories: effects of speciation and matrix diffusion

Performance assessment requires calculating radionuclide discharge for many sets of conditions and scenarios. In general, such calculations use empirical retardation factors that describe the average effect of all radionuclide/fluid/rock interactions. This method, however, may underestimate radionuclide discharges and disguise potential violations of regulatory standards. An alternative approach, coupled reaction-transport models, can be used to obtain a detailed understanding of physicochemical phenomena but the general application of such rigorous models may be impractical in repository performance assessment. The objective of this geochemical sensitivity analysis is to use simple, approximate models to calculate upper bounds for radionuclide discharges and to identify physicochemical conditions where more rigorous theoretical transport calculations must be done. An approximate model is given for bounding discharges of radionuclides in porous rock in scenarios where radioactive production is negligible but decay is appreciable, and several nuclides of an element migrate and undergo a speciation reaction. The model is incorporated into a methodology to determine critical combinations of hydrological and chemical parameter values that result in discharges that violate the HLW standard proposed by the US Environmental Protection Agency (40CFR 191). As an example, the methodology is applied to the discharges of 243Am and 237Np from a reference repository. It also is shown how results from the sensitivity analyses can be used to develop criteria for designing laboratory experiments so that effects of significant speciation reactions will be detected. Approximate methods for calculating radionuclide discharge are given for extending these analyses to transport in fractured rock where diffusion of radionuclides into the matrix may occur. 27 refs., 12 figs., 1 tab