Sorption Characteristics of Mobile Species in Cement Based Waste Forms Using a Linear Sorption Isotherm

The nuclear industry is constantly striving to achieve cost-effective methods for the responsible disposal of radioactive wastes. Cement-based grouts are the most widely used hosts for the immobilization of low-level liquid radioactive waste. Ultimately, the effectiveness of waste immobilization in grouts is determined by the degree to which the waste form protects the environment from the release of radionuclides contained in the waste. For near-surface and sub-surface monolithic waste forms, the most credible transport pathway to the environment is via groundwater movement. A measure of the waste form's ability to retard the release of radionuclides to the environment is its leachability. Hence, leach data provide the source term for the impact or consequence analyses.

The appropriate model for describing the waste form's leachability and the appropriate tests used to measure the critical parameters have long been topics for debate. This lack of consensus is caused by the diversity and the complexity of the interaction mechanisms coupled with transport phenomena that affect radionuclide release.

The purpose of this research is to extend current immobilized waste form transport models by explicitly including effects of species adsorption to internal surfaces and liquid/solid mass transport resistances. The first phase of the research characterized the dynamic leaching behavior of a cement based waste form taking into account mass transfer resistance due to macropore diffusion and external fluid film mass transfer. Experiments carried out on cement disks with forced flow were used to assess the relative effects of diffusion and adsorption on control of release. The second phase of the research utilized the results of the first phase in a numerical study of convective/adsorptive transport. The data were regressed using both an ideal plug flow and a dispersed flow model. These models allow for determination of mass transfer resistance due to macropore diffusion and external fluid film mass transfer.

The first analysis assumed ideal plug flow through the sample. The data did not verify this assumption, so a dispersed flow model was assumed. The predictions from this model more closely approximated the data. These predictions were then compared to results obtained from each of the samples to determine if conditions exist where adsorptive behavior dominates the system. Similarities exist for those samples prepared with either simulated waste or additives.

The addition of ionic species in the aqueous phase apparently affects the hydration process as well as increasing the ionic strength of the porewater. The increase in ionic strength of the pore water resulted in an increase in the retention of the adsorbing species. This increased retention could also be due to the increased complexity of the cement matrix and hydration products. As a result, the breakthrough time was increased and the rate of release was decreased.