Transport of Uranium and Neptunium in the Saturated Volcanic Tuff and Alluvium at Yucca Mountain
Abstract
The saturated volcanic tuff and alluvium located downgradient of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada both have characteristics that can substantially retard radionuclide migration before reaching the regulatory compliance boundary. The partitioning of radionuclides between these heterogeneous materials and groundwater has historically been characterized by Kd values that are estimated from batch sorption and short-term desorption experiments. These Kd values, which typically have narrow ranges, are incorporated into transport models that assume reversible, linear, first-order sorption kinetics. The batch sorption and short-term desorption experiments do not effectively interrogate stronger sorption sites with slow desorption kinetics because they are not conducted long enough to see the effects of slow desorption. In uranium (U) and neptunium (Np) column experiments conducted with Yucca Mountain alluvium, there was an increase in the attenuation of both radionuclides relative to nonsorbing tracers as residence times increased. This type of response cannot be modeled using a single Kd value or a narrow range of Kd values and assuming linear first-order kinetics. We conducted U and Np long-term laboratory desorption experiments with volcanic tuff, alluvium and water collected from boreholes along potential flow pathways between the proposed repository and the regulatory compliance boundary. We used a four-sorption-site kinetic model to fit the long-term desorption data, which resulted in desorption rate constants that decreased with time and spanned over three orders of magnitude. These results suggest that a single partition coefficient (Kd value) derived from sorption or short- term desorption experiments will underestimate uranium and neptunium retardation in these materials. We believe that the effective Kd values that we have calculated more accurately represent the transport of uranium and neptunium in the heterogeneous volcanic tuff and alluvium over long distance and time scales. To corroborate the effective Kd values, a one-dimensional advection-dispersion equation modified to include multiple sorption sites and incorporating the sorption and desorption rate constants estimated from the experimental data was used to model the tracer responses observed in the U and Np column experiments. The modeling results indicate that much more realistic radionuclide transport predictions can be achieved if multiple sorption sites and slow desorption rates are accounted for.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H31B0867D
- Keywords:
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- 1832 Groundwater transport;
- 1835 Hydrogeophysics;
- 1869 Stochastic hydrology;
- 1873 Uncertainty assessment (3275);
- 3265 Stochastic processes (3235;
- 4468;
- 4475;
- 7857)