RETENTION OF NEPTUNIUM IN THE SATURATED ALLUVIAL SEDIMENTS OF FORTYMILE WASH, NYE COUNTY, NEVADA

The saturated alluvium of Fortymile Wash, Nye County, Nevada, may provide an important barrier to the transport of radionuclides that could be released from the potential high-level waste repository at Yucca Mountain. Risk analyses conducted by the U.S. Nuclear Regulatory Commission (NRC) have determined that retardation of radionuclides in the saturated alluvium is highly significant for waste isolation. Neptunium (Np-237) is a potentially important contributor to dose at long time frames (greater than 100,000 years) and is only moderately retarded by sorption processes. Thus, understanding the capability of the saturated alluvium of Fortymile Wash to retard Np-237 transport is important in evaluating potential repository performance.

Current approaches to simulating neptunium sorption with the NRC Total-system Performance Assessment (TPA) code are based on surface complexation modeling interpretations of experiments conducted on single mineral phases (silicates and aluminosilicates) common to the Fortymile Wash system. Recently collected samples of alluvium and groundwater from Fortymile Wash provided an opportunity to conduct Np-237 sorption experiments on natural alluvium mineral mixtures under geochemical conditions directly relevant to those found in the Yucca Mountain region.

Batch-type sorption experiments were conducted using alluvium collected from two boreholes drilled using a sonic-coring technique, which provided more representative sediment samples than previous rotary drilling. The experiments were conducted over a pH range of about 7 to 9 and at equilibrium with atmospheric CO₂(g) using groundwater or lab-prepared artificial groundwater.

The Np-237 distribution coefficients are moderate, ranging from 10 to 35 mL/g over the pH range studied. The NRC TPA code calculates realistically conservative values from approximately 3 to 10 mL/g over the same pH range. Minor to trace amounts of calcite present in the alluvium samples appear to have a significant impact on the magnitude of sorption, and more realistic models of Np-237 sorption may require the inclusion of the influence of calcite in the sediments.

This abstract is an independent product of the Center for Nuclear Waste Regulatory Analyses and does not necessarily reflect the view or regulatory position of NRC.