Simulating Radionuclide Transport Away from an Underground Nuclear Test: Linking Non-Isothermal Flow and Mechanistic Reactive Transport

by Mavrik Zavarin,
Steven F. Carle,
Qinhong Hu,
Annie B. Kersting,
Reed M. Maxwell,
Gayle A. Pawloski,
Eric Ramon,
Sarah K. Roberts,
Dana E. Shumaker,
Andrew F. B. Tompson,
Pihong Zhao,



Document Type: Proceeding Paper

Part of: World Environmental and Water Resources Congress 2008: Ahupua'A

Abstract:

The low-yield (0.75 kiloton) Cambric underground nuclear test situated in alluvium below the water table offers unique perspectives on radionuclide transport in groundwater. The Cambric test was followed by extensive post-test characterization of the radionuclide source term and a 16-year pumping-induced radionuclide migration experiment that captured more mobile radionuclides in groundwater. Discharge of pumped groundwater caused inadvertent recirculation of radionuclides through a 220-m thick vadose zone to the water table and below, including partial re-capture in the pumping well. Non-isothermal flow simulations indicate test-related heat persists at Cambric for about 10 years and induces limited thermal convection of groundwater. The test heat has relatively little impact on mobilizing radionuclides compared to subsequent pumping effects. However, our reactive transport models indicate test-related heat can raise melt glass dissolution rates up to 10⁴ faster than at ambient temperatures depending on pH and species activities. Non-isothermal flow simulations indicate that these elevated glass dissolution rates largely decrease within 1 year. Thermally-induced increases in fluid velocity may also significantly increase rates of melt glass dissolution by changing the fluid chemistry in contact with the dissolving glass.

Subject Headings: Radioactive materials | Flow simulation | Pumping tests | Glass | Water supply systems | Fluid velocity | Water table

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