The United States Department of Energy is conducting a program designed to reclaim or stabilize inactive uranium-mill tailings sites. As part of this program, known as the Uranium Mill Tailings Remedial Action (UMTRA) Program, the Pacific Northwest Laboratory is evaluating existing liner materials for their effectiveness in preventing migration of water-soluble radionuclides and hazardous chemicals over the long term. This report presents the status of the Pacific Northwest Laboratory's

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... on. Proposed regulatory requirements were reviewed to determine material performance criteria; then, potential liner materials were screened for their effectiveness under expected exposure conditions identified in a tailings characterization study. The liner criteria are that the liner must be effective for 1000 yr, have less than 10-7 cm/s permeability, and must be unaffected by leachate contact and weathering conditions. The materials were screened through literature evaluations and industrial contacts. The materials studied included locally available clays and soils, soil treatments, bentonite amendments to soil, asphalt membranes, asphalt concretes, treated grouts and concretes, and elastomeric and plastic membranes. Of these categories, eight liners were chosen for laboratory testing under accelerated exposure conditions. The characterization study, carried out to help define those conditions, identified the wide variability in pH, calcium concentration, anions, and radionuclide content. The status report also presents the information gathered during the field studies at Grand Junction, Colorado. Two liners, a bentonite, sand, and gravel mixture, and a catalytic airblown asphalt membrane, were installed in a prepared trench and covered with tailings. The liners were instrumented and are being monitored for migration of moisture, radionuclides, and hazardous chemicals. The two liner materials will also be subjected to accelerated laboratory tests for a comparative assessment. The eight materials selected for laboratory testing are: • natural soil amended with sodium-saturated montmorillonite (Volclay);