Paper No. 6
Presentation Time: 9:15 AM


SIMMONS, Ardyth M., EP, Los Alamos National Laboratory, Emeritus, P.O. Box 1663, MS-M992, Los Alamos, NM 87501 and NEYMARK, Leonid A., US Geological Survey, Denver Federal Center, Mailbox 25046, MS 963, Denver, CO 80225,

Characteristics of host rocks, secondary minerals, and fluids would affect the transport of radionuclides from a previously proposed high-level nuclear waste repository in a ~700 m thick vadose zone at Yucca Mountain, Nevada. Secondary minerals in the Yucca Mountain tuffs that are important for retarding radionuclides include clinoptilolite, mordenite, clays, and Fe-Mn oxides/hydroxides. Groundwater compositions are controlled by silica and calcite precipitation, and dissolution and ion-exchange reactions. Radionuclide concentrations along flow paths from repository depths could be limited by (1) low waste-form dissolution rates, (2) low radionuclide solubility in groundwater, and (3) radionuclide sorption onto geologic media.

Laboratory and field-scale experiments have shown that fractures provide advective transport pathways for radionuclides in the unsaturated zone. Sorption onto secondary fracture coatings and matrix diffusion, however, may contribute to radionuclide retardation whereas sorption onto mobile colloids may enhance radionuclide transport. Retardation in saturated alluvium would rely on the availability of clays and Fe-Mn-oxides/hydroxides for sorption.

Experiments concentrated on the geochemical mobility of americium, plutonium and neptunium, because these would be the chief sources of radioactivity in spent nuclear fuel at a time when the waste packages might have failed. Other radionuclides were included in some experiments. Solubilities and sorption coefficients were determined experimentally with representative Yucca Mountain waters, rocks, pure minerals, and organic materials. Sorption onto organic materials was not significant for the Yucca Mountain geochemical environment. Batch experiments gave slightly lower retardation factors than those derived from column-breakthrough experiments. This finding indicates that using batch-sorption coefficients to predict radionuclide transport yields conservative results in a performance assessment.