Colloid Transport of Radionuclides: Yucca Mountain Performance Assessment

Abstract

ABSTRACTColloids have the potential to enhance transport of contaminants in the subsurface. Kersting et. al.,[1] have shown that plutonium, as well as europium, cobalt and cesium, were associated with groundwater colloids at the Nevada Test Site.240Pu/239Pu isotopic evidence confirmed that the plutonium was from the Benham nuclear test located 1.3 km away. This observation has lead to an increased concern that colloids may be potential vectors for radionuclide transport from an underground geologic repository [1].The Anaylsis/Model Reports (AMRs) describe the characteristics of colloids for sparingly soluble radionuclides. The types of colloids formed, stability of the colloids as a function of pH and ionic strength, and the attachment of radionuclides to the colloids are modeled by the AMRs. The AMRs attempt to present bounding and conservative estimates for key parameters; however, waste form corrosion data, natural system studies, and experimental data do not always support the assumptions made in the AMRs. Most specifically, all colloids at Yucca Mountain, whether generated by the dissolution of waste glass or spent nuclear fuel or occurring naturally in groundwater, are assumed to be either smectite or iron-oxy-hydroxides. Data from waste form corrosion experiments do not support this assumption: numerous other phases have been identified including weeksite, clay particles with brockite inclusions, schoepite, soddyite, uranophane, boltwoodite, calcite, dolomite, Na-silicates, Al-oxides, absolone, birnessite, analcime, hydrated silicates, and apatite. Furthermore, the response of many of these phases to changes in pH or ionic strength is not known. If fundamental data for these phases, such as the point of zero charge (PZC), is different than that of either smectite or iron oxy-hydroxides, the TSPA may not accurately predict colloid transport of radionuclides.We have analyzed colloid samples from groundwater at the Nevada Test Site using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopic (EDS) elemental mapping, and found that light rare-earth elements (LREE) and Th are associated with monazite. Cesium and uranium are associated with an unidentified phase that is rich in Si and Fe. In addition, Co-60, a fissiogenic element, is associated with a complex metal colloid that includes Fe, Ni, Mo, and Cr. None of these phases are accounted for in the AMRs and their response to changing geochemical conditions is unknown. In addition, the PZC of several Mn-oxide phases was determined and was found to extend beyond the range of PZCs accounted for in the AMRs by smectite and iron oxy-hydroxides. Pu has been shown to preferentially adsorb onto Mn-oxides in the presence of Fe-oxides [2, 3], and the AMRs may not accurately predict Mn-oxide colloid stability and subsequent transport of radionuclides of interest.