Technetium-99 Chemistry in Reducing Groundwaters: Implications for the Performance of a Proposed High-Level Nuclear Waste Repository at Yucca Mountain, Nevada

Performance assessment calculations by the U.S. Department of Energy and the Nuclear Regulatory Commission indicate that Tc-99 is a major contributor to dose to a hypothetical receptor group 20 km downgradient of a proposed high-level nuclear waste repository at Yucca Mountain, Nevada, within the first 10,000 yr after permanent closure. This result is due in large part to the high solubility and low retardation of Tc under oxidizing conditions in the Yucca Mountain environment. Recent site characterization data on the chemistry of saturated zone groundwater at Yucca Mountain and vicinity indicate the presence of locally reducing geochemical conditions, which could decrease the solubility and enhance the sorption and retardation of Tc-99. In this study, a preliminary assessment of the potential effects of reducing conditions on the transport and release of Tc-99 was conducted. Sensitivity analyses using the NRC/CNWRA Total-system Performance Assessment code (TPA Version 3.2) indicate that decreased Tc solubility and increased Tc sorption due to reduction of Tc(7+) to Tc(4+) can significantly delay the arrival of Tc-99 at the receptor group location. Decreased Tc solubility can decrease the Tc-99 dose by three orders of magnitude relative to the TPA 3.2 base case. Enhanced Tc retardation in the tuff aquifer only does not greatly decrease the calculated Tc-99 peak dose, whereas increased Tc retardation in the alluvial aquifer alone prevents Tc-99 from reaching the receptor group in 50,000 yr. The release and transport of other redox-sensitive radioelements could be affected in a manner similar to Tc. Thus, reduced groundwater conditions could significantly enhance the performance of the geologic barrier system and reduce the dose to the receptor group.