Technetium - 99 Chemistry in Reducing Groundwaters : Implications for the Performance Of

This work was conducted as an activity under the Radionuclide Transport (RT) Key Technical Issue (KTI). In the current U.S. Department of Energy (DOE) Total System Performance Assessment (TSPA) for the proposed Yucca Mountain high-level nuclear waste repository, Tc-99 has consistently represented a significant proportion of the dose to the critical group. This is due to the high solubility and low sorption of technetium under oxidizing conditions. As part of its site characterization activities in preparation for a license application, the DOE has obtained Eh-pH measurements at various depths from the saturated zone in a single well in fractured tuff, indicating localized zones of reducing conditions. Under reducing conditions, technetium has low solubility and is strongly sorbed. The work presented in this manuscript used the NRC TPA Version 3.2 code to explore the potential effects of reducing conditions on Tc-99 solubility and transport and the resulting impact on repository performance. This manuscript documents ongoing issue resolution activities in the RT KTI. The sensitivity analyses indicate that localized reducing conditions have the potential to significantly reduce the dose from Tc-99 to the critical group. Although there is some effect in the transport of technetium through the fractured tuff aquifer in TPA Version 3.2, the major effect is due to increased retardation in the alluvium. This is due to treating the alluvial aquifer as a homogeneous porous medium with readily accessible sorption sites. The suggested importance of the alluvium in this analysis underscores the importance of alluvium characterization, either as part of site characterization, through the Nye County Early Warning Drilling Program, or as part of performance confirmation. The results of these sensitivity analyses will be used to investigate alternative conceptual models, provide added realism to TPA, and to develop the flexibility to evaluate changes in future DOE work as new data come out on redox conditions in the SZ and the alluvium. These analyses will also be used to develop the technical bases for acceptance criteria and review methods in the Yucca Mountain Review Plan. If you have any questions about this deliverable, please call me (210) 522-5540 or Dr. David Turner (210) 522-2139. ABSTRACT 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 …