Assessment of Redox Conditions in the Near Field of Nuclear Waste Repositories: Application to the Swiss high-level and
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$VVHVVPHQWRI5HGR[&RQGLWLRQVLQWKH1HDU)LHOGRI1XFOHDU:DVWH5HSRVLWRULHV $SSOLFDWLRQWRWKH6ZLVVKLJKOHYHODQGLQWHUPHGLDWHOHYHOZDVWHGLVSRVDOFRQFHSW Paul Wersin, Lawrence H. Johnson, and Bernhard Schwyn National Cooperative for the Disposal of Radioactive Waste (Nagra), 5430 Wettingen, Switzerland $%675$&7 Redox conditions were assessed for a spent fuel and high-level waste (SF/HLW) and an intermediate-level waste (ILW) repository. For both cases our analysis indicates permanently reducing conditions after a relatively short oxic period. The canister-bentonite near field in the HLW case displays a high redox buffering capacity because of expected high activity of dissolved and surface-bound Fe(II). This is contrary to the cementitious near field in the ILW case where concentrations of dissolved reduced species are low and redox reactions occur primarily via solid phase transformation processes. For the bentonite-canister near field, redox potentials of about -100 to -300 mV (SHE) are estimated, which is supported by recent kinetic data on U, Tc and Se interaction with reduced iron systems. For the cementitious near field, redox potentials of about -200 to -800 mV are estimated, which reflects the large uncertainties related to this alkaline environment. ,1752'8&7,21 The determination of redox conditions in the near field of a nuclear waste repository constitutes an important aspect of safety assessment. Redox conditions strongly affect the mobility and sorption of many safety-relevant radionuclides such as U, Np, Pu, Tc and Se. Redox potentials (Eh) are affected by both the thermodynamics and kinetics of relevant reactions, some of which are not adequately understood. This leads to considerable uncertainty of redox conditions in the repository environment and consequently oversimplified terminology in performance assessment such as ‘reducing’ or ‘oxidizing’ has often been used. The objective of this study is to estimate reasonable Eh values and their uncertainties as basis for determining speciation of redox-sensitive radionuclides in the near field. Thus, we assess redox conditions by a holistic approach that considers all relevant sources of information. This approach is applied to the near field of the two types of repositories foreseen in the Swiss high-level waste program [1]: the spent fuel and vitrified high-level waste (SF/HLW) and the intermediate-level reprocessing waste (ILW) repositories. 0(7+2'2/2*
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