A Model for Redox Control in A Cementitious Repository
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ABSTRACT Calculations have been performed to investigate long timescale redox control in a cementitious deep waste repository, once there has been significant corrosion of the steel containers in the near field. The methodology considers reaction of the porewater with redoxsensitive solids. The solids were selected based on considerations of the repository concept, the waste inventory and the processing of the waste into a form suitable for disposal. A number of factors that may affect the redox potential have been considered. Amongst these are the form of the redox-controlling solids, the effect of changes in pH, the effect of sorption onto the cementitious backfill, and the effect of the association of dissolved radionuclides with complexants arising from the degradation of organic wastes. The calculations suggest that for the United Kingdom Nirex Limited (Nirex) disposal concept and inventory, the redox system in the near field of the repository would be controlled by the iron(Il)/iron(I) redox couple. This would maintain the redox potential in the repository near field at a low value for timescales in excess of 1 million years. INTRODUCTION In the deep repository concept developed by Nirex for the disposal of intermediate-level and certain low-level solid radioactive wastes, vaults would be excavated at depth in a stable geologic environment and the wastes, packaged largely in steel containers, would be placed in these vaults. The space between the waste packages would be backfilled with a cement-based material, the Nirex Reference Vault Backfill (NRVB). The redox potential of the porewater in the near field of a cementitious repository would have a significant effect on the speciation of a number of radioelements present in the waste. The solubilities of redox-sensitive radioelements tend to be lower under more reducing conditions and their sorption tends to be higher. A lower near-field redox potential is, therefore, beneficial for these aspects of repository performance. The issue of redox potential in the near field of the repository has been addressed for the long timescale, once there has been significant corrosion of the steel containers in the near field. This paper does not consider the relatively short timescale immediately following repository closure, when the repository may be considered to be in a transient state with rates of oxygen consumption determining the extent to which the repository is reducing at any given time. METHODOLOGY Iron and steel consume oxygen through the corrosion process and lead to the establishment of reducing conditions. For a system at equilibrium this is represented by the redox potential (Eh) of the repository. Further calculations have been undertaken to establish the controls on Eh once this corrosion process is complete. A methodology has been developed for predicting the long-term redox potential in the repository porewater based on a thermodynamic approach [1]. This initial study showed that the redox potential in the near field of the repository was 1245 Mat. Res.
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