Modelling of the Evolution of Porewater Chemistry in a Cementitious Repository
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MODELLING OF THE EVOLUTION OF POREWATER CHEMISTRY IN A CEMENTITIOUS REPOSITORY.
A. Haworth, S.M. Sharland and C.J. Tweed, Theoretical Physics Division, Harwell Laboratory, Oxfordshire, U.K. ABSTRACT The current concept for disposal of low- or intermediate-level nuclear waste in the UK includes a largely cementitious backfill. An important factor in determining nuclide release rates from the repository is the pH in the near-field. In this paper, we describe development of a coupled chemistry/transport model of cement degradation and its application in determining the evolution of the chemistry within the repository. In particular, the effect of the cement formulation and interaction with groundwater species on the evolution of the pH is examined. The model is sufficiently flexible to consider a number of different repository scenarios. 1.INTRODUCTION. The current concept for disposal of low- or intermediate level waste in the U.K. comprises an underground repository with a largely cementitious backfill. An assessment of safety of such a facility is based on the calculations of transport of nuclides from the repository to the biosphere. Research models are used to test the assumptions made in the safety assessment using information from experiments. An important factor in determining nuclide release rates from the repository is the pH of the near-field. The aqueous phase of the cement is expected to provide a high pH environment in which the solubilities of many nuclides are reduced and the rate of general corrosion is low. In previous modelling, it has been assumed that a high pH will exist for a period of about 106 years[1]. This assumption, however, does not take into full account the chemical interactions between the cement and the groundwater. There may, for example, be local variations within the backfill which produce areas of lower pH through reactions with groundwater species. The evolution of pH in the backfill also depends on the formulation of the cement used. In a previous paper[21, we described the preliminary development of a research model of the degradation of cement in a repository. A thermodynamic description of cement developed by Berner[31 was used in a coupled chemistry/transport model of cement leaching. We now describe further development of the model to investigate both the effect of different cement types and of interactions with groundwater on the evolution of pH in repository. Results from the coupled model are compared with experimental observations on the evolution of pH for different cement types. The experiments formed a separate part of the Nirex Safety Studies program and use a technique called accelerated leaching to simulate the long term effects on the pH of contact with both de-mineralised water and a groundwater[1,4]. Such a comparison increases confidence in the model predictions. The spatial variation in pH and effects such as pore blocking by minerals can then be examined in more detail with the model. The modelling is carried out using the coupled chemistry/transport progra
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