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Technical Report NTB 84-48

The Geochemistry of the Near-Field

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The near-field of a high-level waste repository is an extremely complex environment which results from many simultaneous physical, chemical and hydrological processes. Analysis of this environment is hindered, not only by the number of processes involved but also by their synergistic/competitive interactions and their variations in space and time. This report describes a study of the Swiss disposal concept used in "Project Gewähr 1985" safety analysis which attempts to quantify as many of these processes as possible.

The main components of the near-field are the waste glass matrix, the thick steel canister and the surrounding backfill of compressed bentonite.

In this report it is concluded that mineralogical alteration of the backfill will be negligibly small over the million year period considered. Its physical and chemical properties can thus be relied on for such a period. The canister will retain its integrity for > 103 y and thereafter will act as an Eh/pH buffer. The near-field buffers ensure more alkaline and reducing conditions than in the far-field. Complete degradation of the glass matrix will take > 105 years and nuclide release will be limited by their congruent dissolution although it may be further constrained by low solubility. Diffusion of dissolved nuclides through the backfill is so slow that many species decay to insignificance within it. The large uptake capacity of the bentonite also significantly extends the release duration for longer lived, non-solubility limited nuclides thus decreasing output maxima. Possible perturbing factors such as radiolysis and hydrogen production by anoxic corrosion are of little importance but modelling of speciation / solubility in the near-field and, in particular, colloid formation and mobility are identified as areas in which more work is required.

Although the main analysis aims to err on the side of conservatism, the extent of such pessimism is assessed in a "realistic" appraisal of the near-field. This suggests that the engineered barriers will prevent any radiologically significant releases over periods in excess of a million years which would strengthen their role in the multiple barrier safety concept.

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