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Technical Report NTB 17-04

An evaluation of sulphide fluxes in the near field of a HLW repository

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According to the current reference concept for the deep geological disposal of high-level radioactive waste (HLW) in Switzerland, steel canisters will be used to dispose of the spent fuel and vitrified waste. Surrounding a steel canister, a buffer material will be emplaced to maintain a diffusive transport regime and to retain the released radionuclides after breaching of the canister. Within the buffer, favourable hydrogeochemical conditions guarantee a low canister corrosion rate and thus a long canister life-time. Instead of a steel canister, an alternative concept based on a copper coated canister is currently being developed. Thanks to its negligible corrosion rate in oxygen-free water a copper coating significantly prolongs the canister life-time. In such a case, the presence of sulphide in the near field of the canister requires additional attention, as it is a very effective oxidant for copper. The aim of this report is to investigate the sulphide fluxes in the near field as a function of the buffer material and properties.

The sulphide fluxes in the near field are defined by the hydrogeochemical conditions in the near field, which are to a large part defined by the emplaced buffer material. Three different types of near field are considered in this report: a well-emplaced bentonite buffer, a bentonite buffer with reduced density and a crushed Opalinus Clay buffer. To calculate the sulphide fluxes towards the canister, two different models are used. A simplified transport model in which the diffusive transport of sulphide, produced by the microbial reduction of sulphate, towards the canister is modelled, and a reactive transport model in which the sulphide concentration is defined by the entire chemical environment in the near field (pH, Eh, Fe, microbial activity, …), before being diffusively transported towards the canister. Once the sulphide is at the canister interface, a one-step corrosion mechanism based on the stoichiometry of the simplified corrosion reaction was assumed. This means that the calculated corrosion depths in this report represents an average corrosion depth.

As both models show, the potential corrosion depths after 1'000'000 years in the well-emplaced bentonite buffer scenario are the lowest (0.08 – 0.2 mm), followed by the reduced density bentonite (1.3 – 2 mm) and the crushed Opalinus Clay scenario (3.1 – 3.4 mm). The lower values are provided by the reactive transport model, the upper bound values by the simplified model. With the help of the simplified model, the sensitivity of the sulphide flux to several parameters (e.g. sulphide solubility, anion diffusion coefficient, oxidised pyrite in the EDZ, gypsum concentration in bentonite) could be shown. The most important parameters are shown to be the sulphide solubility, followed by the diffusion coefficient of sulphide and sulphate.

Besides calculating the sulphide fluxes and their impact on corrosion, these models have also illustrated the importance of the sulphide solubility limit. The presence of iron in the near field (arising from goethite in bentonite or siderite in Opalinus Clay) induce the precipitation of FeS minerals, which limits free sulphide in solution. Linked to the solubility limit, are the pH and Eh of the near field, which also influence the free sulphide in solution. Several special cases that influence pH or Eh (such as H2 production from anoxic corrosion of construction steel, presence of cement liner or the effect of pH decrease due to microbial activity) are evaluated by the reactive transport model. In none of these cases does the corrosion depth increase beyond the reference cases.

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