Intercomparison of Ta and Ti solid-electrodes for pH measurements under oxic and anoxic conditions in reconstituted waters of a future nuclear waste disposal - BRGM - Bureau de recherches géologiques et minières Access content directly
Conference Poster Year : 2017

Intercomparison of Ta and Ti solid-electrodes for pH measurements under oxic and anoxic conditions in reconstituted waters of a future nuclear waste disposal

Abstract

In France, the deep geological disposal has been chosen to manage long-lived high and middle activity nuclear waste. This project is supervised by " the Agence National pour la Gestion des Déchets Radioactif" (ANDRA-French national radioactive waste management agency). An underground research laboratory (URL), dedicated to host-rock properties characterization was built at approximately-490m depth in the Callovo-Oxfordian geological formation (COx), which is a potential candidate for nuclear waste disposal. Based on this overall strategy, Andra has analysed the technical requirements that must be met by adapted monitoring equipment. First, these must be able to provide information on key THMCR (Thermal-Hydraulic-Mechanical-Chemical and Radiological) processes, to provide a three-dimensional image of its behaviour and thus to understand the underground installation functioning, in particular the cell interactions with the near-field. This study aims to develop innovative all-solid-state electrodes made of iridium (Ir), ruthenium (Ru), tantalum (Ta), titanium (Ti), tungsten (W), niobium (Nb) and a tin-lead alloy (Sn/Pb), for pH measurement. Moreover, the potentiometric response of these electrodes is mainly based on Metal Oxide (M x O y)/Metal (M) equilibriums, which make them react to changes of oxygen partial pressures. For now, the antimony electrode (Sb) showed excellent results regarding the measure of pH under oxic and anoxic conditions. Nevertheless, it is necessary to multiply the electrode materials considered for pH measurements, for preventing a dysfunction of electrodes which could occur over time. By multiplying the electrode materials for pH measurement, we indirectly increase, firstly, the measurement reliability by giving the possibility to inter-compare the potentiometric responses of the set of the electrodes dedicated to pH, and secondly, the monitoring duration, in the case of electrode surface alterations, which could be caused by variations of the physical and chemical parameters within the nuclear waste repository. In other words, this would provide alternatives electrode materials for pH measurements, in prevention to possible electrode surface alterations. Thus, the influence of pH from 5 to 13, on the potentiometric responses of the several electrodes was investigated by means of pH buffers solutions or by continuous and regulated addition of H 2 SO 4 or NaOH, under both oxic (exposed at air) and anoxic conditions (in glove box: 99% N 2 , 1% CO 2 and [O 2 ] < 2 ppm, 25°C). Titanium (Ti) and tantalum (Ta) electrodes showed a potentiometric linear response to pH variations, as shown on figures 1 and 2. In absence of oxygen, the Ti electrode showed a linear response to pH (from 5.8 to 11), close to the theory. After pH 11, a drop of potential is observed. In presence of oxygen, the response to pH is linear but further to the theory. In the same way, the potentiometric behaviour of the Ta electrode to pH variations is linear, but much more sensitive under anoxic conditions.
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Dates and versions

hal-02165010 , version 1 (25-06-2019)

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  • HAL Id : hal-02165010 , version 1

Cite

Jordan Daoudi, Stéphanie Betelu, Johan Bertrand, Théodore Tzedakis, Ioannis Ignatiadis. Intercomparison of Ta and Ti solid-electrodes for pH measurements under oxic and anoxic conditions in reconstituted waters of a future nuclear waste disposal. Clay Conference 2017 - the 7th International Conference on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Sep 2017, Davos, Switzerland. , 2017. ⟨hal-02165010⟩
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