Na/Ca selectivity coefficients of montmorillonite in Perchlorate solution at different temperatures
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Na/Ca selectivity coefficients of montmorillonite in perchlorate solution at different temperatures Aku Itälä and Arto Muurinen VTT Technical Research Centre of Finland, Otakaari 3 K, Espoo, FI-02044 VTT, Finland ABSTRACT The Finnish spent nuclear fuel disposal is based on the Swedish KBS-3 concept in crystalline bedrock. The concept aims at long-term isolation and containment of spent fuel in copper canisters surrounded by bentonite buffer which mostly consists of montmorillonite. For the long-term modelling of the chemical processes in the buffer, the cation-exchange selectivity coefficients have to be known at different temperatures. In this work, the cation-exchange selectivity coefficients and cation-exchange isotherms were determined in batch experiments for montmorillonite at three different temperatures (25 oC, 50 oC and 75 oC). Five different ratios of NaClO4/Ca(ClO4)2 were used in the experimental solutions. After equilibration the solution and montmorillonite were separated and the solution analysed to get the desired exchange parameters. The experiments were modelled with a computational model capable of taking into account the physicochemical processes that take place in the experiment. INTRODUCTION The Finnish plan for the disposal of spent nuclear fuel is based on the Swedish KBS-3 concept in crystalline bedrock. The concept aims at long-term isolation and containment of spent fuel assemblies in copper canisters with a nodular cast iron insert. The canisters are emplaced several hundred metres deep into the bedrock. Each canister will be separated from the bedrock by a bentonite clay layer (the buffer). The purpose of the buffer material is to maintain the integrity of the canisters by protecting them from detrimental THMBC (thermo-hydromechanical-biological-chemical) processes and to limit and retard the release of any radionuclides from the canisters, should any be damaged [1]. The MX-80 bentonite used in the experiments consists mainly of sodium montmorillonite and small amounts of other minerals [2]. The beneficial properties of bentonite are due to the properties of montmorillonite mineral. The Tetrahedral-Octahedral-Tetrahedral (TOT) layer structure of montmorillonite has a permanent negative charge, which is compensated by the exchangeable cations on the surfaces. In aqueous solutions these cations can exchange with those in a solution. The total permanent negative charge is called cation-exchange capacity (CEC). There are also surface edge sites which can protonate/deprotonate according to the pH in the surrounding solution. Those sites were not investigated in these experiments, however. The cation form affects many essential properties of montmorillonite. In the KBS-3 concept, the maximum temperature limit has been decided to be 90 oC [3]. However, the selectivities for montmorillonite have mostly been studied at room temperatures. Therefore, it is essential to study how the temperature affects the cation-exchange selectivities.
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EXPERIMENT Na-Montmorillonite MX-80 bentonite obtained f
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