In Situ Long-Term Measurements of pH and Redox Potential During Spent Fuel Leaching Under Stationary Conditions-The Meth
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ABSTRACT In order to get a better understanding for the spent fuel corrosion process, the variations of important intensive parameters such as pH and the redox potential (Eh) of the bulk solution have been continuously measured during long term sequential leaching experiments. These data may be used together with standard chemical analytical data for modeling spent fuel corrosion, especially in anoxic or reducing conditions. In order to overcome difficulties caused by the strong radiation field and the long experiment times, a method for in-situ measurements of pH and Eh using a computer controlled system was worked out. The stability of the measuring system over long time periods was then tested; pH values stable within 0.05 pH units/year in buffered systems were measured. The variations of these parameters in a variety of conditions and solution compositions were followed continuously during the spent fuel leaching process. A discussion of the results of spent fuel leaching in anoxic conditions is presented, pointing out the difficulties to realize in laboratory near field conditions. An interesting case of calcite co-precipitation/co-dissolution is also presented. Dissolution experiments show that the calcite precipitated previously during spent fuel leaching experiments in synthetic groundwater contained considerable amounts of actinides and fission products. INTRODUCTION Spent fuel leaching studies have been performed in a variety of conditions and solution compositions since the early 70's [1, 2 and references therein]. An overview of the results accumulated in SKB's fuel leaching experimental program in the hot cells at Studsvik in recent years is provided in [3], while some more recent fuel leaching actinide data are treated in [4]. The aim of these studies is to obtain data on the spent fuel oxidation-dissolution process, which are then used to propose models on the source term evolution during long periods of time. A good understanding of the complex system of spent fuel reactions in deep groundwater is required in order to be able to make such long - term predictions. The high radiation field under which such experiments have to be performed makes difficult the use of more involved experimental setups. This is the reason why in the majority of studies the only parameters measured are the concentrations of the fission products and actinides in the leach solution at various time intervals. In a series of studies the corrosion potential of uranium (IV) oxide and spent fuel electrodes has been measured and used as a basis for the modeling of spent fuel dissolution process [1, 5, 6]. Two main intensive parameters, which influence the corrosion of spent fuel and the state of the released radionuclides, are the pH and the redox conditions under which the spent fuel leaching is carried out. Therefore, all thermodynamic modeling of the near field evolution is performed in Eh-pH coordinates. During the spent fuel oxidation-dissolution process, however, reactions that cause changes in the initial values of these pa
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