Microgravimetric Corrosion Studies on UO 2
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0LFURJUDYLPHWULF&RUURVLRQ6WXGLHVRQ82 Detlef H. Wegen, T. Gouder, M. Eckle, T. Wiss and J.-P. Glatz European Commission, JRC Institute for Transuranium Elements P.O. Box 2340 76125 Karlsruhe, Germany $%675$&7 For the first time at ITU electrochemical quartz crystal micro-balance has been used for the investigation of UO2 thin film oxidation. The combination of cyclic voltammetry with nanogram weight change measurements in-situ has shown at least two steps in the initial stages of UO2 dissolution under oxidising conditions relevant to spent fuel storage. ,1752'8&7,21 Direct storage of spent nuclear fuel in a repository is being considered as one of the options in storage of nuclear waste. It is expected after 1000 years of storage groundwater may penetrate through the engineered barriers to make contact with the fuel pellets through ruptures in the cladding. In the radiation field the penetrating groundwater will undergo radiolysis. Hydrogen peroxide is a major radiolysis product and oxidant of UO2. Therefore studies have been initiated to measure the influence of H2O2 on the dissolution behaviour of UO2. In presence of H2O2 the formation of a surface layer on the UO2 was observed [1, 2]. This layer was investigated by impedance spectroscopy and cathodic sweep voltammetry. Both techniques show an increase of the layer thickness with time, although the thickness values differed by one order of magnitude. To clarify this discrepancy, additional information about the corrosion process such as the mass change is needed. A highly sensitive method to study mass changes is the quartz crystal microbalance (QCM) and is an excellent tool to determine in-situ corrosion rates and inhibition effects. Mass changes in the sub-nanogram range can be detected. On the other side, the total mass, which can be used with the QCM, is restricted; here to some 100 µg. Therefore only thin films of UO2 can be investigated. (;3(5,0(17$/'(7$,/6 Microgravimetric measurements were carried out with the Model 430 electrochemical quartz crystal microbalance (QCM) from CH Instruments. This is a complete system including an electrochemical call, an oscillator and the potentiostat. The system is controlled by a computer running the CHI software. The frequency difference of the working crystal and the reference crystal is measured. This measured frequency shift is converted in to a mass variation relying on the Sauerbrey equation [3]. The diameter of the quartz crystal is 13.7 mm in the centre the crystal is coated with gold (diameter 5 mm). Usually the gold covers the whole oscillating area of the crystal. Mass changes that appear outside this area have only a very little effect on the frequency shift (< 1%). UO2 is sputter-deposited on the crystal using a stencil to achieve a homogeneous layer covering the
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whole gold electrode. The area of the UO2 layer is approximately double the size of the gold electrode. The sensitivity of the crystal due to mass changes is not homogeneous. It decreases from the centre to the outer parts. A mean mass
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