Electrochemical Studies of Corrosion of Simfuel: Simulated Used UO 2 Fuel
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ELECTROCHEMICAL STUDIES OF CORROSION OF SINFUEL: SIMULATED USED U02 FUEL S. SUNDER, D.W. SHOESMITH AND N.H. MILLER AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE ILO ABSTRACT The corrosion of SIMFUEL, simulated high-burnup CANDU (CANadian Deuterium Uranium) fuel, was investigated in 0.1 mol.dm- 3 NaClO 4 solution (pH - 9.5) as a function of dissolved oxygen concentration using electrochemical techniques and X-ray photoelectron spectroscopy (XPS). Electrodes were constructed of SIMFUEL pellets with compositions close to those of a natural UO2 fuel that has undergone burnup of 3 atZ and 6 atZ. The XPS analysis of freshly polished SIMFUEL pellets showed that the extent of uranium oxidation in SIMFUEL was equivalent to that in stoichiometric UO2. The SIMFUEL electrodes showed higher conductivity and electrochemical reactivity than the pure U02 electrodes. A comparison of the open circuit corrosion potentials of the SIMFUEL electrodes with that of a pure UO2 electrode in oxygenated solutions showed a much faster increase in the potential for the SIMFUEL electrodes at short times. This suggests that the initial stages of UO2 oxidation, i.e., UO2 4 U0 2+x U02 . 3 3 , are facilitated H by fission product impurities in the UO2 matrix. However, the "steadystate" oxidation of uranium in SIMFUEL by the dissolved 02 was similar to that observed in pure U02 under similar conditions. INTRODUCTION The concept of direct disposal of used nuclear fuel in a geological vault is being investigated in the nuclear fuel waste management programs of several countries [1,2 and references therein]. The used nuclear fuel is mainly U02 , the remainder being the fission products and other actinides formed during the nuclear fission [2]. It is important to know the effects of these impurities on the corrosion behaviour of UO2 fuel to assess the stability of the used nuclear fuel as a waste form. We have been studying the effects of different parameters on the corrosion of CANDU* fuel using different electrochemical and spectroscopic techniques [1,3-11 and references therein]. These studies were carried out using unirradiated CANDU fuel to avoid the complications due to the radiation fields associated with the fission products and actinides. Consequently the effects of the presence of the fission products and actinides on the corrosion behaviour of used nuclear fuel remain to be determined. In the work reported here, we studied the corrosion of SIMFUEL, simulated high-burnup UO2 nuclear fuel [12,13]. This material provides a possible approach to the study of the effects of impurities formed during burnup on the corrosion of used fuel [12-16]. We studied the corrosion of SIMFUEL in solutions containing dissolved oxygen using electrochemical techniques and X-ray photoelectron spectroscopy. MATERIALS AND METHODS The SIMFUEL pellets were prepared by Chalk River Laboratories of AECL Research. These pellets had compositions close to those of natural U02 fuel that has undergone 3 atZ and 6 at% burnup in a CANDU reactor at a linear p
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