Electrochemical, SECM, and XPS Studies of the Influence of H 2 on UO 2 Nuclear Fuel Corrosion
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Electrochemical, SECM, and XPS Studies of the Influence of H2 on UO2 Nuclear Fuel Corrosion Michael E. Broczkowski, R. Zhu, Z. Ding, J.J. Noël, D.W. Shoesmith Department of Chemistry, The University of Western Ontario London, Ontario, Canada N6A 5B8 ABSTRACT A combination of electrochemical, scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopic (XPS), and time of flight secondary ion mass spectrometry (TOF - SIMS) techniques are being employed to investigate the effects of dissolved hydrogen on the aqueous corrosion of uranium dioxide (UO2) under nuclear waste disposal conditions. Corrosion potential (ECORR) measurements indicate that the oxidation of dissolved hydrogen on noble metal ε-particles polarizes the UO2 nuclear fuel surface to reducing potentials (-300 - -400 mV vs. SCE; i.e., to ECORR values more negative than those observed under anoxic (argonpurged) conditions (-200 mV vs. SCE). A comparison of the behaviors observed on SIMFUEL specimens with and without incorporated noble metal ε-particles indicate that these particles may act as catalytic electrodes for H2 oxidation. It is the galvanic coupling of these particles to the UO2 matrix which suppresses the fuel corrosion potential thereby preventing oxidation of the fuel surface. INTRODUCTION The emplacement of nuclear fuel in a deep geological site is a viable option for the disposal of spent nuclear fuel. One option for the management of used fuel in Canada is to bury it in a dual-walled container, 500 - 1000 m deep in the granitic rock of the Canadian Shield [1]. The prospects for long-term containment using copper containers is very good, and corrosion models predict only minimal damage, insufficient to cause corrosion failures, will be sustained [2, 3]. However, it is judicious to analyze the consequences for the fuel of failure when groundwater would contact both the waste form and the carbon steel liner. A reasonable assumption is that the fuel will not be wetted while γ / β radiation fields are significant (i.e. the container will last a minimum of 1000 years) and that, consequently, only α-radiolysis need be considered, alpha radiation fields remaining significant for ~ 10-6 years. In sealed repositories, the corrosion of carbon steel should cause substantial hydrogen pressures, leading to dissolved hydrogen concentrations in the 10-2 to 10-1 mol·L-1 range [4]. This introduces the possibility that H2, produced by steel corrosion, will scavenge radiolyticallyproduced H2O2 by reaction to produce H2O. This would limit fuel corrosion, and delay radionuclide release. The expected behaviour of a UO2 surface as a function of surface redox conditions (expressed as a corrosion potential, ECORR) is summarized in Figure 1 [5].
Figure 1: Expected behaviour of a UO2 surface as a function of redox conditions (expressed as a fuel corrosion potential). The corrosion potential is that potential at which the current for the anodic oxidation and / or dissolution of a material is balanced by the current for the cathodic reduction of reactin
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