An Experimental Basis for a Mixed Potential Model for Nuclear Fuel Corrosion within a Failed Waste Container
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An Experimental Basis for a Mixed Potential Model for Nuclear Fuel Corrosion within a Failed Waste Container D.W. Shoesmith and J.J. Noël Department of Chemistry University of Western Ontario London, ON Canada N6A 5B7 F. Garisto Ontario Power Generation 700 University Avenue Toronto, ON Canada M5G 1X6 ABSTRACT A mixed-potential model to predict the corrosion behaviour of nuclear fuel inside a failed carbon steel-lined copper waste container in a granitic repository is briefly described. A number of experiments underway to improve the mechanistic form of the model and to provide the necessary input data are discussed. A primary emphasis is placed on the consequences of the accumulation of corrosion product deposits on the fuel surface on the development of aggressive local chemistries, the cathodic reduction of H2O2 and potential for scavenging of H2O2 by the products of carbon steel corrosion (in particular H2). INTRODUCTION UO2 Fuel
Aqueous Solution In A Failed Container
UO2
UO22+
Fuel Dissolution
H2O
2e-
Carbon Cu Steel Liner
O2
H2O2
H2O
FeIII(s)
FeII(aq)
Redox Scavenging
H2O
H2O
H2
Fe 2e-
Steel Corrosion α
H2O
H2O Water Radiolysis
Figure 1: Corrosion scenario within a failed Cu waste container.
A primary requirement in the development of performance assessment models for the permanent disposal of nuclear fuel is a model to predict the corrosion rate of nuclear fuel inside a failed waste container. The prospects for long term containment using a copper waste container are very good [1]. Nevertheless, it is judicious to analyze the potential consequences of failure while the production of radiolytic oxidants within a groundwater-flooded container still exists. Since gamma (γ) and beta (β) radiation fields become insignificant for times >103 years, it seems reasonable to consider only the potential effects of alpha (α) radiation [2]. Recently [3], we have described a mixed potential model to predict the corrosion behaviour of fuel in α-radiolytically decomposed water within a failed copper nuclear waste container internally supported with a carbon steel liner or containing a carbon steel
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insert. Figure 1 summarizes the key reactions included in this model. A fuller description of the reactions included as well as the details of its mathematical formulation, are described elsewhere [3]. The model consists of two corrosion fronts, one on the fuel surface and a second on the steel surface, interconnected by diffusion processes in the groundwater. Depending on the time of container failure and, hence, the alpha dose rate available at the fuel surface, the redox gradient between these two surfaces (expressed as a difference in corrosion potentials) could be between −600 and −900mV. Thus, the diffusive mixing of corrosion products will lead to homogeneous redox reactions which could scavenge radiolytic oxidants thereby reducing the fuel corrosion potential and suppressing fuel corrosion and radionuclide release. Sensitivity calculations using this model indicate that key issues in
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