Modeling the Distribution of Acidity within Nuclear Fuel (UO 2 ) Corrosion Product Deposits and Porous Sites
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Modeling the Distribution of Acidity within Nuclear Fuel (UO2) Corrosion Product Deposits and Porous Sites W-J. Cheong, P. G. Keech, J. C. Wren, D. W. Shoesmith, and Z. Qin The University of Western Ontario, London, Ontario, N6A 5B7, Canada ABSTRACT A model for acidity within pores within corrosion products on anodically-dissolving UO2 was developed using Comsol Multiphysics 3.2 to complement ongoing electrochemical measurements. It was determined that a depression of pH within pores can be maintained if: electrochemically measured dissolution currents used in the calculations are attenuated to reflect very localized pores; corrosion potentials exceed -250 mV (vs. SCE); and pore depths are >1 µm for 300 mV or >100 µm for -50 mV (vs. SCE). Mixed diffusional-chemical equilibria control is suggested through deviations in the shapes between pH-potential and pH-pore depth plots. INTRODUCTION One possibility for the management of Canadian nuclear fuel wastes is their permanent disposal in a deep geologic repository [1]. In such a repository the used fuel would be sealed in metallic containers and emplaced in bentonite clay with excess space within the repository filled with a mixture of clay and crushed rock. While container failure by corrosion in under 106 years is a very remote possibility [2], an experimental and modeling program is underway to assess the consequences of failure on fuel corrosion/dissolution and the release of radionuclides. Assuming container failure does not occur until β/γ radiation fields have decayed to insignificant levels, the only source of oxidants within a failed container will be the α-radiolysis of water. Our experiments and calculations indicate that the redox conditions established will not be particularly aggressive [3], and will be strongly suppressed by the reducing effects of H2 produced by the anoxic corrosion of the carbon steel inner shell of the container. Despite this optimism there is a residual concern that the presence of corrosion product deposits on the fuel surface could both isolate corroding sites from H2 and allow the development of aggressive acidified conditions at faults (cracks, pores) within the fuel. The source of this acidity is the hydrolysis of dissolved UO22+, Equation (1). nUO22+ + yH2O → (UO2)n(OH)y(2n-y)+ + yH+
(1)
Its development would introduce a pH gradient within the fuel flaws (or pores in a corrosion product deposit) and, hence, a chemical driving force (based on the pH dependence of UVI solubility [4]) which would maintain porosity and sustain corrosion, Figure 1. The development of such acidity has been demonstrated electrochemically [5] and is suspected to occur at high αradiation dose rates [6]. However, it is proving experimentally very difficult to determine whether or not acidification is possible at the low dose rates anticipated, and given these difficulties, we are attempting to model the process. The primary goal of such a model is to determine whether or not acidity, leading to enhanced fuel corrosion rates, can occur withi
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