Redox Reactions of Iron and Uranium Dioxide in Simulated Cement Pore Water Under Anoxic Conditions
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II9.11.1
REDOX REACTIONS OF IRON AND URANIUM DIOXIDE IN SIMULATED CEMENT PORE WATER UNDER ANOXIC CONDITIONS Daqing Cui1, Kastriot Spahiu2 and Paul Wersin3 1
Studsvik Nuclear AB, SE-611 82 Nyköping, Sweden, SKB, P. O. Box 5864, SE-102 40 Stockholm, Sweden, 3 NAGRA, Hardstrasse 73, CH-5430 Wettingen, Switzerland 2
Results on the chemical behavior of Fe(0) and UO2(s), as well as the interaction between fresh and corroded iron with U(VI) in simulated cement contacting alkaline solution are reported. Batch experiments were conducted under anoxic conditions at different alkalinities and salt concentrations to investigate: (a) the corrosion of iron foils (b) the U(VI) removal by fresh, aged and pre-treated (with FeS or Fe3O4 layers) iron surfaces in a simulated cement pore fluid, (c) the dissolution rates of newly reduced UO2.00 slices in simulated cement pore fluid and KOH solutions (pH 12.7) and (d) the isotope exchange reactions between dissolved 235 U(VI) and 238UO2(s). The reacted iron and UO2(s) surfaces were analyzed by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectra (SEM-EDS), laser Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). INTRODUCTION Concrete is used as cask material for low and intermediate level nuclear waste drums and has been proposed as a construction material in some planned high level nuclear waste repositories. Concerns have been expressed with regard to the compatibility of high pH concrete waters with UO2(s) [1]. Spent nuclear fuel is largely UO2(s), with only a small fraction of transuranium elements and fission products. Deep groundwaters are usually reducing and it has been shown that post-closure oxygen is quickly consumed by reducing minerals [2], as well as bacteria [3]. Under such conditions uranium dioxide is stable. However, in the high pH region (pH>12), much lower redox potentials (< -300 mV) are required to stabilize UO2(s) than in the neutral region [4]. On the other hand, the stability of U(VI) species in this pH region is not precisely known [5], which may give rise to large errors in solubility estimations. For this reason we carried out an experimental study on the stability of UO2(s) under anoxic conditions in the high pH region. Progress in understanding the mechanism of UO2(s) dissolution was recently made by investigating the interaction between U(VI) and UO2.0 surfaces [6]. Under anoxic synthetic groundwater conditions (10 mM NaCl and 2 mM NaHCO3), the concentration of U(IV) was found to decrease in solution when freshly reduced UO2(s) was added to the solution. Here we present results on the interaction of U(VI) with freshly reduced UO2(s) surfaces under anoxic conditions in the high pH region of simulated cement contacting waters. The iron system is one of the major contributors to the redox properties of deep groundwaters. The corrosion behavior of Fe(0) and redox reaction between Fe(0) and U(IV) under anoxic synthetic groundwater (pH 8.5) have been recently investigated in our laboratory [7]. The iron stability and the ro
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