Oxidation of UO 2 Fuel by Radicals Formed During Radiolysis of Water
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OXIDATION OF U0 2 FUEL BY RADICALS FORMED DURING RADIOLYSIS OF WATER.
S. SUNDER, D.W. SHOESMITH, H. CHRISTENSENa, N.H. MILLER AND M.G. BAILEY, Geochemistry and Waste Immobilization Division, Atomic Energy of Canada Ltd. Whiteshell Nuclear Research Establishment, Pinawa, Manitoba, Canada ROE ILO; aStudsvik, S-611 82 Nykoping, Sweden.
ABSTRACT
The effects of radicals, formed during radiolysis of water, on the oxidation and dissolution of U02 were investigated as a function of dose rate and total dose. The U02 oxidation rate during radiolysis was monitored by recording the corrosion potential of a U02 electrode as a function of time. Changes in the surface of U02 were determined using cathodic stripping voltammetry (CSV) and X-ray photoelectron spectroscopy (XPS). Our studies suggest that the oxidation of U02, in irradiated de-oxygenated solutions consists of two stages. The first stage consists of the growth of a surface film of composition close to U02. 3 3 and of thickness similar to that obtained (over longer exposure periods) in unirradiated oxygenated solutions. The rate of growth of this film appears to be proportional to the square root of the dose rate. The second stage consists of oxidative dissolution of this film (as U02+). This step mainly occurs at higher doses. Relative rates of oxidation of U02 to U02 . 3 3, the first stage, were obtained in four different solutions favouring the formation of selected radicals.
INTRODUCTION
An assessment of the concept of the disposal of used nuclear fuel in a geological vault requires an understanding of its interactions with groundwater [1-3]. Used fuel is mainly U02 (> 95 Z), the remainder being fission products and other actinides formed during nuclear fission. U02 has a very low solubility in water under reducing conditions. However, the solubility of "uranium dioxide" in water greatly depends on the oxidation state of its surface [1,4]. Although the groundwaters at the depth of the proposed disposal vault (500-1000 m) in the granitic rocks are generally reducing [5], the redox conditions near the fuel surface may be altered due to the radiolysis of water by the ionizing radiation associated with the fuel. Therefore, we are studying the effects of the radiolysis of groundwater on U02 oxidation [5-8]. This information is essential to develop an understanding of the dissolution behaviour of used fuel in a geologic disposal vault. Although the radiation fields associated with the used fuel at disposal times beyond 500 y will be dominated by a-radiation [6], there will also be a significant contribution by fl-radiation at very long disposal times, i.e., beyond 50 000 y [9,10]. As the G-values for radicals formed during the radiolysis of water are almost one order of magnitude higher for fl-radiation than for a-radiation, the effects of fl-radiation and, hence, of radicals on U02 oxidation and dissolution may be significant. Also, our previous studies have shown that the radicals are much more effective in causing U02 oxidation than the molecular oxidants [8]. The
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