Reducing the uncertainty of nuclear fuel dissolution: an investigation of UO 2 analogue CeO 2
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Reducing the uncertainty of nuclear fuel dissolution: an investigation of UO2 analogue CeO2 Claire L. Corkhill1, Daniel J. Bailey1, Stephanie M. Thornber1, Martin C. Stennett1 and Neil C. Hyatt1 1
The Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, UK, S1 3JD. ABSTRACT
In this investigation, CeO2 analogues, which approximate as closely as possible the characteristics of fuel-grade UO2, were characterised after dissolution under a wide range of conditions. Powdered samples were subject to a range of aggressive and environmentally relevant alteration media with different solubility controls, and reacted at 70 °C and 90 °C. Dissolution kinetics were monitored through analysis of the coexisting aqueous solution. Monolith samples were monitored for development of surface defects such as pores and dissolution pits, in addition to morphological changes at grain boundaries and surface pores upon dissolution under aggressive conditions. The surfaces were analysed using confocal profilometry, vertical scanning interferometry and scanning electron microscopy. Dissolution rates were found to be greatest in low pH solutions and at higher temperatures. Preferential dissolution appears to occur at grain boundaries and on particular grains, suggesting a crystallographic control on dissolution. INTRODUCTION In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater [1]. Therefore, to assess the performance of the repository after infiltration of groundwater and contact with spent fuel, the dissolution characteristics must be determined. The use of spent nuclear fuel, or its main component UO2 is problematic due to safety and redox sensitivity issues. As such, in experiments using UO2, special care must be taken to avoid oxidation of the sample surfaces [2, 3]. It is useful to use a non-redox sensitive analogue for UO2 with the same fluorite-type structure for investigations of surface alteration, such as CeO2 [4]. In spent nuclear fuel, high energy sites occur at grain boundaries and within the material as naturally occurring surface defects. Current studies of spent nuclear fuel dissolution have not considered the effect of high energy surface sites within the material structure. In this preliminary investigation, CeO2 analogues were powdered and subjected to a range of aggressive and environmentally relevant alteration media with different solubility controls. Dissolution was monitored through analysis of the aqueous solution. To complement these experiments, the dissolution of monoliths of CeO2 is described. These samples were monitored for the evolution of high energy sites, including pores, steps and dissolution pits. Preliminary results are discussed, which will form the basis for future investigation.
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METHODOLOGY CeO2, prepared as described in [4] was ground to a powder (75-150 μm) according to the product consistency test ASTM standa
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