High Temperature Thermodynamic Studies On The Transuranium Oxides And Their Solid Solutions

Author : Benjamin Edward Hanken
Publisher :
ISBN 13 : 9781267758972
Total Pages : pages
Book Rating : 4.7/5 (589 download)

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Download or read book Computational and Experimental Study of the Thermodynamics of Uranium-cerium Mixed Oxides written by Benjamin Edward Hanken and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The thermophysical properties of mixed oxide (MOX) fuels, and how they are influenced by the incorporation of fission products and other actinides, must be well understood for their safe use in an advanced fuel cycle. Cerium is a common plutonium surrogate in experimental studies of MOX, as it closely matches plutonium's ionic radii in the 3+ and 4+ oxidation states, and is soluble in fluorite-structured UO2. As a fission product, cerium's effects on properties of MOX are also of practical interest. To provide additional insights on structure-dependent behavior, urania solid solutions can be studied via density functional theory (DFT), although approaches beyond standard DFT are needed to properly account for the localized nature of the f-electrons. In this work, DFT with Hubbard-U corrections (DFT+U) was employed to study the energetics of fluorite-structured U[subscript 1-y]Ce[subscript y]O2 mixtures. The employed computational approach makes use of a procedure which facilitates convergence of the calculations to multiple self-consistent DFT+U solutions for a given cation arrangement, corresponding to different charge states for the U and Ce ions in several prototypical cation arrangements. Results indicate a significant dependence of the structural and energetic properties of U[subscript 1-y]Ce[subscript y]O2 on the nature of both charge and cation ordering. With the effective Hubbard-U parameters that reproduce well the measured oxidation-reduction energies for urania and ceria, it was found that charge transfer between U4 and Ce4 ions, leading to the formation of U5+ and Ce3+, gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of the formula unit, depending on the nature of the cation ordering. In conjunction with the computational approach, high-temperature oxide-melt drop-soltuion calorimetry experiments were performed on eight samples spanning compositions of y = 0.119 to y = 0.815. Room temperature mixing enthalpies of U[subscript 1-y]Ce[subscript y]O2 determined from these experiments show near ideal behavior, in line with the expectations of the computational results. Combination of these theoretical and experimental results yields insight into the plausible degrees of charge transfer within U[subscript 1-y]Ce[subscript y]O2 ; the comparison suggests that although charge transfer between uranium and cerium ions is disfavored energetically, it is likely to be entropically stabilized at the high temperatures relevant to the processing and service of urania-based solid solutions.