Structural Contributions to the Third-Law Entropy of Uranyl Phases
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INTRODUCTION
Uranyl phases are found as the alteration products of uraninite under oxidizing conditions [1,2]. They are also expected to be the dominant alteration products of U0 2 in spent fuel in an oxidizing environment, such as at the proposed nuclear waste repository at Yucca Mountain [3-7]. Also, radionuclides released from the altered nuclear fuel may become incorporated into the alteration products [7-9]. Thus, the precipitated uranyl phases formed during the alteration of spent fuel will become the source term for the release of uranium and other radionuclides. Therefore, the thermodynamic stability of these phases is of critical importance in evaluating the long-term behavior of spent nuclear fuel under oxidizing conditions. Calorimetric measurements are commonly used to obtain the entropies of solid phases. By this method, the Gibbs free energy of formation (AG°) for a compound is calculated using the equation: AGf° = AHf° - T ASO. Where AHf° and ASO are enthalpy of formation and third-law entropy change for the formation reaction, respectively. Thus, the Gibbs free energy of a compound obtained from thermal data depends largely on the third-law entropy. * Permanent address: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China. Corresponding author. 1017 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
For any given reaction, we can calculate the Gibbs free energy change at any temperature (T) using the following equation [10]: 0 AG° 0 AGr 0 - ASr0 + ACp0dT - TJ AC d In T where AGt and AS,' are, respectively, the changes of Gibbs free energy and third-law entropy for the reaction at the reference temperature Tr which is 298.15 K by convention. Therefore, the reliability of the extrapolation of experimentally determined thermodynamic data and stability relations largely depends on a knowledge of the third-law entropies of the relevant phases. However, due to the contributions of residual entropies that cannot be extracted by calorimetric measurements, the true third-law entropy may be different from the values derived from thermal data. The residual entropies are caused by site-mixing, disorder of polar molecules and hydrogen bonds, and the lack of significant magnetic ordering at temperatures reached by heat capacity measurements. Ulbrich and Waldbaum [11] have made a detailed examination of the possible contributions to the third-law entropy of silicate minerals and concluded that the published entropies for many silicate minerals should be checked against structural and magnetic information. The entropies of uranyl phases compiled in the widely adopted uranium thermodynamic database developed by Grenthe et al. [12] are obtained based on heat-capacity data. Residual contributions to the calorimetric entropies of uranyl phases should, however, also be examined. Entropy due to isotopic mixing cancels across a balanced reaction; thus, this effect can be neglected. The same is true for the nuclear spin-configurational entropy. The magnetic
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