Measurement of the oxygen partial pressure and thermodynamic modeling of the U–Nd–O system

Abstract

Fission products greatly impactthe properties of fuel necessitating a thorough understandingof the thermochemical properties of oxide fuels with fission products. However, thermochemical data for the U-Nd-O system is insufficient even though neodymium is a major fission product. As neodymium will likely be present as a solute in UO 2 , this research focuses on the study of (U 1-y Nd y)O 2±x. Experimental measurements and analyses of the oxygen partial pressure (pO 2)-temperature-oxygen to metal ratio (O/M ratio) relationships were performed using a thermogravimetric analyzer (TGA) and an oxygen analyzer.Thermodynamic computational modeling was performed using the CALPHAD (CALculation of PHAse Diagrams) method with the FactSage software. The Gibbsenergy of the (U 1-y Nd y)O 2±x solid solution was described by the compound energy formalism (CEF), which is based on earlier thermodynamic modeling data of the binary U-O system from Guéneau et al.. The thermodynamic and phase diagram data of the U-Nd-O system produced in this work show good agreement with the experimental data.

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions: Lack of miscibility gap

“…The FCC structure is maintained in all cases. At all compositions and annealing temperatures, the system shows only a single fluorite-like phase, which agrees with what is proposed by several authors for hypo-stoichiometric conditions [8,29,[32][33][34][35]. Therefore, the results presented in this work differ from those published by Desgranges et al and Dottavio et al [18], [30], [36] where a miscibility gap was identified: no peak splitting is observed in the samples object of this study and therefore there is no evidence for a phase separation.…”

“…This means that the local distances of the first shell will vary depending if it is U or Nd, which is consistent with the observation of the variation of the k space mentioned earlier in the document. Similar conclusions were seen in [27,35]. Barabash et al argued that the atoms of Nd, which are inserted randomly on the cationic sub lattice, are able to push the surrounding atoms of oxygen out of their regular position, creating a local structural disorder along with the distortion created by the size mismatch between the Nd 3+ , U 4+ and U 5+ cations.…”

“…Finally, if the sample has over 50% of trivalent dopants, all the uranium has been oxidized from U 4+ to U 5+ and the system will have no option but to oxidize the U 5+ to U 6+ . behaviour was evidenced in other UO 2 doped systems [37][38][39] and it is explained by the destructive 306 interferences due to opposites phases and similar intensities for the U-U and U-Nd paths [35]. In 307 contrast, no relevant differences can be observed between the intensities of the samples T0 and TT 308 with equal concentration of Nd.…”

Thermodynamic and Structural Modelling of Non-Stoichiometric Ln-Doped UO2 Solid Solutions,Ln = {La, Pr, Nd, Gd}

“…This force at the thermodynamic equilibrium is proportional to the chemical potential of oxygen gas that is required for attaining a certain degree of hypo- or hyper-stoichiometry. Indeed, a large experimental effort has been invested in studying effects of Ln -doping on the chemical potential of oxygen in equilibrium with a given degree of hypo- or hyper-stoichiometry at a fixed temperature ( Hagemark and Broli, 1967 ; Tetenbaum and Hunt, 1968 ; Javed, 1972 ; Saito, 1974 ; Une and Oguma, 1983a ; Nakamura and Fujino, 1987 ; Lindemer and Sutton, 1988 ; Yoshida et al, 2011 ; Lee et al, 2016a ; McMurray and Silva, 2016 ). These experiments invariably show that the larger the doping, the higher is the chemical potential, or, equivalently, the partial pressure of oxygen, at which a given degree of non-stoichiometry can be attained.…”

Thermodynamic assessment of the oxygen rich U–Ce–O system

“…Therefore, all condensed phases were assumed to be immiscible as was done for UeGd [34], UeLa [39], UeY [40], and UeNd [41]. A tentative diagram of the UeCe binary is shown in Fig.…”