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

Abstract

Available data on the dependence of the equilibrium chemical potential of oxygen on degrees of doping, z, and non-stoichiometry, x, y, in U1-zLnzO2+0.5(x-y) fluorite solid solutions and data on the dependence of the lattice parameter, a, on the same variables are combined within a unified structural-thermodynamic model. The thermodynamic model fits experimental isotherms of the oxygen potential under the assumptions of a non-ideal mixing of the endmembers, UO2, UO2.5, UO1.5, LnO1.5, and Ln0.5U0.5O2, and of a significant reduction in the configurational entropy arising from short-range ordering (SRO) within cation-anion distributions. The structural model further investigates the SRO in terms of constraints on admissible values of cation coordination numbers and, building on these constraints, fits the lattice parameter as a function of z, y, and x. Linking together the thermodynamic and structural models allows predicting the lattice parameter as a function of z, T and the oxygen partial pressure. The model elucidates contrasting structural and thermodynamic changes due to the doping with LaO1.5, on the one hand, and with NdO1.5 and GdO1.5, on the other hand. An increased oxidation resistance in the case of Gd and Nd is attributed to strain effects caused by the lattice contraction due to the doping and to an increased thermodynamic cost of a further contraction required by the oxidation.

Benefits of using multiple Raman laser wavelengths for characterizing defects in a UO2 matrix

“…Recent computational studies on lanthanide-doped UO 2 [22] have shown that trivalent cation size influences oxidation, in that the strain associated with small cations results in stabilization from further contraction upon oxidation; this result is supported by our previous investigations where Yb:UO 2 showed delayed oxidation compared with Nd:UO 2 . [1] In the case here, there may be a different effect, because Zr is considerably smaller than Th and results in worse oxidation, but the charge compensation mechanism is not present, so the effects of oxygen vacancies and interstitials will be different for the tetravalent dopants.…”

“…[ 21 ] The effect of dopants, which in fuels may be fission products, on the oxidation behavior of UO 2 has been shown to be significant and variable with the dopant. [ 1,16,22 ]…”

“…[21] The effect of dopants, which in fuels may be fission products, on the oxidation behavior of UO 2 has been shown to be significant and variable with the dopant. [1,16,22] As the main matrix for oxide nuclear fuel, UO 2 is irradiated in power reactors, creating fission products (such as lanthanide rare-earth metals, zirconium, etc. ), which can be incorporated in the lattice and cause various oxygen stoichiometry defects that ultimately affect the long term aqueous chemical corrosion.…”

Accelerated dissolution of doped UO2-based model systems as analogues for modern spent nuclear fuel under repository conditions

“…Apparently, much less H 2 O 2 is consumed for the oxidation of UO 2 due to doping with trivalent Nd 3+ ions. It seems that the doping of Nd 3+ is stabilising the UO 2 structure and prevent U(+ IV) from oxidation [16].…”

Cation interdiffusion in uranium–plutonium mixed oxide fuels: Where are we now?

“…Indeed, increasing the number of oxygen vacancies induces a reduction of An(IV) to An(III) to keep electroneutrality. These trivalent cations trap the oxygen vacancies, forming uncharged cluster defects (Anderson, 1971;Ando and Oishi, 1983;Bevan et al, 1986;Nakayama and Martin, 2009;Yoshida et al, 2011;Vinograd, Bukaemskiy, Modolo, Deissmann, Bosbach).…”