Oxidation and anion lattice defect signatures of hypostoichiometric lanthanide-doped UO₂

Highlights

• Oxidation characteristics for hypostoichiometric Ln-doped UO_2-x revealed.

• Two Raman lasers provide spectrally selective information on UO_2-x.

• This is the first oxidation study on hypostoichiometric Yb-doped UO_2-x.

• Yb-doping strongly inhibits U₃O₈ formation by 180 °C relative to pure UO₂.

• New spectroscopic and oxidation signatures compared with prior studies of UO_2-x.

Abstract

A series of sintered UO₂ pellets doped with lanthanide (Ce, Nd, Yb) elements were investigated using powder X-ray diffraction, Raman spectroscopy, thermogravimetric analyses and differential scanning calorimetry. A combination of electron microprobe and thermogravimetric analyses, for oxygen content, enabled precise determination of the hypostoichiometry for lanthanide-doped samples at 1 and 5 atom percent. Two Raman laser wavelengths (785 and 455 nm) have afforded greater sensitivity to spectroscopic signatures of the phonon bands (1LO and 2LO) associated with oxidation of (U_1-yM_y)O_2-x and the anion defects introduced by lanthanide substitution. Oxygen hypostoichiometry forces a reduction in the average coordination number surrounding (U,M) sites, which is compensated by a decrease in U–O bond length, and concomitantly the lattice parameter, consistent with the obtained Raman spectra. The evolution of O/M ratio up to (U_1-yM_y)O₂ after oxidation was also examined using Raman spectroscopy, revealing that the ‘defect band’, including a component attributed to oxygen vacancies (∼540 cm⁻¹) and the 1LO phonon (∼575 cm⁻¹) increased in intensity with increasing dopant concentration and upon oxidation. The lanthanide dopants inhibited oxidation to U₃O₈, most prominently for Yb 5 at%, having been delayed by ∼180 °C. Thermogravimetric analyses reveal an early oxidation feature that may be related to influx of O to satisfy hypostoichiometry up to (U_1-yM_y)O₂, possibly stabilizing a U₄O₉ or U₃O₇ intermediate, delaying oxidation to U₃O₈.

Introduction

Uranium dioxide nuclear fuel that has undergone fission is the most chemically complex material known, at first containing hundreds of radionuclides with a range of half-lives [1,2]. The UO₂ structure is able to incorporate certain of these fission products, predominantly the lanthanides, as well as transuranic elements produced by neutron capture reactions. However, as a nuclear fuel package ages, its chemical composition, radiation field, and emitted heat evolves dynamically. The gaseous fission products incompatible with the UO₂ structure become confined to bubbles distributed between fuel grains which eventually escape through cracks and cladding, while other elements exsolve as metallic particles and oxide phases, producing complex chemical and microstructural features that have been nearly wholly reproduced in the laboratory (i.e., as SIMFUEL) [[3], [4], [5]]. Understanding the stability and alteration behavior of used nuclear fuel (UNF) remains one of the greatest challenges to ensure safe reuse and long-term disposal, as well as environmentally sound stewardship of the legacy and future UNF generated by nuclear activities.

As a result of their high fission yield, lanthanides (Ln) are among the most abundant, stable and miscible elements found in UNF [6,7]. Although these and other fission products compatible within the UO₂ structure can stabilize it from oxidation, various defects are introduced to the cation and anion sublattices (hypostoichiometry) due to their different charges and sizes compared to U^IV, which can force local redox reactions to satisfy charge balance, chiefly, via U^IV → U^V [8]. Stable configurations occur for hypostoichiometric UO_2-x up to x = 0.34–0.35 [9,10], with other experiments revealing that oxygen vacancies are stabilized by lanthanide dopants [[11], [12], [13]], and are highly mobile and randomly distributed [14,15] in contrast to the Willis-type cuboctahedral clustering of interstitial O preferred in hyperstochiometric UO_2+x [[16], [17], [18]]. Used fuel is known to have different oxidation and dissolution characteristics than fresh fuel due to the complexity of its fission product inventory [19], and many studies have shown these effects are dictated by the size and charge of the substituting elements [9,[20], [21], [22], [23], [24], [25]]. In general, trivalent cation dopants slow UO₂ oxidation by hindering U₃O₈ formation. As described by Desgranges et al. (2011), a random distribution of trivalent dopants limits the formation of cuboctahedra, preventing the necessary shear-transformation of the crumpled sheets in α-U₄O₉ into the U₃O₈-type sheet structure [26].

Using a coprecipitation method we have synthesized and sintered a series of UO₂ pellets containing low concentrations (1 and 5 at%) of homogeneously distributed lanthanide elements (Ce, Nd, and Yb) with hypostoichiometric oxygen content. By combining powder X-ray diffraction (PXRD) and Raman spectroscopic analysis with two laser wavelengths, we identify the effect of lanthanide size on the hypostoichiometric anion defect structures and detail how their spectroscopic trends and signatures change with oxidation, as monitored during thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments. A majority of the data presented here for Nd- and Yb-doped UO_2-x, as far as we know, have not yet appeared in the literature. Furthermore, we observe a profound effect of delayed oxidation in 5 at% Yb doped UO_2-x, and compare these results to previous studies of Ln-doped UO₂ with small trivalent dopants, Y, Lu, and Gd.