Cr2+ solid solution in UO2 evidenced by advanced spectroscopy

Abstract

Advanced Cr-doped UO2 fuels are essential for driving safe and efficient generation of nuclear energy. Although widely deployed, little is known about their fundamental chemistry, which is a critical gap for development of new fuel materials and radioactive waste management strategies. Utilising an original approach, we directly evidence the chemistry of Cr(3+)2O3–doped U(4+)O2. Advanced high-flux, high-spectral purity X-ray absorption spectroscopy (XAS), corroborated by diffraction, Raman spectroscopy and high energy resolved fluorescence detection-XAS, is used to establish that Cr2+ directly substitutes for U4+, accompanied by U5+ and oxygen vacancy charge compensation. Extension of the analysis to heat-treated simulant nuclear fuel reveals a mixed Cr2+/3+ oxidation state, with Cr in more than one physical form, explaining the substantial discrepancies that exist in the literature. Successful demonstration of this analytical advance, and the scientific underpinning it provides, opens opportunities for an expansion in the range of dopants utilised in advanced UO2 fuels.

Oxidative dissolution of Cr-doped UO2 nuclear fuel

“…Complete digest of the pellets sintered at 1700 °C showed that the Cr content was below the expected, nominal, Cr concentration (Fig. 1b), which is attributed to volatilisation at the high sintering temperature 9,31,32 .…”

“…The microstructure and defect chemistry of the Cr-doped UO 2 materials utilised in the dissolution experiments was detailed fully by Smith et al 9 . Briefly, for all concentrations of Cr-doped UO 2, the sintered pellets presented a homogenous distribution of Cr 2+ within the UO 2 matrix, charge compensated by O v defects.…”

“…Briefly, for all concentrations of Cr-doped UO 2, the sintered pellets presented a homogenous distribution of Cr 2+ within the UO 2 matrix, charge compensated by O v defects. Above the solubility limit of Cr within UO 2 (700-1200 ppm) [25][26][27][28] , precipitates of Cr were observed to reside within grain boundaries, hypothesised to be Cr 3+ 2 O 3 9,29 . When sintered at 1700 °C, the presence of these precipitates in Cr-doped UO 2 resulted in a reduction in grain size when compared to undoped UO 2 (Fig.…”

“…This mechanism is governed by the availability of oxygen to diffuse through the lattice, enhanced by the presence of oxygen vacancies (O v ) 8 . In the new alernative nuclear fuel, Cr-UO 2 , it has been shown that Cr doping of UO 2 results in the substitution of Cr 2+ onto U 4+ sites in the lattice, with the concurrent formation of O v and U 5+ to form a compound with a stoichiometry of 9 . As such, the presence of O v and U 5+ defect species should tend to increase the oxidative dissolution of doped UO 2 , which may reduce durability in a geological disposal facility when compared with conventional, undoped, UO 2 fuel types.…”

Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies

“…It has also been proposed the possibility of Cr +2 occurring within the UO 2 lattice matrix itself with some redox mechanisms between it and Cr +3 which should be observable at ambient conditions. 8 , 48 We note relevant studies to this argument have only investigated bulk material at ambient conditions which carries significant deconvolution in Cr measurement, as demonstrated by Fig. 2 b and c regarding the difference between bulk and lattice structure, respectively.…”