Chromium-doped uranium dioxide fuels: A review

TL;DR

The paper addresses the potential of Cr-doped UO2 as a near-term accident-tolerant fuel by synthesizing decades of experimental and modeling work on Cr incorporation, grain growth, and fission-gas behavior. It clarifies that Cr is mainly substituted in the lattice with Cr-rich precipitates and grain-boundary segregation, whose balance is governed by Cr2O3/Cr redox chemistry and the ambient oxygen potential. The dominant grain-growth mechanism is liquid-assisted sintering via a CrO–Cr2O3 eutectic at grain boundaries, which can offset Cr-induced diffusivity increases under reactor conditions; however, the exact diffusion and irradiation responses remain nuanced and condition-dependent, with modeling predicting up to a diffusion enhancement of about $3$ under certain operating regimes. The review identifies gaps in understanding Cr charge-state dynamics, space-charge effects at grain boundaries, and direct GB potential measurements, proposing integrated irradiation experiments, advanced spectroscopy and ML-enabled simulations to accelerate deployment. Overall, Cr-doped UO2 shows promise for safer, higher-burnup operation, but requires sustained, multi-scale validation under realistic irradiation and storage conditions.

Abstract

UO2 doped with parts per million Cr2O3 powder is considered a potential near term accident tolerant fuel candidate. Here, the results of decades of industry and academic research into Cr-doped UO2 are analyzed and their shortcomings are critiqued. Focusing on the incorporation mechanisms of Cr into the fuel matrix, we explore a mechanistic understanding of the characteristic properties of Cr-doped UO2, notably, enhanced fission gas retention attributed to enlarged grain sizes following sintering, along with marginal improvements in the thermophysical properties. The findings of recent X-ray Adsorption Near Edge Spectroscopy studies were compared and put into conversation with historic data regarding the incorporation of Cr in UO2. On the basis of defect mechanisms, the case is made for the substitutional incorporation of Cr governing the lattice solubility but not the enhanced U diffusivity. Instead, Cr/Cr2O3 redox chemistry in a well-defined oxygen potential explains the differences in the U diffusivity and O/M ratio. The primary mechanism of doping-enhanced grain growth is found to be liquid assisted sintering due to a CrO(l) eutectic phase at the grain boundaries. The role of inhomogeneities in Cr concentration in UO2 at various length scales across the materials microstructure is highlighted and connected to promising experimental and modeling work to fill in the gaps in the current understanding of Cr-doped UO2. The review ends with an outline of future works that combine meticulous irradiation studies and high resolution experiments with next generation modeling and simulations techniques empowered by machine learning advances to accelerate the fabrication and adoption of Cr-doped UO2 light water reactors.

Figures (8)

• Figure 1: Cr solubility in $\text{UO}_2$ as a function of temperature from literature KLEYKAMP1997103leenaers2003solubilitycardinaels2012chromiaBOURGEOIS2001313RIGLETMARTIAL201463kashibe1998effect. Stable phase of Cr-O phase diagram for sintering conditions is annotated in the legend. All but Bourgeois et al. used EPMA to characterize their Cr content after sintering, whereas Bourgeois reported the minimum Cr composition at which grain growth slowed due to solute drag. Solubilities are in wppm Cr per $\text{UO}_2$ rather than the expected phase.
• Figure 2: Lattice parameter of Cr-doped $\text{UO}_2$ (a) versus weight percent $\text{Cr}_2\text{O}_3$ added initially, (b) versus measured concentration of $\text{Cr}_2\text{O}_3$ in weight percent equivalent. Values taken from milena2021ramankegler2020chromiumTERRICABRAS2024155022SMITH2022leenaers2003solubilityCARDINAELS2012289silva2021evaluationMurphy2024
• Figure 3: XANES spectra for Cr doped $\ch{UO_2}$ taken from Riglet Martial RIGLETMARTIAL201463, Smith SMITH2022, Mieszczynki mieszczynski2014microbeam, Murphy MURPHY2023, and Terricabras TERRICABRAS2025156114
• Figure 4: Cr doped $\text{UO}_2$ O/M ratio under reported sintering parameters versus grain size for available literature BOURGEOIS2001313cardinaels2012chromiayang2012effectkegler2020chromiumzhong2021preparationsilva2021evaluationTERRICABRAS2024155022 and calculated with equation 35 from reference watanabe2023oxygen.
• Figure 5: Sintering conditions from previous studies of Cr doped $\text{UO}_2$kegler2020chromiumTERRICABRAS2024155022BOURGEOIS2001313leenaers2003solubilityyang2012effectzhong2021preparationcardinaels2012chromiasilva2021evaluationMURPHY2023RIGLETMARTIAL201463 (a) focuses on the boundary between the various Cr-O phases which are annotated from toker1991equilibrium (b) shows the full range with the theoretical O/M ratio for $\text{UO}_2$ for the specified conditions calculated from reference watanabe2023oxygen.