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Investigated accident tolerant nuclear fuels are fuels with enhanced thermal conductivity, which can withstand the loss of coolant for a longer time by allowing faster dissipation of heat, thus lowering the centerline temperature and preventing the melting of the fuel. Traditional nuclear fuels have a very low thermal conductivity and can be significantly enhanced if transformed into a composite with a very high thermal conductivity components. In this study, we analyze the thermal properties of various composites of mixed oxides and thoria fuels to improve thermal conductivity for the next generation safer nuclear reactors.

Grain growth is prevalent in nanocrystalline (NC) materials at low homologous temperatures. Solute element addition is used to offset excess energy that drives coarsening at grain boundaries (GBs), albeit mostly for simple binary alloys. This thermodynamic approach is considered complicated in multi-component alloy systems due to complex pairwise interactions among alloying elements. Guided by empirical and GB-segregation enthalpy considerations for binary-alloy systems, a novel alloy design strategy, the "pseudo-binary thermodynamic" approach, for stabilizing NC-high entropy alloys (HEAs) and other multi-component-alloy variants is proposed. Using Al 25 Co 25 Cr 25 Fe 25 as a model-HEA to validate this approach, Zr, Sc, and Hf, are identified as the preferred solutes that would segregate to HEA-GBs to stabilize it against growth. Using Zr, NC-Al 25 Co 25 Cr 25 Fe 25 HEAs with minor additions of Zr are synthesized, followed by annealing up to 1123 K. Using advanced characterization techniques- in situ X-ray diffraction (XRD), scanning/transmission electron microscopy (S/TEM), and atom probe tomography, nanograin stability due to coupling self-stabilization and solute-GB segregation effects is reported in HEAs up to substantially high temperatures. The self-stabilization effect originates from the preferential GB-segregation of constituent HEA-elements that stabilizes NC-Al 25 Co 25 Cr 25 Fe 25 up to 0.5T m (T m -melting temperature). Meanwhile, solute-GB segregation originates from Zr segregation to NC-Al 25 Co 25 Cr 25 Fe 25 GBs; this results in further stabilization of the phase and grain-size (≈14 nm) up to ≈0.58 and ≈0.64T m , respectively., (© 2024 The Authors. Small published by Wiley‐VCH GmbH.)

High-entropy alloys (HEAs) represent a promising class of materials with exceptional structural and functional properties. However, their design and optimization pose challenges due to the large composition-phase space coupled with the complex and diverse nature of the phase formation dynamics. In this study, a data-driven approach that utilizes machine learning (ML) techniques to predict HEA phases and their composition-dependent phases is proposed. By employing a comprehensive dataset comprising 5692 experimental records encompassing 50 elements and 11 phase categories, we compare the performance of various ML models. Our analysis identifies the most influential features for accurate phase prediction. Furthermore, the class imbalance is addressed by employing data augmentation methods, raising the number of records to 1500 in each category, and ensuring a balanced representation of phase categories. The results show that XGBoost and Random Forest consistently outperform the other models, achieving 86% accuracy in predicting all phases. Additionally, this work provides an extensive analysis of HEA phase formers, showing the contributions of elements and features to the presence of specific phases. We also examine the impact of including different phases on ML model accuracy and feature significance. Notably, the findings underscore the need for ML model selection based on specific applications and desired predictions, as feature importance varies across models and phases. This study significantly advances the understanding of HEA phase formation, enabling targeted alloy design and fostering progress in the field of materials science., (© 2023. Crown.)

The interfacial interaction of U 3 Si 2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U 3 Si 2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U ) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H 2 formation mechanisms. The adsorption strength decreases in the order U 3 Si 2 {001} > U 3 Si 2 {110} > U 3 Si 2 {111} for both molecular and dissociative H 2 O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H 2 O on the oxygen-covered U 3 Si 2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H 2 O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

Cerium dioxide (CeO 2 ) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (k L ), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Grüneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the k L , measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured k L , the effect of porosity on k L by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO 2 . Since CeO 2 films find application in various industries, the dependence of thickness on the in-plane and cross-plane k L for an infinite CeO 2 thin film was also reported.

Uranium-containing metallic systems such as U 3 Si 2 are potential Accident Tolerant Fuels (ATFs) for Light Water Reactors (LWRs) and the next generation of nuclear reactors. Their oxidation behaviour, especially in oxygen and water-enriched environments, plays a critical role in determining their applicability in commercial reactors. In this work, we have investigated the oxidation behaviour of U 3 Si 2 experimentally and by theoretical computation. The appearance of oxide signatures has been established from X-ray diffraction (XRD) and Raman spectroscopic techniques after oxidation of the solid U 3 Si 2 sample in synthetic air (oxygen and nitrogen). We have also studied the changes in the electronic structure as well as the energetics of oxygen interactions on the U 3 Si 2 surfaces using first principles calculations in the Density Functional Theory (DFT) formalism. The detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO 2 and SiO 2 on the U 3 Si 2 {001} surface as well as UO 2 alone on the U 3 Si 2 {110} and {111} surfaces. The formation of the peroxo (O 2 2- ) state confirmed the dissociation of molecular oxygen before U 3 Si 2 oxidation. Core experimental analyses of the oxidized U 3 Si 2 samples have revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidation of U-Si metallic fuel compounds.