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A novel kind of composite constructed by uranium dioxide and reduced graphene oxide (RGO) was developed via a one-step hydrothermal reduction method, named as UO2@G. The introduction of graphene oxide (GO) to the reaction system brought significant changes: in the control experiment without GO, the reductant of ethylenediamine itself could only reduce uranyl ions to mainly U3O8, and the final solid products appear in the form of flakes; by introducing GO to the system, the final products are pure UO2 in homogenous nanospheres. By employing UO2@G as an additive in the UO2 fuel pellet (the final volume ratio of graphene materials was controlled to be around 10%), the thermal conductivity is increased by 35.4%, which is significantly higher than that of the pellet prepared by simply mixing UO2 powders and commercial graphene nanosheets. The simulation results show that the core temperature of the pellet containing the UO2@G composite additives is much lower than that of the one without UO2@G composite additives, showing the potential as novel kind of accident tolerant fuel.

Large grain UO2 is considered as an accident tolerant fuel with great application potential due to its competitive advantage of good fission gas retention. In this paper, the influence of preparation parameters such as sintering atmosphere, mixing process, powder pretreatment and grain growth additives on the grain size of UO2 is systematically studied. The result shows that the factors mentioned above have different effects on the grain size of UO2. The grain growth of UO2 pellet sintered in oxidizing atmosphere is better than those in reducing atmosphere. The wet mixing process has a significant advantage over the dry mixing process. In addition, the powder pretreatment has little effect on grain growth while the influence of additives plays the main role. Large grain UO2 pellets with uniform grain size up to 150 μm are successfully prepared. Finally, the thermo-physical properties of the pellets are investigated.

A series of monodisperse Y2WO6: Dy3+, Gd3+ hierarchical microspheres were synthesized via a facile surfactant-assisted hydrothermal method followed by heat treatment. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence (PL) spectra. XRD patterns of the samples with Gd3+ codoping exhibit a merging of (− 232) and (232) peaks, indicating the distorted deformation in Y2WO6 host structure. SEM images demonstrate that the Y2WO6: Dy3+, Gd3+ microspheres are well-dispersed and assembled by many irregular nanoparticles. Upon ultraviolet (UV) excitation at 291 nm, the emission peaks of Dy3+ ions is observed at 480 nm (blue), 579 nm (yellow) and 669 nm (red), corresponding to the characteristic transitions of 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 of Dy3+, respectively. The optimal PL intensity was obtained in Y2WO6: 2.5 mol% Dy3+ microspheres which can be further enhanced with the codoping of Gd3+. The strongest luminescence is achieved at Gd3+ concentration of 40 mol% with a quantum yied (QY) of 34.45%. In addition, the thermal stability of the sample was also investigated. The CIE of the investigated samples exhibit little change by varying the concentration of Gd3+ ions. The calculated CIE values are all around (0.37, 0.38) and located at the white region, suggesting that the Y2WO6: Dy3+, Gd3+ microspheres could be used for white LEDs.

Ti3SiC2 with a continuous network structure was introduced to UO2 to improve the thermal conductivity of UO2. A dense microstructure with a clean Ti3SiC2-UO2 interface was obtained using the spark plasma sintering (SPS) method. Ti3SiC2 exhibited high thermal conductivity at elevated temperature, and a coefficient of thermal expansion (CTE) close to UO2. Therefore, the thermal conductivity of Ti3SiC2/UO2 was greatly improved compared to that of UO2, particularly at high temperature. The enhancing effect of Ti3SiC2 even exceeded those of SiC and BeO above the working temperature of fuel pellets. This improvement in thermal conductivity is of great importance for enhancing the fuel safety under accident conditions.

Graphite flakes (GFs) were introduced into UO2 via spark plasma sintering to improve its thermal conductivity (TC). The effect of GFs on the sintering behavior and TC of GFs/UO2 was systematically investigated. GFs/UO2 composites exhibited a highly oriented structure; thus, the radial TC was significantly higher than the axial TC. GFs/UO2 was crack-free below a sintering temperature of 1673 K. At higher temperatures, some cracks were formed owing to the reaction at the interface. Accordingly, the radial TC of GFs/UO2 reached its peak value at 1673 K. The radial TC presented a nearly linear relationship with the GF fraction. The radial TC of GFs/UO2 composites was averagely improved by 340% from 298 to 1273 K with 15 vol.% GFs addition. The radial TC continuously increased with the diameter of the GFs. In contrast, the volume fraction and diameter of the GFs had a relatively inconspicuous effect on the axial TC. This was attributed to the thermal resistance of the interface and deflection of GFs from the radial direction.

In order to increase the radial thermal conductivity (TC) of UO2 pellets, highly-oriented graphite flakes (GFs) were first introduced to UO2 pellets by spark plasma sintering. The radial TC of GFs/UO2 agreed well with Hatta-Taya model, which was closely related to the well-bonded interface and highly-oriented structure. Compared with UO2, the radial TC of GFs/UO2 was greatly improved by about 200 % within a wide temperature range, higher than all the other reported value until now. Such enhancement in TC would significantly improve the safety of UO2 fuel pellets.

Ho3+ doped Yb2O3 upconverstion (UC) luminescent nanorods have been prepared using a simple hydrothermal method followed by an annealing treatment. The as-prepared samples were investigated by powder x-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectra for their structural, morphologic and UC luminescent properties. The XRD patterns indicate that all samples are crystallized in the cubic phase of Yb2O3. SEM images reveal that the samples have a rod-shape morphology with the length ranging from 0.5 to 1.0 μm and the diameter ~100 nm. Under 980 nm excitation, the as-obtained nanorods exhibit bight orange emission which is attributed to the characteristic transitions of 5F4, 5S2 → 5I8, 5F5 → 5I8 and 5F4, 5S2 → 5I7 of Ho3+ ion, respectively. The UC emission intensity initially increases as Ho3+ contents increase, reaching a maximum at 2.0 mol%, and then decreases due to the concentration quenching effect. The possible UC mechanisms and energy transfer process were also discussed.