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Uranium tetraboride (UB4) was successfully synthesized by boro/carbothermal reduction of UO2 with B4C and carbon as combined reduction agents under flowing argon. The effects of processing temperature and mole ratios of starting materials on phase evolution were studied. XRD results demonstrated that UB4 was obtained with 3.75 mol% B4C in excess based on the UO2/B4C/C molar ratios of 1:1:1 at 1500°C. SEM observation revealed that submicrometer‐sized quasi‐spherical UB4 particles cemented together to be an aggregate. The laser particle size analysis showed that the particle size was in the range of 1‐10 μm. The oxidation behavior of UB4 was also investigated by TG and XRD. The oxidation of UB4 started at about 500°C and it showed better oxidation resistance than other basic uranium nuclear fuels (UO2, UC, UN and U3Si2). The oxidation chemical process of UB4 was presented as a three‐step process: (a) the formation of U3O8 and B2O3 oxidation products; (b) the formation of UB2O6 intermediate product by the interaction of U3O8 and B2O3; (c) the decomposition of UB2O6 to get U3O8. [ABSTRACT FROM AUTHOR]

Abstract: Uranium monocarbide (UC) was successfully synthesized by the Pechini‐type in situ polymerizable complex technique (IPC) with the organic matter as the only carbon source. In the aqueous process, a mixture of citric acid (CA) and mannitol with UO 2 2 + was polymerized to form a spongy‐like organic polymeric precursor without any precipitations. The structural evolution and formation mechanism of the precursor were investigated using XRD, DSC‐TG, SEM (EDX), TEM, and FT‐IR. XRD results demonstrated that UC was obtained with the UO 2 2 +/mannitol/CA molar ratios of 1.0/0.3/1.0 at a low temperature of 1400°C. SEM and TEM analyses revealed that the UO2 nanoparticles were uniformly distributed in the carbon matrix to form UO2/C nanocomposites, and submicrometer‐sized ellipsoidal UC particles cemented together. FT‐IR showed that a UO 2 2 +‐CA chelated structure was firstly obtained, achieving the molecular scale mixing of uranium and C. Then the in situ charring guaranteed the intimate contact of UO2 and C, leading to a low reaction temperature in carbothermal reduction owing to a short diffusion distance. [ABSTRACT FROM AUTHOR]