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During the production of uranium oxide microspheres with carbon using internal gelation, a small fraction of microspheres will not meet the required size or sphericity specification. The uranium microspheres with carbon can be rejected after they have been air-dried or converted into uranium carbide and uranium oxide (UCO) kernels. The next step for the rejected spheres was an air oxidation for carbon removal. The air-dried spheres became triuranium octoxide (U3O8) spheres, which were sometimes ground into powder. The UCO kernels became U3O8 powder during the air oxidation. The next recycle step was nitric acid dissolution of the U3O8 spheres or powders to produce acid-deficient uranyl nitrate (ADUN) solutions, which were used to make new uranium oxide microspheres with carbon and subsequently UCO kernels. The kinetics of the acid dissolution process were compared, and the impurity levels in the different ADUN solutions were determined. X-ray diffraction results for the UCO kernels from the initial and recycled ADUN solutions indicate that changes in the impurity levels can impact the uranium carbide to uranium dicarbide ratio in the UCO kernels.

Three important failure mechanisms that must be controlled in high-temperature gas-cooled reactor (HTGR) fuel for certain higher burnup applications are SiC layer rupture, SiC corrosion by CO, and coating compromise from kernel migration. All are related to high CO pressures stemming from O release when uranium present as UO 2 fissions and the O is not subsequently bound by other elements. In the HTGR kernel design, CO buildup from excess O is controlled by the inclusion of additional uranium apart from UO 2 in the form of a carbide, UC x and this fuel form is designated UCO. Here general oxygen balance formulas were developed for calculating the minimum UC x content to ensure negligible CO formation for 15.5% enriched UCO taken to 16.1% actinide burnup. Required input data were obtained from CALPHAD (CALculation of PHAse Diagrams) chemical thermodynamic models and the Serpent 2 reactor physics and depletion analysis tool. The results are intended to be more accurate than previous estimates by including more nuclear and chemical factors, in particular the effect of transmuted Pu and Np oxides on the oxygen distribution as the fuel kernel composition evolves with burnup.