Growth of Gd₂O₃ coherent layers on UO₂ microsphere surface via sol-gel process

In this work, we synthesized and characterized UO₂-Gd₂O₃ nuclear fuel via three routes, aiming to solve the problems arising from the addition of Gd₂O₃ in UO₂ matrix. By the industrial route, the mixture of powders (UO₂, <90 μ and 6 wt% Gd₂O₃ <10 μm) results in pellets with 91% TD at 1677 °C/H₂/4 h. By the mixed route, the formation of Gd₂O₃ coherent layers on UO₂ powder (particles <90 μ) and microsphere (225 μm) surface produced UO₂ - 6 wt% Gd₂O₃ pellets with 95% (powder; 1625 °C/H₂/4 hr) and 83% (microsphere; 1677°C/H₂/4 hr) TD. By the sol-gel route, we obtained UO₂ - 6 wt% Gd₂O₃ in a deagglomerated (powder; <70 μm) or agglomerated microsphere 232 μm) form whose pellets reached > 97% (powder) and >98% (microsphere) TI) at 1677 °C/H₂/4h. According to XRD, OM, and SEM/EDS analysis, the referred three routes do not form a complete solid solution of UO₂-Gd₂O₃ at the temperatures and time of sintering used; Gd₂O₃ granule islands are present in the pellets originating from these routes. The obtained results suggest that the topological arrangement and the deficient nanostructuring of UO₂ and Gd₂O₃ phases, either in the raw material (powder and microsphere) as in their compacts, are the cause of low densification and irregular distribution of Gd₂O₃ in UO₂ matrix; mixing of U and Gd at the molecular level does not form a solid solution; and the mixed route is a good alternative to the industrial route. (author)