Usage of cellulose ashless filter paper for direct denitration to fabricate high density (U_0.7Ce_0.3)O₂ pellets with homogeneous U/Ce cation distribution

Direct thermal denitration of aqueous mixtures of metal (M = Ln, An) nitrate with uranyl nitrate have been widely reported to result in glassy (mixed) oxide chunks. Due to the very low specific surface area, pellets pressed from such powders do not achieve the desired sintered density. In this work, the batch-wise direct thermal denitration of aqueous mixtures of uranyl nitrate and (NH₄)₂Ce(NO₃)₆, Ce(NO₃)₃ or CeO₂ dissolved in HNO₃:H₂O₂, with targeted 30 mol% Ce content was investigated. Each aqueous solution was soaked onto pure cellulose ashless filter paper, dried in air and subjected to direct thermal denitration in air for 1 h at 723 K. The resulting mixed-metal oxide sheets were manually crushed into fine powder and reduced for 4 h at 873 K in 5 %H₂:95 %Ar. The resulting powders were characterized by BET and SEM. All three powders yielded well-pressed green pellets which were sintered for 10 h at 1923 K in a 500 ppmO₂:Ar atmosphere into high-density (≥95 %TD) ceramics and subjected to an equilibration treatment at 1073 K to obtain an O/M ratio of 2.00. Ambient X-Ray powder diffraction patterns from crushed pellets revealed very sharp fcc (Fm-3 m) patterns of (U,Ce)O₂. EPMA analysis and line scans on axial cross-sections of a Ce(NO₃)₃ derived pellet confirmed the homogeneous U/Ce distribution.