Porous Stainless Steel Microsphere Synthesis by a Nonconventional Powder Metallurgy Process Useful in the Cermet-Type Advanced Nuclear Fuel Fabrication

The fabrication of SS (stainless steel)-UO 2 cermet-type advanced nuclear fuel pellets suitable for use in power reactors depends on the development of metallic (SS), ceramic (UO 2 ), and cermet (SS-UO 2 ) microspheres with special characteristics. In this work, a nonconventional powder metallurgy process was developed to produce porous SS microspheres aiming to contribute to solve the bottlenecks found in the SS-UO 2 cermet pellet manufacturing. SS, UO 2 , and SS-UO 2 microspheres and SS-UO 2 cermet pellets were fabricated and characterized (XRD, EDX, EDS, and SEM). Hard (153 ± 5 µ m; 132.2 ± 24.7 MPa; 72% TD) and soft (216 ± 10 µ m; 1.3 ± 0.4 MPa; 17% TD) SS, hard (176 ± 6 µ m; 147.4 ± 25.0 MPa; 99% TD) UO 2 , and cermet (SS-UO 2 ) microspheres were obtained. The soft porous SS microspheres did not micronize properly in situ , but their high compressibility favors the compaction of the green SS-UO 2 cermet pellet; in this pellet, the UO 2 microspheres behaved as rigid inclusions. This favored the obtainment of sintered SS-UO 2 cermet pellets with high geometric densities (93% TD), excellent metal–ceramic interaction, and the preservation of the physical integrity of the UO 2 microspheres. The usage of high fractions of the SS-UO 2 cermet microspheres obtained mixed with low fractions of the said soft porous SS microspheres is already under development. This will enable the fabrication of SS-UO 2 cermet pellets with a volume fraction greater than 42 vol% UO 2 , a homogeneous distribution of UO 2 microspheres in the metallic matrix, and null connection between them. The oxide–metal reduction mechanisms were discussed. The applicability of the process is already being explored in the manufacture of porous NdFeB microspheres.