Interactions of Silicon Carbide and Stainless Steel in Cermet Wasteforms

The development and deployment of Gen IV reactors using TRISO and related fuels is on-going, both for large-scale and small modular reactors. The back-end of a TRISO fuel cycle poses new challenges to waste immobilization science, with the complex TRISO particle structure comprising layers of porous carbon, pyrolytic carbon and silicon carbide present alongside the fuel kernel (UO₂, UC, UCO, <i>etc.</i>) being unique among fission fuels. Cermets, composites of ceramic components dispersed throughout a metallic matrix, have been proposed as high-density, durable wasteforms, with the utilization of metallic waste streams (<i>e.g.</i> contaminated stainless steels, Ni-based alloys) as components resulting in extremely high waste loadings and lowering the amount of non-active feedstock materials necessary. In this work, characterization of the interactions between SiC and 316L stainless steel in cermets prepared by hot uniaxial pressing (HUP) will be presented. Cermets with 50:50 volume loadings of SiC and 316L stainless steel were produced (HUP cycle of 1000 °C, 30 mins, 39 MPa), and the impact of a range of SiC sources and particle sizes was examined. Processing routes for the production of bulk SiC from relevant forms of C have been appraised for their applicability to TRISO immobilization, with simple high temperature reactions being the most feasible given the need for scale-up.<br/>Low porosity was observed in the final products; however, significant interaction between SiC and steel was observed in all materials, resulting in formation of regions of C and metal silicides of approximately the same size as the initial SiC particle size. The produced microstructures were characterized by Raman spectroscopic mapping and electron microscopy techniques, with both giving complementary information with respect to identifying regions containing C, steel, SiC, and a range of metal silicides and carbides. Segregation of the metallic components was observed, with Cr and especially Ni appearing to form separate phases, rather than mixed Fe/Ni/Cr silicides or carbides. The observed fractions of C and SiC varied as a function particle size, with larger initial particle sizes resulting in less reaction with the stainless steel matrix, forming relatively lower fractions of metal silicides and carbides. The source of SiC (or C used to synthesize it) was less important to the produced phase assemblage, other than effects attributable to particle size. A preliminary sample comprising a 50:25:25 volume ratio of 316L:SiC:UO₂ has also been prepared as a better simulant of a wasteform for a TRISO fuel mix.