Modeling Forced Flow Chemical Vapor Infiltration Fabrication of SiC-SiC Composites for Advanced Nuclear Reactors

Silicon carbide fiber/silicon carbide matrix (SiC-SiC) composites exhibit remarkable material properties, including high temperature strength and stability under irradiation. These qualities have made SiC-SiC composites extremely desirable for use in advanced nuclear reactor concepts, where higher operating temperatures and longer lives require performance improvements over conventional metal alloys. However, fabrication efficiency advances need to be achieved. SiC composites are typically produced using chemical vapor infiltration (CVI), where gas phase precursors flow into the fiber preform and react to form a solid SiC matrix. Forced flow CVI utilizes a pressure gradient to more effectively transport reactants into the composite, reducing fabrication time. The fabrication parameters must be well understood to ensure that the resulting composite has a high density and good performance. To help optimize this process, a computer model was developed. This model simulates the transport of the SiC precursors, the deposition of SiC matrix on the fiber surfaces, and the effects of byproducts on the process. Critical process parameters, such as the temperature and reactant concentration, were simulated to identify infiltration conditions which maximize composite density while minimizing the fabrication time. 1. Introduction Advanced nuclear reactor concepts promise significant improvements over current technology, including increased efficiency, higher fuel burn-up, and longer core life. However, these features put increasing demands on the performance of fuel cladding and other reactor components, and materials must be developed for these reactors that are both resistant to high levels of irradiation damage and offer accident tolerant behavior. Silicon carbide fiber/silicon carbide matrix (SiC-SiC) composites offer many desirable properties, and are being considered for use in advanced nuclear reactor designs, such as the General Atomics Energy Multiplier Module (EM2) concept. Experiments on monolithic silicon carbide have shown that it maintains excellent mechanical performance in harsh, high temperature, and high irradiation rate environments, but its low toughness limits its application [1–3]. High purity and high quality SiC fiber-reinforced composites have shown similar performance under harsh conditions but offer improved toughness to address this limitation. In these composites, a silicon carbide matrix is deposited within a preform composed of high purity, near-stoichiometric silicon carbide fibers, such as Tyranno-SA fibers (Ube Industries, Ube,