Study on the densification process of SiC_f/SiC composite cladding tube for nuclear grade

Aiming at the densification technical challenges of long-size nuclear-grade SiC_f/SiC composite cladding materials, this paper uses the Fluent numerical analysis method to study the flow field distribution in the CVI (chemical vapor infiltration) chemical vapor deposition reaction chamber, and determines the optimal process parameters for the carrier gas (H₂). Based on the existing pyrolytic carbon (PyC) interlayer deposition, chemical vapor infiltration (CVI), and chemical vapor deposition (CVD) processes, the preparation of SiC_f/SiC composite cladding is completed. The phase structure, chemical composition, and microstructural characteristics of deposits at different pipe diameters are investigated. The results show that when the carrier gas flow rate is 1200 mL/min, the MTS/HCl concentration distribution is uniform, providing a suitable deposition atmosphere. Using C₃H₆ as the precursor deposition gas at 1000℃ for 10 h, a pyrolytic carbon (PyC) layer of approximately 288 nm is obtained on the SiC fiber surface. TEM analysis indicates that the structure of the PyC layer is an amorphous structure. The thickness of the CVI-SiC densification layer is 618–723 μm in the MTS/H₂ system at 1000℃ for 400–500 hours. The density of CVI-SiC at equidistant positions is ≥ 2.75 g/cm³, and TEM analysis shows a β-SiC phase with an FCC structure (space group F43m). The thickness of the CVD-SiC coating is 630–671 μm at 1250℃ for 50–60 h. XRD indicates a pure β-SiC phase with uniform thickness and high densification. In conclusion, the existing densification processes are stable, enabling the production of double-layer SiC composite cladding materials with uniform deposition thickness.