Candidate structural materials for in-core VHTR application

Selection of VHTR in-core structural material will depend on the ability for the material to withstand extreme temperatures and irradiation. The use of Alloy 800H for control rod applications is preferred, but will require designing with a relatively low strength material which becomes brittle upon modest irradiation. Presently, more fidelity in both design and the available Alloy 800H database are required to judge the lifetime this alloy will have in service. The only currently envisioned alternative to Alloy 800H are the CFC and SiC/SiC composites, which are currently challenged by code qualification issues. As with Alloy 800H, the composite irradiation data-base is inadequate, though it appears that the CFC service lifetime will be limited due to its anisotropic dimensional change under irradiation to doses somewhat higher than Alloy 800H (1-5×10²⁵ n/m²), while SiC/SiC composites appear to retain their mechanical properties to doses in excess of the VHTR lifetime dose. As the path to qualification of both CFC and SiC/SiC are similar, it appears that SiC/SiC is more attractive for high-dose Gen IV application unless high thermal conduction is required.