Advanced Metallic-Silicon Carbide Composite Claddings for Improved Damage Tolerance

The recent events at the Fukushima nuclear power plant highlight the need for enhanced accident tolerance. Of particular concern is the overheating of standard zirconium alloy cladding in a loss of coolant accident (LOCA). One of the leading candidates for future claddings is a silicon carbide (SiC) composite. However, it is clear that due to the inherent brittle nature of ceramics and the intrinsic porosity of composites, the retention of fission products within a SiC-based cladding is an issue. Some SiC clad designs seek to overcome this issue through one or more ceramic coatings such as CVD SiC. However, the community has recently raised concerns regarding hydrothermal corrosion of SiC. Both fission product retention as-compromised by thermomechanical loading or irradiation, or corrosion leading to an unacceptable level of silica in the reactor coolant, suggest the need for an alternative concept to the fullyceramic SiC/SiC clad. One such design is the hybrid design, using a thin metallic coating on the exterior of the composite. In theory, this coating could provide fission product retention under normal operating conditions and the required environmental barrier coating (EBC) against corrosion. During this investigation, an innovative coating technique has been developed for producing the corrosion resistant metallic EBC, i.e., Vacuum Plasma Spray – Thin Film (VPS- F). The VPS-TF process enables the production of dense, thin film coatings at lower pressures with higher plasma power than conventional VPS processing. As a result, the feedstock material is evaporated or partially evaporated resulting in a very fine coating microstructure of splats and columnar growth, which possess improved strain tolerance similar to physical vapor deposition (PVD). However, unlike PVD, VPS-TF has relatively high deposition rate and the ability to economically coat large SiC/SiC on a production basis. Using these techniques, corrosion resistant metallic coatings such as FeCrAl, 310SS, and Cr, were successfully deposited on SiC/SiC tubes during Phase I. Preliminary analysis and testing of these coatings at MIT have produced very encouraging results including improved hydrothermal corrosion resistance as compared to uncoated SiC/SiC. During Phase II, fabrication techniques will be optimized. Prototypical fuel rods with the improved cladding will be fabricated and tested at relevant reactor conditions.Commercial Applications and Other Benefits Fuel rods for safe commercial nuclear power in USA and throughout the world. A significant secondary benefit is that the primary manufacturing process being developed under this work, the VPS-TF, is likely to find application is a wide array of other applications such as EBCs for turbines or similar thermal protective applications.