Spacer Grids for Accident-Tolerant Fuel Assemblies

Past DOE SBIR programs have shown that accident-tolerant cladding like silicon carbide (SiC) fuel rod cladding can be a durable, less reactive alternative for zirconium alloy cladding for light water reactors.The higher temperature performance and increased safety potential of accident tolerant fuel rod cladding during a loss of coolant accident (LOCA) is expected to both improve the sustainability of the existing fleet of commercial light water reactors and serve as a key building block for future generation reactors.Grid-to-rod fretting is one of the leading causes of fuel rod failure in modern pressurized water reactor (PWR) fuel assemblies.SiC cladding, being significantly harder than the zirconium alloys currently used for PWR & BWR fuel rod cladding, should be less prone to fuel rod failure due to fretting if used in conjunction with metallic spacer grids.In fact, it is so hard that the opposite problem now becomes a significant concern; that the vibrating fuel rod will wear away the spring contacts and stops on the zirconium alloy intermediate spacer grids and PWR flow mixing grids, thereby compromising the fundamental positioning function of those grids.Also, attaching zirconium alloy spacer grids to the guide tubes in a PWR can be a challenge if those guide tubes are likewise converted to SiC.Although ongoing efforts have rightly placed priority on proving the accident-tolerant fuel rod cladding viable, concerns over the potential fretting and structural integrity issues associated with the accidenttolerant fuel assembly need to be addressed and resolved in parallel to ensure that implementation is not delayed.An all-SiC structure also simplifies the assembly process for PWR fuel assemblies.The purpose of this project is to complete development of the manufacturing processes required for a SiC grid assembly and to perform testing that will be required to approve the grid for use in reactors.The brazing and grid assembly processes developed in Phase I will be refined to incorporate lessons learned and to improve the overall product.Additional analysis including seismic analysis and fretting and fuel assembly distortion evaluations will be performed in Phase II.Finally, grid spring testing, pressure drop testing and grid impact tests will be performed.