Developing Techniques for Joining SiC to Metal Components

Several different Generation IV molten salt reactor (MSR) designs are in the development stages. Some designs are based on using fluoride salt with the fuel dissolved in the salt and others are based on using solid fuel forms with clean salt as the coolant. Many of these designs will operate with outlet temperatures in the range of 700-800°C. However, advanced very high-temperature MSRs are also being examined where molten salt is used as the coolant, with outlet temperatures in the 900-1000°C. Traditionally, reactor temperatures were limited, in part, by the use of water for heat transfer. While liquid metals offer a way to increase operating temperatures, they cannot raise temperatures sufficiently to allow alternative power conversion technologies. Molten salt reactors can operate at higher temperature ranges to effectively utilize super critical CO2 Brayton cycle or even standard Brayton cycle power conversion systems. Continuous silicon carbide fiber/silicon carbide matrix (SiC/SiC) composites are especially suitable for use in a number of MSR core components, including control rods/blades, tie rod support, and fuel assembly structural members due to the ability of SiC composites to withstand harsh operating conditions, such as high temperatures, high neutron flux and lifetime fluence, and long-term contact with corrosive molten salt. SiC composites may also be used for components outside of the core, such as piping, valves, and pipe connections. However, there are several technology gaps that need to be addressed in order to allow the application of this material in MSRs. One critical gap is the joining of SiC composites to appropriate metal alloys that are suitable for use in the high-temperature, corrosive molten salt environment.Such joints between materials with dissimilar properties must not only be chemically stable, but must also incorporate sufficient compliance to minimize stresses. Although, joining SiC composites to limited metallic materials has been demonstrated in the past, further research and development is needed to ensure strong, reliable, leak-tight joints between SiC composites and metal alloys used in MSRs. The objective of this project will be to develop the design and process for joining a SiC composite pipe to a metallic pipe that includes a flexible bellows. The plan will be to braze metal flanges to the outside of SiC pipes and perform tests followed by metallurgical evaluation. The Phase I activities are focused on developing a conceptual design of a prototype SiC-metal joint which will be used to confirm solutions to key technical issues for high temperature molten salt applications. The tasks include development of an engineering specification, material evaluation, design of the metal-SiC joint, development of braze processes, component fabrication and testing. The engineering specification will identify a specific application in a molten salt reactor along with the requirements for the joint in that application. This specification will be used to evaluate the best materials to use for the metal flanges. Several conceptual joint configurations will be modeled and analyzed using finite element analysis. In parallel with joint design and analysis, brazing trials will be performed to determine the optimum process parameters for material, time and temperature. Finally samples will be fabricated and tested to confirm the best configuration to use for the application.