High Temperature Alloy-Interface Characterization in Molten Salt Reactors by Radiation Resistant Fiber Optic Sensors

In order to deploy clean, affordable, domestic energy sources of the future, the United States must improve the safety and reliability of high energy systems.Engineering materials for service in nuclear reactors, such as the higher efficiency molten salt reactors, are under development that will extend their safe operating lifetimes to at least four to five decades to achieve long-term energy security.Molten salt nuclear reactors (MSRs) have highly corrosive salts that are liquids at operating temperatures, and continuous, in-situ character­ization of the structural integrity of their conduits is essential. High-temperature capable transducers are desired for continuously monitoring the MSR containment vessels, such as high melting point (> 1500 °C) fused silica optical fibers.Innoveyda proposes to build on successful Phase I feasibility demonstration results and, in the follow-up Phase II, to develop structural diagnostic technology for continuous, autonomous in-situ monitoring of MSR conduits with corrosive molten salts. High-temperature- and radiation- resistant fused silica fiber optic sensors will measure the effects of corrosion on alloy metal tubes.Structural material changes in alloys containing the molten salts will be measured via ultrasound guided waves in alloy pipes that are detected by high-temperature capable fiber optic sensors.Distributed fiber optic sensors will localize structural changes by measuring position-sensitive ultrasound responses in corroded alloy pipes, at ultra-high temperatures. In Phase I, Innoveyda compared and analyzed pre-corrosion and post-corrosionultrasound response data of MSR-specific alloy pipe sections provided by Oak Ridge National Laboratory.Changes in the properties of the pipe samples due to molten salt interactions were successfully measured by select guided ultrasound modes and found to be in reasonable agreement with those predicted by numerical models developed by Innoveyda’s team for the specific sample alloy and geometry. Ultrasound responses of the corroded samples were also measured by fiber optic sensors, demonstrating the Phase-I proof of concept. In Phase II, a complete prototype autonomous TR-FOSIM system to measure real-time corrosion induced effects in MSR-specific alloy pipes will be designed, developed, and data analyzed, based on mathematical models tuned to a select guided mode propagation in the specific pipe geometry. This complete prototype diagnostic system will be developed in Phase II with hardware and dedicated data analysis software by our team of experts. The major tasks to reach this Phase II goal include: (a) launching guided modes at ultra-high temperatures in a select alloy pipe in a thermal convection loop; (b) detecting pipe ultrasound responses (specifically frequency dependent phase and group velocities) at candidate corrosion-prone locations, and (c) determining time dependent material properties of the pipe with autonomous software controlling data acquisition, over 500 hours of operation at near 700 °C, in a thermal convection loop at ORNL facilities. Thermal convection loop studies shall form the basis of diagnostic systems for more complex piping structures that measure in-situ alloy material changes at the ultra-high MSR operating temperatures. The diagnostic data obtained from these measurements are expected to assist in the selection of: (i) less corrosion-prone alloys and/or (ii) salt mixtures for improving the safe operating lifetimes of high-temperature MSRs presently under development. The ultimate goal is to measure in-situ changes in material properties at corroded interfaces and degree of damage classified for disaster prevention. Upon achieving material/geometry-specific mathematical modeled response data that provide a better understanding of pipe guided waves and effect of cracks/damage detected by fiber transducers, the proposed project will enable commercialization of the high performance, cost-reducing automated pipe structural health monitoring system.The final goal is to predict remaining useful life of alloy structures.The business model incorporates structural health monitoring for alternative energy solar thermal power plant conduits where corrosive salts are used for heat storage, for oil, gas, and refinery operation pipelines, as well as for corrosion-prone pipes in nuclear reactors that use corrosive fluoride salts.