On Measuring Material Changes at Molten Salt Reactor Temperatures in a Thermal Convection Loop with Guided Ultrasound Waves

Molten salt reactors (MSRs) for nuclear power generation contain liquid halide salts that may be highly corrosive at operating temperatures. The safety and economic viability of MSRs rely upon the continuous, in-situ characterization of the structural integrity of their conduits. Off-line nondestructive inspections of components are simply too expensive, making structural health monitoring in this harsh environment a critical technology. High-temperature capable transducers, such as fused silica optical fibers with a softening point of about 1600 °C, can be utilized to continuously monitor MSR containers. We are using a thermal convection loop (TCL) at Oak Ridge National Laboratory (ORNL) 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 chloride salt corrosion on alloy 600 tubing. Structural material changes in alloys containing the molten chloride salts will be measured via guided ultrasound waves in alloy tubes, detected by high-temperature capable fiber optic sensors. Distributed fiber optic sensors can localize structural changes with position-sensitive measures. The described tasks are: • launching guided wave modes at elevated temperatures in alloy 600 tubing in a thermal convection loop, and • detecting tube ultrasound responses at corrosion-prone locations in the TCL piping with fiber optic sensors. TCL studies at MSR operating temperatures could provide a basis for diagnostic systems that measure material changes in more complex structures.