SHM system development for molten salt test loop

Ultrasonic structural health monitoring (SHM) systems can detect degrading material properties based on changing wave speeds. In harsh environments like molten salt reactors and allied test loops, the transducers must be sufficiently robust to survive, and guided waves facilitate minimizing the number of transducers. The L (0, 2) guided wave mode can propagate long distances in the Ni-based alloy 600 tubing used for a molten salt thermal convection loop. The sensitivity of the L (0, 2) group velocity to material degradation associated with the corrosive attack of molten salt is assessed by modeling and ex situ guided wave speed measurements from alloy 600 tubing used in static salt tests. Guided wave testing is performed on dry tubing at room temperature. Microscopy is performed after guided wave testing to document the corrosive attack. At driving frequencies of 490 and 680 kHz, the group velocities are shown to decrease and increase, respectively, for small amounts of material degradation on the inner diameter of the tubing. These opposite trends in group velocity are anticipated to provide a sensitive characteristic for SHM. Although the data are somewhat limited, the wave speed of the more dispersive 680 kHz frequency appears to be more sensitive to corrosive attack, with a possible threshold in the 50–100 µm range.