Quantifying biases and limitations of two-dimensional DIC during LOCA burst testing of fuel cladding to support deployment in hot cell environments

Abstract

Accurately characterizing nuclear fuel cladding deformation under postulated loss-of-coolant (LOCA) conditions is essential for refining reactor safety limits and qualifying accident tolerant fuel (ATF). Nondestructive hot-cell techniques are particularly valuable for providing real-time measurements of scarce irradiated specimens without compromising integrity. Digital image correlation (DIC) was recently integrated into the hot-cell Severe Accident Test Station to monitor in-situ strain evolution during simulated accident burst testing. Both stereo and two-dimensional (2D) configurations were implemented, with stereo DIC providing higher fidelity and 2D DIC serving as a backup when stereo operation is impractical. Given the high cost of hot-cell experiments, validating the fidelity of the 2D approach is essential. Out-of-cell benchmarking was performed on bare and Cr-coated Zircaloy-4 specimens using simultaneous stereo and 2D DIC during transient burst and creep tests. Average strain curves showed strong agreement, with discrepancies generally between 0.05 and 1.5% strain. Modified Allan variance confirmed that noise in both systems was dominated by low-frequency optical disturbances, with differences limited to error magnitude rather than type of error. Line-scan comparisons revealed larger discrepancies in ballooned regions where non-uniform strain introduced projection errors, whereas analysis restricted to central regions during creep testing produced nearly identical results. Stereo DIC remains the preferred approach for capturing high-fidelity strain maps. However, 2D DIC with a telecentric lens provides a practical, reliable backup for average strain measurements in hot-cell experiments.