Multisensor Characterization of Process Behavior During Underwater Laser Cutting of 100 mm Thick STS 304 Stainless Steel

Underwater laser cutting is a promising remote segmentation method for highly activated reactor internals because it enables non-contact cutting while maintaining water shielding. Although previous studies have demonstrated the feasibility of thick-section underwater laser cutting, process monitoring remains challenging because bubbles, optical scattering, local gas-cavity behavior, water cooling, and complex melt ejection can alter the relationship between the cutting state and externally measured signals. Therefore, the monitoring characteristics of different sensing modalities during underwater thick-section cutting need to be clarified. This study synchronized a camera, a broadband photodiode, and a triaxial accelerometer during underwater laser cutting of 100 mm thick STS 304 stainless steel. Camera observations were used as the reference for identifying process states. During partial penetration, a broad lateral plume developed, followed by transient lower-lateral ejection and pronounced downward discharge at full penetration. The photodiode mean decreased by 88.8% and 93.0% from partial to full penetration in Experiments 1 and 3, respectively, and remained low afterward. Accelerometer responses around full penetration were less consistent, whereas in Experiment 2, late-stage abnormal behavior was accompanied by a 128.8% increase in the mean PSD in the 75–245 Hz band. These results indicate distinct monitoring characteristics of the three sensing modalities.