Study on GRADIUM Lens Based Fiber Imaging for Reduction of Debris During Nd:YAG Laser Cutting and Dismantling

摘要

This paper reports on experimental study of pulsed Nd:YAG laser cutting of various contaminated parts and structures in nuclear facilities for dismantling, decommissioning and refurbishment purposes. Laser cutting has been reported as a well-established and very specific technique for dismantling. During laser cutting process, laser beam melts the material and an assist gas blows it out. Some amount of the material is lost in the form of debris ejection from the cut width. Most of the time, this debris contains highly radioactive contamination and is hazardous for environment. An experiment has been performed for the reduction of the cut width and loss of materials (debris) during laser cutting process using a GRADIUM lens based half imaging of optical fiber. These GRADIUM lenses are made from an advanced axial gradient index glass, which reduces spherical aberration in focusing. A 500 W average power, long pulse Nd:YAG laser with 400 μm core diameter fiber optic beam delivery has been deployed in the experiment. The mass of debris ejection during the cutting of 4 mm thick stainless steel sheet has been reduced by an amount of ～40% after using GRADIUM lenses as compared to conventional fused silica lenses. It was also observed that GRADIUM lens enhances the cutting speed and reduces the heat-affected zone significantly as compared to laser cutting with conventional fused silica lenses. A compact laser-cutting nozzle of 15 mm outer diameter with coaxial gas jet has been developed to perform laser cutting operation at remote locations in nuclear facilities with limited working space.