Parametric characterization of underwater laser ablation vis-a-vis laser-assisted standard defect simulation in sintered UO2 pellets

Abstract We report here on the generation of simulated defects on sintered uranium dioxide fuel pellets by taking advantage of the underwater laser ablation procedure and their subsequent characterization. This work, we believe, can play a role towards the validation of the performance of fuel pellet inspection machines. A repetitive fiber laser, capable of delivering pulses of nanosecond duration, in conjunction with a galvo-scanner served as the machining tool in the experiment. The study of the dependence of mass ablation rate on laser fluence, water column height, repetition rate, and beam scanning speed formed the bulk of this work. A water column of height ∼3 mm above the pellet surface in combination with a laser fluence of lying within 6–7 J/cm2 yielded the maximum ablation rate. The generated defects were analyzed using optical and electron microscopy. Clean defects, e.g. longitudinal cracks, circumferential cracks, pits, and end caps of different sizes and depths were created on the pellet surface by varying the laser parameters, beam travel trajectory, and beam scanning parameters. The mechanical pressure arising out of shock wave generated as a result of the confinement of the laser-produced plasma and the collapse of cavitation bubbles together with the microfluid jet formed due to the implosion of the bubbles pushed the molten ablated products from the interaction zone thereby facilitating the generation of cleaned machined surfaces. An insight into the physical processes’ operative in the underwater laser machining process has been offered, albeit qualitatively.