Underwater cutting using a 1 μm laser source

There are several potential benefits of using laser cutting in aspects of nuclear (and other) decommissioning processes. These include the speed of the cut, the light weight of the cutting head, the flexibility offered by optical fiber beam delivery, the minimal reaction force on the part being cut, and the high degree of remote automation possible with this process. Such benefits have already been described, for cutting in air, by TWI and others. This paper will focus on laser cutting underwater, where it will be shown that some of these benefits are just as applicable underwater as in air and indeed, particularly for the application of nuclear decommissioning, some additional benefits accrue for the case of underwater cutting. The cutting head used employed a series of jets of compressed air to remove water and maintain an effective dry area in the region of the interaction of the laser beam and the surface of the material being cut. An additional, more conventional, central gas jet was used to remove molten material from the kerf of the cut. A series of trials are described to investigate the effects of parameters used for cutting C-Mn steel and stainless steel underwater, using a 5 kW laser beam. The parameters investigated primarily involved optimization of the positions of the laser beam focus and the cutting nozzle tip, with respect to the surface of the material being cut and the relative pressures in the gas jets. With 5 kW of laser power, it was possible to cut 32 mm thick stainless steel underwater, at a speed of 100 mm/min.