In order to verify practical effectiveness of fiber laser cutting technology to reactor decommissioning, towards over 150 mm thickness, laser cutting experiment of thick steel plate is conducted by using 10(4+6) kW fiber laser system. As it stands now, laser cutting of over 100 mm thickness steel plate isn't achieved. There are several possible reasons why thick steel plate can't be cut. One of them, we consider is a difficulty of dross (molten metal) ejection to the back side of steel plate. A cutting kerf is small in width, and assist gas flow decay with increasing kerf depth. Therefore thermohydraulic interaction between assist gas and dross takes on an important role for a formation of the steel kerf. Numerical simulation code, based on multi-phase thermo-hydraulics, has been being developed with a goal of a control and prediction for the laser cutting process. In order to analyze the dross ejection characteristics, the code solves mass, momentum, and energy conservation equations simultaneously in a finite difference form with a series of physical models of the laser cutting process, such as heat input by laser, phase-change, and three-phase surface capturing. In this way, the laser cutting simulation code was build on the concept of multi-purpose multi-phase thermohydraulic applications. A thermohydraulic numerical simula- tion of the laser steel cutting was carried out to confirm an assist gas and cutting speed effect to the cutting performance. The performance was evaluated, based on temperature profile and cutting front formation. Simulation results were as follows. If there was no effect of dross ejection by assist gas, a laser light was absorbed into molten steel stagnated in the kerf. Therefore, there was less laser heat input to a solid surface directly. Then, heat transport to the back side of the steel plate got delayed. In the case of faster cutting speed, delay of heat conduction and failure cut were confirmed at behind the cut starting position of the steel plate. Failure cut at the position was observed in our experiments. From these results, it was concluded that the ther-mohydraulics in the kerf takes important role for not only dross ejection but also promotion of heat input at solid surface.
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