Hi community! Could anyone help me to get the meaning of the phrase "once in action" in the following extract from the article about a nuclear facility? The extract: "For six weeks, Sellafield’s engineers prepared for the task, rehearsing on a 3D model, ventilating the cell, setting up a stream of air to blow away the molten metal, ensuring that nothing caught fire from the laser’s sparks. ONCE IN ACTION, the snake took mere minutes to cut up the vat. But then the pieces were left in the cell. No one had figured out yet how to remove them. The snake hasn’t been deployed since 2015, because other, more urgent tasks lie at hand." Does it mean that they strated doing it for real, or it was their another time of training on the 3D model? I would REALLY appreciate your help!