Unexpected events during maintenance operations of a fusion plant like JET or ITER may require rescheduling, suspending the execution of the current procedure to pick it up later. In this paper, we look at how the introduction of slack influences the port plug maintenance schedule robustness. We introduce a metrics to measure the robustness. Simulating disruptions we evaluate the effect on a schedule. Finally, we show how the robustness is affected when introducing different amounts of slack.