A segment of a structure mitigates flow of fluid therethrough. In one embodiment the segment includes an opening for the fluid flow and the modified structure may include a ferromagnetic wall defining the opening and a plurality of permanently magnetized particles. Some of the permanently magnetized particles are attached to the wall by magnetic forces. A system is also provided for injecting magnetic particles into a cavity to impede movement of fluid through the cavity. A method is also described for mitigating a flow of fluid through an opening in a wall. In one embodiment, the method includes positioning a plurality of first magnetic particles along the wall and about the opening and attaching a plurality of second magnetic particles to the first magnetic particles wherein some of the second magnetic particles collectively extend across the opening to cover the opening.