A method and apparatus for removing solid or semi-solid material from within a container, for example removing an energetic material from the casing of an explosive shell. The method is applicable to the demilitarization of aged or obsolete explosive ordnance, for example mortar rounds, and uses ultrasonic energy rather than heat like previous methods. An item to be demilitarized and cleaned is immersed in a fluid contained within a vessel. An ultrasonic probe or wand is inserted into the submerged item, in proximity with the material to be removed. The fluid is the medium for transmitting sonic energy from the probe to the material to fracture and mobilize the material. Mobilized particulates are expelled from the casing, and settle to the bottom of the fluid vessel for collection. At the conclusion of the process, the emptied container shell, now relatively clean, is retrieved from the vessel and salvaged as desired.