Abstract: Material such as fissile material (2) requiring to be safeguarded is stored inside a container (1) which is subsequently sealed, for example by a closure comprising a cover (3) and sealing element (30). The whole structure of the container (1) is ultrasonically scanned initially using piezo electric transducers (T1, T2) to obtain an output from which is derived a distinctive identity for the container (1), determined by the particular characteristics of the internal structure of at least a portion of the container, and also a reference signal indicative of the whole container structure when its integrity is intact. Subsequent ultrasonic scanning of the whole container structure, for example intermittently, produces a monitor signal which if different to the reference signal indicates the occurrence of a breach in the container's integrity. The distinctive identity for the container (1) may be derived from a distinctive internal structure of the cover (3) which preferably contains a material matrix, e.g.