[en] Operational critical assemblies are subject to the same standards of materials control and accountability (MC and A) as other fissile materials. Typically these assemblies are not easily dismantled and the fuel components cannot be inventoried by traditional methods of destructive or non-destructive materials assay. Because of the large mass or unusual geometry of the fuel components these parts are often categorized as difficult-to-measure (DTM) items. The use of criticality to infer fissile inventory, in conjunction with physical security and two-person control procedures, provides accurate and cost-effective MC and A for these large critical assembly components. This method is being implemented at the Los Alamos Critical Experiments Facility (LACEF) and at several national nuclear research centers in Russia