Access to the reactor containment must be through doors or hatches that have locks of substantial construction to offer penetration resistance and impede both surreptitious and forcible entry. All doors and locks should be properly installed and not have excessive play between door and frame, and lock and strike. Six-Tumbler Cylinder Lock Equivalent . Pushbutton mechanical locks are generally not equivalent to six-tumbler locks and are not recommended for use at nuclear facilities. Manipulation Resistance .
Most combination locks are vulnerable to manipulation.
Masterkeying at each level further decreases the effectiveness of the lock, because a larger number of keys will operate the lock. When a key system has been masterkeyed, determine if the licensee is aware of all crosskeying possibilities and has appropriate controls in effect. Crosskeying occurs because of the increased number of tumbler splits created. A crosskey is a combination of tumbler levels between the change and master key. When a higher level of masterkeying, such as grand or great grand master is used, the problem of crosskeying becomes acute.