Abstract:The Plutonium Facility (TA-55) at Los Alamos National Laboratory (LANL) has been in operation for almost 20 years. The Operations Center of TA-55 is the nerve center of the facility where operators are on duty around the clock and monitor several thousand data points using the Facility Control System (FCS). The FCS monitors, displays, alarms, and provides some limited control of several systems, including: HVAC, lire detection and suppression, radiation detection, and electrical. The FCS was failing and needed to be replaced expeditiously. This paper will discuss the human factors aspects of the design, installation, and testing of the new FCS within the above noted constraints. Particular items to be discussed include the functional requirements definition, operating experience review, screen designs, test program, operator training, and phased activation of the new circuits in an operational facility.