Use of Human Performance Tools Resolves Blocked Valve Gearing

While performing a valve line up on the By-Pass Demineralizer (BPD) system at the Advanced Test Reactor (ATR), operators noticed a pressure change in the Primary Coolant System (PCS). Normally this procedure would not impact the flow of the PCS. Faced with an unexpected situation the operators stopped the procedure and placed the system in a safe configuration. The cause for the problem was discovered after a bit of detective work using both situational awareness and a questioning attitude.