While operating at rated power the unit was scrammed by the AZ-1 emergency protection due to in-turn tripping of Reactor Coolant Pumps, RCPs-2,1,3,4 respectively. Primary circuit cooldown was performed in the regime of natural circlation. No deviations from safe operation limits and conditions were observed. The cause of RCP trip was the increase of RCP component cooling circuit temperature up to RCP tripping setpoint (45'C) due to detachment of the control valve's cut-off device in the service water supply line for cooling the heat exchanger of RCP component cooling circuit. RCP component cooling circuit temprature increase went on during 18 min. and remained unnoticed because of lack of appropriate alarms. Root cause of the event was deficiency of the service water conrol valve maintenance procedure.
BASIS FOR RATING: Safety function availability - full. According to Table II (A1) - level 0.