05000361/LER-2009-001
| San Onofre Nuclear Generating Station Unit 2 | |
| Event date: | 09-13-2009 |
|---|---|
| Report date: | 11-12-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3612009001R00 - NRC Website | |
Event Date: September 13, 2009 Reactor Vendor: Combustion Engineering Mode: Mode 1 Power: 94 percent On 9/13/09 (Event Date) at approximately 12:42 PDT, Unit 2 automatically tripped from 94 percent power on low condenser vacuum [JC] caused when Circulating Water [NN] Heat Treatment Recirculation Gate 5 stopped in mid-position. On 9/13/09 at 13:12, Southern California Edison reported this occurrence to the NRC in accordance with 10CFR50.72(b)(3)(iv)(A)(NRC Event Log #45347) for an event that resulted in a valid actuation of the Reactor Protection System, including a reactor trip. This LER is being submitted in accordance with 10CFR50.73(a)(2)(iv)(A).
Background
Heat treatment of the intake water tunnels is periodically performed to control marine growth. Intake structure recirculation gates allow reversal of the normal flow path, directing heated water from the condenser discharge through the inlet tunnel, and seawater through the outlet tunnel to the condenser inlet. Circulating Water Heat Treatment Gate 5 is one of four gates operated for heat treatment. Gate 5 is opened to divert the heated condenser discharge to the inlet tunnel.
Gate 5 is 15' 4" wide, 14' 5" high, and 1' 9" thick, made of reinforced concrete with a rubber edge seal, and weighs about 78,000 lbs. The gate is positioned using two 25 ft. long six inch diameter threaded shafts (stems) driven by a Limitorque SMB-5 actuator through two geared right angle drives.
Gate 5 was operated on 9/5/09 during a sequence of operations to prepare the intake for heat treatment.
When the gate was closed two loud bangs were heard. Latent damage to the drive shaft coupling keys was not discovered at that time. Maintenance personnel replaced missing set screws on a gate shaft (stem) stop nut; however, the gate was not sufficiently realigned and leveled. Optional procedure steps to level the gate and adjust the stop nuts were not performed.
On 9/9/05 another attempt was made to complete the Unit 2 heat treatment. With the power reduced to about 85 percent, the heat treatment was halted when Gate 5 stopped at about 30-35 percent open.
Operators were able to reverse the heat treatment sequence and successfully recover the unit before a substantial loss of vacuum. The reduced power level and relative intake gate positions provided sufficient time for effective operator action.
Subsequent investigation by maintenance personnel revealed that the Gate 5 actuator thermal overload had tripped, the drive was damaged, and both drive shaft keys were sheared. The gate was slightly out of position, but it was believed debris in the intake had prevented the gate from seating fully closed; it was not recognized that the gate was off level and had been dragged against the guide channels until the motor tripped. The gate was repaired and successfully stroked to about 10% open as a test of the repair (the gate cannot be moved through the full range with the unit on line and the normal condenser inlet / outlet alignment).
Description of Event:
On 9/13/05 at12:22 PDT operators performing a heat treatment of the Unit 2 intake were in the process of reversing the circulating water tunnel flow through a sequence of gate operations. Power had been reduced from 100 percent to approximately 94 percent for the heat treatment. At 12:31 Gate 5 stopped moving at the 45 percent open position. With Gate 5 open and the other intake structure gates repositioning to reverse the tunnel flow, heated water from the condenser discharge was partially diverted to the condenser inlet. The increasing condenser circulating water inlet temperature caused the condenser vacuum to decrease.
Control Room operators responded by reducing turbine load and directing field operators to realign the gates. Gate 5 could not be closed; as a result the condenser vacuum continued to decrease. At 12:40 the "Lo Vacuum Pretrip" alarm annunciated, followed about one minute later by an automatic turbine trip on loss of condenser vacuum and Unit 2 reactor trip.
Cause of the Event:
The direct cause was a misalignment of Gate 5 which initially occurred on 9/5/09. The corrective maintenance did not level the gate, leading to the failure on 9/13/09. The work process did not employ a formal troubleshooting plan, the gate was not fully tested, and there was an inadequate preparation for a potential gate failure before proceeding with the heat treatment.
Corrective Actions
The applicable maintenance procedures were revised to include a positive method of securing both set screws, and the circulating stop gate inspection procedure was revised to require removal of the stem covers to determine the stem heights.
Maintenance revised the "Troubleshooting Plant Equipment and Systems" procedure to require the approval of an SRO for high risk activities and communicated the expectations to follow and observe the formal procedure for troubleshooting. Maintenance also formed an incident response team with multi disciplined skills to lead future troubleshooting efforts.
Operations enhanced the heat treatment procedure to incorporate a rapid response strategy for each intake structure gate and revised procedures to require a manual reactor trip when any plant parameter is at a valid pretrip setpoint and continuing to degrade.
Safety Significance
The safety significance of the event was minimal. All plant safety systems and components operated as required following the Unit 2 automatic trip. Unit 3 remained on line at 100 percent power and was unaffected by the event. The intake structure also supplies the Salt Water Cooling (SWC) System which removes heat from the safety related Component Cooling Water (CCW) System. An evaluation determined there were no deleterious impacts to either system from the increased intake structure temperatures. The heat treatment was sufficiently completed to minimize marine growth in the CCW Heat Exchanger. Safety System Components (SSCs) were not adversely affected by the event, and the event remained bounded by the UFSAR Section 15.2.1.3 "Loss of Condenser Vacuum.
Operating Experience (OE) 12/31/07 SONGS Unit 2 Gate 5 was damaged during maintenance when the gate bottomed out on support cribbing, caused by improper management of the repair task. Corrective action was taken to review the lessons learned and the procedure "Circulating Water Stop gate and Operator Inspection," was revised to include an instruction on how to verify the gate is level prior to assembly and a requirement to ensure the gate is lowered prior to disassembly. Had the optional procedure step of removing the stem covers been performed to verify the gate is level, the gate misalignment could have been averted, precluding the Unit 2 trip on 9/13/09.