05000333/LER-2012-007
| James A. Fitzpatrick Nuclear Power Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3332012007R00 - NRC Website | |
BACKGROUND
At approximately 9:41 pm on November 4, 2012, Operations personnel were performing ST-21F, "Main Turbine Overspeed Trip Device and Mechanical Trip Valve Test." The purpose of ST-21F is to test the main turbine overspeed trip device and mechanical trip valve without causing a main turbine trip or affecting turbine speed.
During performance of the surveillance, operators noted that annunciator 09-5-2-40 (Main Turbine Overspeed Lockout) remained lit following completion of step 8.2.5 where it should have cleared. Approximately 1 minute after step 8.2.5 had been performed, a full reactor scram occurred.
EVENT DESCRIPTION & ANALYSIS On November 4, 2012, at 9:53 pm, with the plant operating at 100% power in Mode 1, the James A. FitzPatrick Nuclear Power Plant (JAF) experienced a reactor scram. Immediately after the scram, the main turbine tripped [El IS System Identifier: TA]. The sequence of events (SOE) log determined the reactor scram was caused by the Main Turbine Stop Valves [EllS Component Identifier: SHV] moving greater than or equal to 15% closed from full open. The root cause evaluation (RCE) and troubleshooting later determined that the scram was due to a failure of the main turbine emergency lockout valve (9450V-LV) [EllS Component Identifier: PSV].
9450V-LV is a solenoid operated, 4-way valve used in a 3-way configuration. During Main Turbine Overspeed Testing, the internal spool pieces are aligned in such a way so as to lockout pressure from the Mechanical Trip Valve and therefore prevent a loss of pressure to the Master Trip Solenoid Valve. Following the scram, it was noted that 9450V-LV was degraded by evidence of overheating. During subsequent troubleshooting, the plastic casing started to melt and char due to excessive heat. The valve was removed and shipped offsite to determine the internal failure mechanism.
At the start of this event, there were no systems, structures, or components inoperable that contributed to this event. All control rods fully inserted, all primary containment isolations occurred as designed, the High Pressure Coolant Injection (HPCI) [EllS System Identifier: BJ] and Reactor Core Isolation Systems (RCIC) [EllS System Identifier: BN] initiated as expected. The RCIC system injected into the Reactor Coolant System (RCS) and HPCI did not; also as expected.
9450V-LV had been recently replaced in September 2012. Following replacement, surveillance test ST-21F was successfully performed as preventative maintenance during JAF's refueling outage 20.
CAUSE OF EVENT
The most probable cause is that the emergency trip lockout valve stuck in an abnormal position which caused the solenoid to fail. After the solenoid failed, it is postulated that the spool(s) inside the lockout valve moved to a position that allowed hydraulic fluid to port off thereby causing a pressure loss to the master trip solenoid. This loss of pressure then allowed all four main turbine stop valves to begin to close. Once the main turbine stop valves reached 85% open, a reactor scram occurred. No contributing causes to this event were identified.
EXTENT OF CONDITION
A review of the Electro-Hydraulic (EHC) control system [El IS System Identifier: TG] was completed to determine if this failure mechanism could occur elsewhere in the system, resulting in multiple turbine valves repositioning, and causing a reactor scram. There is a similar shuttle valve called the Relay Trip Valve that is in series with the Emergency Trip Lockout Valve. The Relay Trip Valve uses EHC pressure from the pumping skid and relays it into Relay Emergency Trip System (RETS) pressure. This pressure maintains the disc dump valve on each of the four turbine control valves and four intercept valves closed thus allowing the valves to remain open and be controlled by the EHC system.
The relay trip valve is a mechanical device only, the shuttle (spool) is actuated via EHC oil pressure. It has no electrical interface and is not cycled at any time after the turbine has been reset. Therefore, the extent of condition for a reactor scram caused by a similar failure of the relay trip valve was deemed not applicable. In addition, the specific model used for the Emergency Trip Lockout Valve, 94S0V-LV is not used in any other location in the plant.
FAILED COMPONENT IDENTIFICATION
Description: Main Turbine EHC Emergency Trip Lockout Valve Manufacturer: Denison Hydraulics (A014) Model/Part Number: D1VW030HVYG591XB962 FitzPatrick Component ID: 94S0V-LV
CORRECTIVE ACTIONS
Completed Actions
- 94S0V-LV was replaced with the same model valve.
- The lockout circuit was statically and dynamically tested to ensure there were no other issues with the circuit.
- ST-21F was performed satisfactory with the Main Turbine offline, with the Main Turbine at 1800 RPM but not synched to the grid, and with the Generator synched to the grid.
Future Actions
- Perform an equipment failure analysis on 94S0V-LV to determine failure mode.
- Review Entergy and original equipment manufacturer recommended PM scope and frequency; revise the PM if necessary based on this review.
ASSESSMENT OF SAFETY CONSEQUENCES
Radiological & Industrial Safety There were no Radiological or Industrial Safety concerns associated with this event.
Nuclear Safety There were no nuclear safety concerns associated with this event. The main turbine generator system is a non safety related system. However, there are eight safety related position switches in the main turbine steam stop valves that will actuate to provide a reactor scram signal to the reactor protection system. As discussed in this LER, this system operated as designed.
SIMILAR EVENTS
The Entergy corrective action database was searched for events in which a reactor scram occurred due the closure of all main turbine stop valves. There were no previous events at JAF that had a match to this event, although two events were found to be similar in the Entergy fleet.
REFERENCES
- JAF Condition Report: CR-JAF-2012-07901, Root Cause Evaluation