05000416/LER-2007-002

LER-2007-002, Reactor SCRAM due to Turbine Trip caused by Loss of Condenser Vacuum

Docket Number
Event date:	05-19-2007
Report date:	07-16-2007
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4162007002R00 - NRC Website

On May 19, 2007 at 1127, an automatic Reactor SCRAM occurred due to a Main Turbine trip on low Condenser vacuum. Failure of a Condenser expansion joint resulted in Condenser vacuum decrease that led to a Turbine trip and actuation of the Reactor Protection System (RPS). This report is being submitted pursuant to 10CFR50.73(a)(2)(iv).

B. INITIAL CONDITIONS

At the time of the event, the Reactor was in OPERATIONAL MODE 1 with Reactor power at 100 percent. There were no additional inoperable structures, systems, or components at the start of the event that contributed to the event.

C. DESCRIPTION OF OCCURRENCE

On May 19, 2007, at 1124, GGNS was in normal operation at 100 percent power when the Control Room received the alarms "Off-Gas Trouble" alarm and "Condenser Turbine Expansion Joint Level Low" alarm. The HP Condenser expansion joint (1N19B007A) seal level alarm was soon followed by a decrease in Condenser vacuum and an increase in Offgas flow. In response to these conditions, Operators entered the Off-Normal-Event Procedure (ONEP), 05-1-02-V-8, "Loss of Condenser Vacuum" and reduced Reactor power to approximately 78% by reducing core flow.

As vacuum was decreasing operations established a decision point for a manual Scram prior to reaching the automatic Turbine trip setpoint of 21 inches. At 1127, a Main Turbine trip on a low Condenser vacuum occurred at 24.9 inches. This was prior to the established value for the manual scram. The early trip of the Main Turbine was the result of improper calibration of the Turbine vacuum trip setpoint. The loss of Condenser vacuum was a result of an HP Condenser boot seal failure. No SRVs actuated and Turbine Bypass Control and steam system maintained pressure until much later in the event.

After the Turbine trip, vacuum continued to lower and a decision was made to initiate a cooldown to Cold Shutdown.

Turbine Bypass Control valves were used to begin depressurizing. Main Steam Isolation Valves (MSIVs) remained open for significant length of time, allowing Feedwater system to remain in service until Reactor pressure was lowered enough to provide makeup to the Reactor with Condensate Booster pumps. Additionally Control Rod Drive System (CRD) was available to provide makeup to the Reactor. Approximately five hours after the Scram, at about 200 psig Reactor pressure, MSIV's and drains were manually closed prior to the 9 inches Condenser vacuum-low MSIV isolation signal.

With the MSIVs closed one SRV was used to continue cooldown until Shutdown Cooling was in service. No heatup/cooldown limits were exceeded. Suppression Pool water level was greater than the allowable 18.81 feet after the event due to water added from SRV actuations. 05-S-01-EP-3, "Containment Control", was entered as required to lower Suppression Pool level to less than 18.81 feet. All safety-related equipment functioned as required and there was no safety system functional failures. This event is considered a Scram with a loss of normal heat removal because an SRV was used prior to placing Shutdown Cooling in service. Additionally based on the review of the new Performance Indicator criteria, this will not constitute a complicated Scram.

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (6-2004)

D. APPARENT CAUSE

The cause of the event was failure of the HP Condenser expansion joint between the Main Turbine and Condenser. A failure analysis is being performed on the expansion joint; therefore, the Root Cause of the expansion joint failure is indeterminate (Reference Root Cause Analysis Report 2007-05).

E. CORRECTIVE ACTIONS

Immediate Corrective Actions:

• Plant shutdown was conducted in accordance with the requirements of Off-Normal-Event Procedures 05-1-02-V­ 8, Loss of Condenser Vacuum; 05-1-02-1-1, Reactor Scram; 05-1-02-1-2, Turbine and Generator Trips and Integrated Operating Instruction (101) 03-1-01-4, Scram Recovery.

• Removed and replaced failed expansion joint.

• Re-calibrated Turbine vacuum trip setpoint Long Term Corrective Actions - Condition Report GGN-2007-02743 was written to address any additional actions.

F. SAFETY ASSESSMENT

Nuclear Safety was not compromised during the event. The plant is designed for a loss of vacuum event. After MSIVs were closed, cold shutdown conditions were achieved using one Safety Relief Valve and the Residual Heat Removal (RHR) Shutdown Cooling system. Use of standby ECCS systems was not required to maintain Reactor inventory. The MSIVs remained open for significant length of time, allowing the Feedwater system to remain in service until Reactor pressure was lowered enough to provide makeup to the Reactor with Condensate Booster pumps. Additionally, Control Rod Drive System was available to provide makeup to the Reactor. MSIVs were then closed and Reactor pressure was then lowered with a Safety Relief Valve. During this event, even though not needed, there was sufficient high pressure makeup to the Reactor available with the Reactor Core Isolation System (RCIC) and High Pressure Core Spray System (HPCS). After Reactor pressure was lowered to below 135 psig, Shutdown Cooling was placed in service and Reactor taken to Cold Shutdown.

Radiological Safety was not affected due to failure of Condenser neck seal and subsequent scram. There was no radiological release during the event.

For above reasons, there was no challenge to Nuclear Safety.

G. ADDITIONAL INFORMATION

Previous Similar Events — Pursuant to 10CFR50.73(b)(5) there has been no similar events in the past two years at Grand Gulf Nuclear Station.