05000353/LER-2009-002
| Limerick Generating Station, Unit 2 | |
| Event date: | 05-24-2008 |
|---|---|
| Report date: | 07-07-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3532009002R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On February 15, 2008 all Unit 2 main turbine bypass valves (BPVs) (EIIS:SO) opened momentarily during stop valve testing. The event investigation determined that the potential cause was an unexpected change in the load limit circuit output. Based on the event it was determined that this could be prevented by lifting a lead to defeat this logic.
On April 26, 2009, all Unit 2 main turbine bypass valves (BPVs) opened momentarily. The event investigation determined that the cause was an unexpected change in the load limit circuit output.
On May 8, 2009 upon further investigation the troubleshooting team discovered that actions taken to lift the lead after the February 15, 2008 event, adversely affected the main turbine bypass system response time. The action of lifting the lead rendered the turbine bypass system inoperable.
Technical Specification (TS) 3.7.8 action statement refers to TS 3.2.3 Minimum Critical Power Ratio (MCPR), which requires the main turbine bypass system to be operable. TS 3.2.3 Action "c" requires a determination, within one hour, that MCPR is greater than or equal to the rated MCPR limit specified by the Core Operating Limits Report (COLR) BPVs inoperable curve.
A review of main turbine valve testing activities for the last three years identified three occasions (5/24/08, 9/13/08 and 12/14/08) on Unit 2 when the stop valve load limit logic was intentionally disabled by lifting a lead for periods that exceeded one hour and the BPVs were not declared inoperable. Therefore TS 3.2.3 action "c" was not completed within the one hour allowed outage time (AOT).
This event resulted in a condition prohibited by TS.
Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. MCPR was verified to be greater than the operating limit adjusted for turbine bypass valves out of service for all three events. The system would have responded appropriately for the pressure control function, but the anticipatory function for stop valve closure was not available during the brief times when the lead was lifted during testing.
Cause of the Event
The condition prohibited by TS was caused by inadequate site documentation for a critical design basis function regarding an adverse effect on the main turbine bypass system response time when a lead was lifted to disable the stop valve load limit logic during testing.
Corrective Action Planned Design Basis Document LS-S-45 "Main Steam, Turbine, and Extraction Steam Systems" will be revised to include the design basis function of the stop valve load limit logic and its impact on main turbine bypass system response time.
Previous Similar Occurrences Three events are being reported in this LER. There are no additional previous occurrences of unplanned main turbine bypass system inoperability that resulted in a condition prohibited by TS in the last five years.
Component Data:
Equipment: Turbine Bypass Valves Component Number: BPV-001-1-9 Manufacturer: G080 General Electric Model Number: 823E891