05000353/LER-2011-004
| Limerick Generating Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46903 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3532011004R00 - NRC Website | |
CONTINUATION SHEET
Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 75% power. Main turbine control valve CV-3 was closed due to in-progress corrective maintenance and the "Al" channel of the reactor protection system (RPS) was in the tripped condition due to the valve maintenance.
Description of the Event
On Sunday, May 29, 2011, Limerick Unit 2 was operating at 75% power.
The "Al" channel of the reactor protection system (RPS) (EIIS:JC) was in the tripped condition due to in-progress corrective maintenance on main turbine control valve CV-3 (EIIS:V). At 0502 hours0.00581 days <br />0.139 hours <br />8.300265e-4 weeks <br />1.91011e-4 months <br />, during restoration of CV-3, the electrohydraulic control (EHC) (EIIS:TG) relayed emergency trip supply (RETS) valve (031-2013) to CV-3 was opened. A pressure drop in EHC RETS oil pressure was sensed at adjacent main turbine control valve CV-1 which actuated the "Bl" channel of the reactor protection system. A full scram resulted and all control rods inserted. All safety significant systems functioned as expected. The 2A and 2B reactor recirculation pumps (EIIS:AD) tripped as designed.
The operating crew stabilized the plant using the appropriate Transient Response and Operating Procedures. The lowest transient reactor level observed on the plant monitoring system (PMS) was plus 4 inches. The digital feedwater level control system (DFWLC) responded as designed. There was no emergency core cooling system (ECCS) actuation. The only automatic isolation actuated was the expected plus 12.5 inch isolation signal for Group IIB. All main turbine supervisory functions and main generator protective relaying functioned as designed.
An investigation determined that an air void in the EHC RETS oil line caused a decrease in EHC RETS oil pressure at the CV-1 RETS pressure switch when the EHC supply valve was throttled open during restoration of CV-3.
A 4-hour NRC ENS notification was required by 10CFR50.72(b)(2)(iv)(B) for an actuation of RPS when the reactor was critical. An 8-hour NRC ENS notification was required by 10CFR50.72(b)(3)(iv)(A) for a valid actuation of RPS. The ENS notification (#46903) was completed on Sunday, May 29, 2011 at 07:10 ET. This event involved an automatic actuation of RPS. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(iv)(A).
JRC FORM 3613A (10-2010) LICENSEE EVENT REPORT (LER) U.S. NUCLEAR REGULATORY COMMISSION
CONTINUATION SHEET
2. DOCKET
Analysis of the Event
There were no actual safety consequences associated with this event.
The potential safety consequences of this event were minimal. The plant equipment performed as designed during the transient. The operators effectively stabilized reactor parameters.
On May 28, 2011, CV-3 failed to re-open during the quarterly main turbine control valve exercise and RPS functional testing (ST-6-001- 765-2). Initial troubleshooting identified flow noise indicating that the disc dump valve was not seated. The fast acting solenoid and shutoff valve associated with CV-3 were identified as potential causes for the valve failure. Based on operating experience, a plan was developed to replace the fast acting solenoid and shutoff valve and restore them to service including a controlled restoration of fluid pressure and flushing to remove air entrapped during component replacement. During the maintenance the "Al" RPS channel was placed in the tripped condition to satisfy the TS 3.3.1 RPS instrumentation action for the inoperable turbine control valve fast closure channel.
The sequence used to restore EHC pressure involved opening the EHC oil supply ball valve 031-2013. The valve was opened slowly by sequentially opening the valve to approximately 10% open and allowing time for pressure to equalize. During the third evolution to slowly open valve 031-2013 the full scram signal occurred when the EHC oil pressure switch associated with CV-1 actuated. A postulated void in the EHC BETS oil line was pressurized when valve 031-2013 was reopened resulting in a pressure transient within the EHC BETS oil line.
The EHC oil supply valve that was being opened at the time of the event is located on the BETS line to CV-3. This header supplies EHC oil to the turbine control valves and intercept valves and seats the disc dump valves of the control valves after passing through the fast acting solenoid valve.
Cause of the Event
The root cause of the event was a void in the electrohydraulic control oil supply line that resulted in a perturbation of the oil supply pressure at the adjacent control valve.
Corrective Action Completed Main turbine control valve CV-3 was repaired and restored to service.
Corrective Action Planned A procedure will be developed and implemented to provide specific restoration steps for turbine control, stop, and combined intermediate valves.
Previous Similar Occurrences There was no similar event in the last 3 years.
Component data:
Equipment:
Main. Turbine Control Valve Component Number: CV-001-3 Manufacturer: G080 General Electric Company Model Number: 823E838 DWG Equipment: Main Turbine Control Valve Operator Component Number: CV-001-3-0P Manufacturer: G080 General Electric Company Model Number: 944D549 DWG Equipment: CV-001-3 RETS EHC Isolation Valve Component Number: 031-2013 Manufacturer: W165 Whitey Company Model Number: SS-65ESW16T Equipment: CV-001-1 RETS Pressure Switch Component Number:
PS-001-202C Manufacturer:
B070 Barksdale Control Division / Delaval Inc Model Number:
TC9622-3-V