05000352/LER-2012-001
| Limerick Generating Station, Unit 1 | |
| Event date: | 02-20-2012 |
|---|---|
| Report date: | 04-19-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3522012001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 3 (Hot Shutdown) with reactor coolant temperature at approximately 331 degrees Fahrenheit and reactor pressure at approximately 90 psig. A reactor cooldown was in progress to support the beginning of refueling outage 1R14. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Monday, February 20, 2012, Limerick Unit 1 was performing a planned shutdown to enter refueling outage 1R14. The main steam isolation valve (MSIV) (EllS:ISV) cold shutdown valve test (ST-6-041-202-1) was in progress. The test records the closing times of the eight MSIVs. Technical Specification (TS) 3.4.7, Main Steam Isolation Valves, requires that two MSIVs per main steam line shall be operable with closing times greater than or equal to 3 and less than or equal to 5 seconds.
This event involved a condition prohibited by TS regarding the failure of a surveillance test on three MSIVs. The three valves' as-found hot stroke times exceeded the Technical Specification required closing time of less than or equal to 5 seconds. This event also involved a common-cause inoperability of independent trains. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B) and 10CFR50.73(a)(2)(vii).
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequences of this event were minimal. A technical evaluation (IR 1329463-02) the 3 MSIVs that failed to meet the TS requirement were bounded by the containment analysis for a steam line break. Three of 4 main steam lines had at least one operable MSIV. Both of the "B" main steam line MSIVs failed the surveillance test.
The "as-found" ST was performed with a main steam average temperature that exceeded 300 degrees Fahrenheit; therefore, the ST directs recording the stroke times on Attachment 1 which provides the "hot" test minimum and maximum closing stroke time limits. The MSIV "light to light" (LTL) and "switch to light" (STL) closing times are recorded in the test. The LTL closing time is verified to be greater than the ST minimum limit of 3.1 seconds. The STL closing time is verified to be less than the ST maximum limit of 4.8 seconds. No LTL stroke time data was obtained for one valve due to a stopwatch error. The missed stroke time was later determined to be satisfactory at 3.4 seconds using the valve position computer point. The times recorded in the surveillance test (ST) were as follows:
Inboard MSIVs�LTL�STL HV-041-1F022A no data 4.59 HV-041-1F022B�3.55�5.03 * HV-041-1F022C 3.35 4.51 HV-041-1F022D 3.64 � 4.70 Outboard MSIVs HV-041-1F028A 3.77 5.16 * HV-041-1F028B 3.36 5.11 * HV-041-1F028C 3.53 4.57 HV-041-1F028D 3.28 4.64 * = exceeded ST limit The "as-left" ST was performed on March 8, 2012, with a main steam average temperature that was less than 300 degrees Fahrenheit; therefore, the ST directed recording the stroke times on Attachment 2 which provides the "cold" test minimum and maximum closing time limits. The MSIV LTL and STL closing times were recorded in the test. The LTL closing times were verified to be greater than the ST minimum limit of 3.4 seconds. The STL closing times were verified to be less than the ST maximum limit of 5.0 seconds. Therefore, all 8 MSIV closing times were verified to be within the ST limits. The times recorded in the ST were as follows:
Inboard MSIVs LTL STL HV-041-1F022A 3.56 4.88 HV-041-1F022B 3.45 4.70 HV-041-1F022C 3.47 4.85 HV-041-1F022D 3.75 4.86 Outboard MSIVs HV-041-1F028A 3.51 4.70 HV-041-1F028B 3.46 4.97 HV-041-1F028C 3.47 4.77 HV-041-1F028D 3.46 4.94 The MSIV TS Bases states the following:
"Double isolation valves are provided on each of the main steam lines to minimize the potential leakage paths from the containment in case of a line break. Only one valve in each line is required to maintain the integrity of the containment; however, single failure considerations require that two valves be OPERABLE. The surveillance requirements are based on the operating history of this type valve. The maximum closure time has been selected to contain fission products and to ensure the core is not uncovered following line breaks. The minimum closure time is consistent with the assumptions in the safety analyses to prevent pressure surges.
UFSAR 15.6.4 Steam System Piping Break Outside Primary Containment describes the analysis of a main steam line break. The total integrated mass leaving the RPV through the steam line break is 108,785 lb; however, for the radiological consequence evaluation, a total mass of 140,000 lb is assumed. This analysis bounds the as-found MSIV closing times effect on the main steam line break accident radiological release analysis.
Cause of the Event
The apparent cause of this event was that valve stroke times were not optimized following test failures. A contributing cause was inherent inaccuracies in valve stroke timing practices. An additional contributing cause is the HV-041-1F028B MSIV pneumatic manifold is causing an extended delay time prior to valve motion. The extended delay time effect on the valve closing time was verified to be acceptable during the final valve timing ST since the valve closed in 4.97 seconds. In addition the inboard valve (HV-041-1F022B) in this steam line closed in 4.70 seconds which is well within the 5 second requirement.
Corrective Action Completed The MSIV cold shutdown valve test was performed successfully prior to restart.
Corrective Action Planned The MSIV cold shutdown valve test (ST-6-041-202-1) will be revised to require valve stroke time adjustment if the as-found stroke time exceeds pre-established test limits. This will ensure the as-left closing times provide adequate margin to the TS limits.
The HV-041-1F028B MSIV pneumatic manifold has been scheduled for replacement in the next Unit 1 refueling outage (1R15) in spring 2014.
MSIV stroke timing method improvements for timing accuracy will be investigated, benchmarked and incorporated as necessary.
Previous Similar Occurrences There were no previous similar occurrences of multiple MSIV slow closing time ST failures in the prior three years.
Component data:
System: Reactor Containment Component: HV-041-1F022B Main Steam Isolation Valve Manufacturer: A585 Weir Valves & Controls USA INC Model: 48223-706-7503