05000259/LER-2009-004
| Event date: | 07-24-2009 |
|---|---|
| Report date: | 09-27-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2592009004R01 - NRC Website | |
Prior to the event, Unit 1 was operating in Mode 1 at approximately 100 percent of rated thermal power (3458 megawatts thermal). Units 2 and 3 were also at 100 percent power (3458 Megawatts thermal) and unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On July 24, 2009, with Unit 1 in Mode 1, at approximately 100 percent of rated thermal power, reactor temperature at 529 degrees F, and reactor pressure at 1034 psi, Operations was conducting a surveillance on the HPCI [BJ], 1-SR-3.5.1.7, "HPCI Main and Booster Pump Set Development Head and Flow Rate Test at Rated Reactor Pressure." HPCI had been declared inoperable for the performance of the surveillance test. At approximately 1415 CDT, during the prerequisite steps of this surveillance, 1-PCV-073-0018C, the HPCI Turbine Stop Valve Mechanical Trip Hold Valve (Robert Shaw model VC-210), developed a tear in the diaphragm.
This tear caused oil from the HPCI control oil system to spill onto the floor at a rate visually observed to be approximately 0.25 - 0.5 gpm. The leak was a concern due to the fact that pressurization of the control oil system is the motive force required to open the HPCI stop valve. If oil pressure were to fall low enough, the stop valve would close and the HPCI turbine would decelerate until the valve reopens. In this event, the concern was that a severe oil leak could potentially make it impossible to pressurize the control oil system. If the oil system could not be pressurized, the HPCI Stop Valve could not be opened, which would render HPCI inoperable. On July 24, 2009, at approximately 2100 CDT, the torn diaphragm was removed and replaced with a new one. Systems Engineering was with Maintenance at the time of removal and examined the diaphragm. It was at this time that the tear was discovered. The failure was determined to be caused by a manufacturing defect. As such, personnel actions were not a contributor to the failure.
At 1750 CDT on July 24, 2009, BFN made notification 45227. Since there were concerns regarding the ability of HPCI to fulfill its safety function, BFN made the eight hour notification in accordance with 10 CFR 50.72(b)(3)(v)(B) and 10 CFR 50.72(b)(3)(v)(D).
On July 25, 2009, at approximately 0125 CDT, Operations declared HPCI operable after the successful completion of post maintenance testing for Work Order 09-719727-000.
The TVA is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(v)(B) as a condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat and 50.73(a)(2)(v)(D) mitigate the consequences of an accident; and 50.73(a)(2)(i)(B) as any operation or condition which was prohibited by the plant's Technical Specifications.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Ma or Occurrences:
July 24, 2009 1415 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.384075e-4 months <br /> CDT During the performance of 1-SR-3.5.1.7, valve 1-PCV-073-0018C developed an oil leak.
HPCI was inoperable at the time of discovery due to performance of the surveillance and continued to be inoperable due to the oil leak that developed.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
A 0.25 to 0.5 gallon per minute oil leak was visually observed during routine scheduled testing activities.
F. Operator Actions
None.
G. Safety System Responses
None.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of the event was a failed diaphragm in 1-PCV-073-0018C.
B. Root Cause
A material defect in the diaphragm of 1-PCV-073-0018C, HPCI Turbine Stop Valve Mechanical Trip Hold Valve, allowed the diaphragm to tear under normal system pressure and operating conditions after being installed for 2 years and 8 months.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
A material defect in the diaphragm of 1-PCV-073-0018C HPCI, the Turbine Stop Valve Mechanical Trip Hold Valve, allowed the diaphragm to tear under normal system pressure and operating conditions after being installed for 2 years and 8 months.
Once the leak developed on the 1-PCV-073-0018C, the surveillance test of HPCI (1-SR-3.5.1.7) was halted. As an immediate corrective action WO# 09-719727-000 was written, planned, and worked within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. This WO directly corrected the apparent cause by removing the torn diaphragm and replacing it with a new diaphragm. Systems Engineering retrieved the torn diaphragm from Maintenance immediately after it was removed and had it sent to TVA Central Labs for evaluation. The result of the TVA Central Lab evaluation is that no fibers were present inside of the Buna-N material in the region of the tear.
These fibers are required for the diaphragm to perform its intended function. The diaphragm is a two ply, fabric reinforced Buna-N diaphragm. The lack of fibers discovered by the central lab is a material defect.
The completion of the work order that replaced the diaphragm allowed the Unit 1 HPCI system to be returned to operable status and allowed satisfactory completion of the surveillance.
BFN will verify that diaphragms from the same lot as the failed diaphragm are not currently installed on Units 1, 2, or 3, PCV-073-0018C, HPCI Turbine Stop Valve Mechanical Trip Hold Valve.
A Preventative Maintenance request currently exists to replace this diaphragm once every 6 years due to known industry failure rates. Researching the most recent replacement of the diaphragm on Unit 1 prior to this event shows that the torn diaphragm had been installed by WO# 06-712568-000 on 12/21/2006.
The research also shows that WO# 04-710016-000 installed a new diaphragm on Unit 2 on 04/16/2005 and WO# 03-010776-000 installed a new diaphragm on Unit 3 on 03/22/2004. The diaphragm on BFN, Unit 3, was replaced by WO# 111148388 on 09/08/2010, and the BFN, Unit 2, diaphragm was replaced by WO# 111148387 on 09/16/2010. I Unknown at the time of this event, RHR Loop II was inoperable due to the RHR Loop II Low Pressure Injection (LPCI) flow control valve 1-FCV-074-0066 being inoperable (LER 50-259/2010-003-02) and the Loop I RHR 1C pump was also inoperable (LER 50-259/2010-004-01) due to a rotor/shaft bow in the RHR pump 1C motor. The period of concurrent inoperability existed from approximately November, 2008 until November, 2010.
BFN Unit 1 TS 3.5.1, ECCS — Operating, requires, in part, that each ECCS injection/spray subsystem and the Automatic Depressurization System (ADS) function of six safety/relief valves be OPERABLE in Modes 1, 2, and 3. With two or more low pressure ECCS injection/spray subsystems inoperable, TS 3.5.1 Condition H requires entry into TS LCO 3.0.3.
Because the condition of RHR Loop II LPCI and Loop I RHR pump 1C were unknown at the time, Unit 1 was not placed into LCO 3.0.3 as required by TS 3.5.1 Required Action H.1.
10 CFR 50.73(a)(2)(i)(B) requires the reporting of:
"Any operation or condition which was prohibited by the plant's Technical Specifications except when:
(1) The Technical Specification is administrative in nature; (2) The event consisted solely of a case of a late surveillance test where the oversight was corrected, the test was performed, and the equipment was found to be capable of performing its specified safety functions; or (3) The Technical Specification was revised prior to discovery of the event such that the operation or condition was no longer prohibited at the time of discovery of the event.
As none of the three exceptions to 10 CFR 50.73(a)(2)(i)(B) apply in this case, the conditions are reportable.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The Emergency Core Cooling Systems (ECCS) are designed, in conjunction with the primary and secondary containment, to limit the release of radioactive materials to the environment following a loss of coolant accident (LOCA). The ECCS uses two independent methods (flooding and spraying) to cool the core during a LOCA. The ECCS network consists of the HPCI System, the Core Spray (CS) System, the Low Pressure Coolant Injection (LPCI) mode of the Residual Heat Removal (RHR) System, and the Automatic Depressurization System (ADS).
A Probabilistic Risk Assessment (PRA) was performed for the past concurrent inoperability of the RHR Loops I and II for the time period that HPCI was inoperable considering the following: 1 Risk significant systems (or portions of risk significant systems) unavailable for extended periods of time reduces the margin of safety in the plant. The PRA calculated the Incremental Core Damage Probability Deficit (ICDPD) and Incremental Large Early Release Probability Deficit (ILERPD). The ICDPD was determined to be 4.02E-8 and the ILERPD was determined to be 3.96E-9.
Based on the PRA, this event posed minimal reduction to public health and safety.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
The diaphragm on 1-PCV-0073-0018C was replaced.
B. Corrective Actions to Prevent Recurrence
The corrective actions are being managed by BFN's corrective action program.
1. BFN assessed spare diaphragms on site in order to determine if they were from same lot as the diaphragm that failed. It was determined that one spare suspect diaphragm remained in inventory. The suspect diaphragm was quarantined and a replacement was ordered.
2. BFN verified that diaphragms from the same lot as the failed diaphragm are not I currently installed on BFN, Units 1, 2, or 3, PCV-073-0018C, HPCI Turbine Stop Valve Mechanical Trip Hold Valve.
VII. ADDITIONAL INFORMATION
A. Failed Components
HPCI Turbine Stop Valve Mechanical Trip Hold Valve, 1-PCV-073-0018C (Robert Shaw Model VC-210). I
B. Previous Similar Events
None.
C. Additional Information
Corrective action document for this report is PER 177206.
D. Safety System Functional Failure Consideration:
This event does involve a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideration:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.