05000269/LER-2011-007, Regarding Inoperable Containment Isolation Valve
| ML11272A036 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 09/26/2011 |
| From: | Gillespie T Duke Energy Corp |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 11-007-00 | |
| Download: ML11272A036 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2692011007R00 - NRC Website | |
text
Duke T.
PRESTON GILLESPIE, JR.
Vice President
- SEnergy, Oconee Nuclear Station Duke Energy ON01 VP / 7800 Rochester Hwy.
Seneca, SC 29672 864-873-4478 864-873-4208 fax T. Gillespie@duke-energy. corn September 26, 2011 U.S. Nuclear Regulatory Commission Document Control Desk Washington, DC 20555
Subject:
Oconee Nuclear Station Docket No. 50-269 Licensee Event Report 269/2011-07, Revision 0 Problem Investigation Program Nos.: 0-11-0218 and 0-11-8854 Gentlemen:
Pursuant to 10 CFR 50.73 Sections (a)(1) and (d), attached is Licensee Event Report 269/2011-07, Revision 0, regarding Oconee Unit I operating in a condition prohibited by Technical Specification (TS) from January 14, 2011 until April 2, 2011. The prohibited condition was an inoperable containment isolation valve for a period of time which exceeded the completion time allowed by TS 3.6.3, "Containment Isolation Valves",
Required Action A.1. Also, Oconee Nuclear Station violated TS 3.0.4 when exiting the forced outage. Unit 1 changed modes on January 14, 2011 with an inoperable containment isolation valve.
This report is being submitted in accordance with 10 CFR 50.73 (a)(2)(i)(B). Cause analysis for this event is not yet complete. The root cause evaluation results and corrective actions will be provided in a supplement to this report.
There are no regulatory commitments contained in this report. This event is considered to be of no consequence with respect to the health and safety of the public.
Any questions regarding the content of this report should be directed to Oconee Regulatory Compliance, Sandra N. Severance, at 864-873-3466.
Sincerely,
- - F
--. &I,k T. reston Gillespie, Jr.
Vice President Oconee Nuclear Station Attachment www. duke-energy, com
Document Control Desk September 26, 2011 Page 2 cc:
Mr. Victor McCree Administrator, Region II U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Mr. John Stang Project Manager U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Mail Stop 8 G9A Washington, DC 20555 Mr. Andrew Sabisch NRC Senior Resident Inspector Oconee Nuclear Station INPO (Word File via E-mail)
Abstract
On January 8, 2011, Oconee Unit 1 containment isolation valve, 1 HP-5, was declared inoperable, and Unit 1 initiated a shutdown. Investigation identified galling between the gland ring and valve body of 1 HP-5. Corrective actions included replacement of the valve gland ring with one made from material that is less susceptible to galling and increasing the clearance between the gland ring and valve body. After successfully completing post-maintenance valve stroke testing, Unit 1 was returned to service, entering Mode 4, the Mode of Applicability for TS 3.6.3, on January 14, 2011.
However, continuing investigation identified that a valve seat material change in 2003 resulted in a reduction in actuator margin such that valve 1 HP-5 could not be assured of closing. On June 2, 2011, inadequate actuator sizing was confirmed via testing. Therefore, 1 HP-5 was inoperable, and in a condition prohibited by Technical Specifications, from January 14, 2011, until Unit 1 entered Mode 5 for the planned refueling outage, April 2, 2011. Additionally, Oconee Unit 1 violated TS 3.0.4 because the Unit 1 operators unknowingly changed modes with the containment isolation valve inoperable.
This event is associated with the failure of 1 HP-5 previously reported under LER 269/2011-02 and is similar to the failures of 2HP-5 and 3 HP-5 reported under LER 270/2011-01. This event is considered to have no consequence with respect to the health and safety of the public.
NRC FORM 366 (10-2010)
BACKGROUND 1HP-5 is a containment isolation valve [EIIS:ISV] in the Oconee Unit 1 Letdown System [EIIS:CB].
This air operated valve (AOV) has an instrument air operated piston actuator and goes to the closed position on loss of air. The valve is normally open when High Pressure Injection (HPI)
[EIIS:CB] is in service to allow letdown flow from the Reactor Coolant System (RCS)[EIIS:AB]. The valve serves as the outside containment isolation valve and is automatically closed by an Engineered Safeguards (ES)[EIIS:JM] signal. ES channel 2 automatically de-energizes a solenoid valve to bleed off air, allowing HP-5 to close by spring force.
For Technical Specification (TS) Operability, 1 HP-5 is credited to close during Large Break LOCA, Small Break LOCA, and Rod Ejection Accident events. The inability of 1 HP-5 to fully close in all design basis accident scenarios from January 14, 2011, to April 2, 2011, constitutes an inoperable containment isolation valve for a period of time which exceeded the completion time allowed by TS 3.6.3, "Containment Isolation Valves", Required Action A.1. Therefore, this issue is reportable per 10 CFR 50.73(a)(2)(i)(B), operation prohibited by Technical Specifications. Additionally, because 1 HP-5 remained inoperable following the forced shutdown, TS LCO 3.0.4 was violated since Unit 1 was unintentionally returned to service with 1 HP-5 still inoperable.
EVENT DESCRIPTION
On January 8, 2011, during performance of ES logic testing for Unit 1 ES digital channel 2, the wiring jumper, intended to prevent travel of letdown line containment isolation valve 1 HP-5, became dislodged and the signal to close the valve became active. Upon investigation of the unintended valve closure, 1 HP-5 was found to be approximately 25 percent open. When the jumper was reinstalled, 1 HP-5 returned to the fully open position. However, because the valve did not fully close, it was declared inoperable.
TS 3.6.3 Required Action A.1 entry conditions were met at 0148 hours0.00171 days <br />0.0411 hours <br />2.44709e-4 weeks <br />5.6314e-5 months <br /> on January 8, 2011, for an inoperable containment isolation valve. LER 269/2011-02 (ML110690973) documents this event.
Investigation identified galling between the gland ring and valve body of 1HP-5. Corrective actions included replacement of the valve gland ring with one made from material that is less susceptible to galling and increasing the clearance between the gland ring and valve body. After successfully completing post-maintenance valve stroke testing, Unit 1 was returned to service, entering Mode 4 on January 14, 2011.
As the root cause investigation continued, it was determined that valve seat material changes from the original seat material of ethylene propylene diene monomer (EPDM) to ARLON 1260, in 2003, represented an increase in seat coefficient of friction (COF) of 2.5 times the original seat COF used in the sizing calculation. In March 2011, Oconee personnel identified that there was very little closing margin for this valve. Oconee personnel, working with a vendor, identified that the sizing analyses used in the 2003 time frame under predicted required valve torque. On June 2, 2011, the vendor calculation revealed that the actuator margin for 1 HP-5 was negative; thus, containment isolation valve 1 HP-5 would not have been able to perform its required function from the time of the seat material replacement in 2003 until the issue was corrected in June 2011.
This event was initially reported under LER 269/2011-02. What was not recognized at this time of the initial report was that, although post-maintenance valve stroke testing was successfully completed, Oconee Unit 1 was returned to service on January 14, 2011, with 1 HP-5 still being unable to perform its design function in all required design basis event scenarios. The corrective actions to replace the valve gland ring with one made from material that is less susceptible to galling and increasing the clearance between the gland ring and valve body were not sufficient to ensure proper valve performance. When the low margin concern was first identified, actions were initiated to replace the valve spring with a stronger spring to restore margin. When the negative actuator margin for 1 HP-5 was identified, replacing the spring was now a required action to restore the valve to an operable condition.
The event date for this LER is documented as January 14, 2011. It was on this date that Oconee Unit 1 entered Mode 4 from the forced outage, unknowingly, with an inoperable containment isolation valve. Although the apparent cause of the inoperability had been addressed, the lack of actuator margin issues had not yet been identified or addressed. On April 2, 2011, Oconee Unit 1 entered Mode 5 for a scheduled refueling outage. It was not until June 2, 2011, that the continued valve inoperability was identified. Focus was then applied to extent of condition determinations (LER 270/2011-01 (ML11215A196) for 2HP-5 and 3HP-5), restoring 1 HP-5 to an operable status, and completing the cause investigation. It was not until July 26, 2011, that Oconee personnel recognized that 1 HP-5 inoperability for the period from January 14, 2011 to April 2, 2011 should be reported separately since this was an additional violation of the requirements of TS 3.6.3.
Summary Sequence of Events January 8, 2011 - 1 HP-5 failed to close. Oconee Unit 1 shut down for inoperable containment isolation valve. Reported under LER 269/2011-02.
January 14, 2011 - Oconee Unit 1 entered Mode 4, returning to operation from the forced outage caused by 1HP-5 inoperability. (Note: It is now known that 1HP-5 remained inoperable.)
" April 2, 2011 - Oconee Unit 1 entered a scheduled refueling outage.
June 1, 2011 - 1 HP-5 modification to install a stronger actuator spring during the refueling outage complete.
June 2, 2011 - vendor calculation revealed valve actuator margin issues which resulted in declaration of Units 2HP-5 and 3HP-5 inoperability. Reported under LER 270/2011-01.
July 26, 2011 - Oconee personnel identified the need for an additional LER for the period between January 14, 2011 and April 2, 2011, following exhaustive physical testing.
LER 269/2011-07 (this LER) initiated.
This event is considered to have no consequence with respect to the health and safety of the public.
CAUSAL FACTORS The technical cause for the inoperability of 1 HP-5 has been determined and addressed in LER 269/2011-02. However, this event report results from a human performance error resulting from an
inadequate root cause during initial failure investigative efforts. Because the true root cause of the failure of 1 HP-5 had not been revealed during the initial cause evaluation, the inoperable valve was inappropriately returned to service. The human performance root cause into this event is not yet complete. If necessary, the final results of the root cause of the initial incorrect determination of the root cause of the valve failure and any additional corrective actions will be provided in a supplement to this report.
Root Cause:
- 1. The root cause of 1 HP-5 is insufficient actuator margin following a valve seat material change due to an inadequate modification design change implemented in October 2003.
Basis:
The root cause has been identified as improper level of analysis and testing following a valve seat material change. Seat material in the DMV-971 valves was changed due to system requirements. The new seat material resulted in a higher seat coefficient of friction increasing the required closing forces. Current analysis performed confirmed that the actuator had negative closing margin using the newer valve seat material. The modification process and post-modification testing failed to identify the reduction in closing torque margin when the valve seat material was changed. Lack of engineering rigor during the equivalency change that installed the ARLON 1260 seat material failed to identify the sizing calculation as requiring update due to the seat material change.
- 2. Investigation into the initial incorrect determination of root cause of the valve failure continues. Factors contributing to the difficulty in determining the technical cause of the failure include informal design change documentation from many years previous. Without the design change documentation being retrievable from the expected locations, the adverse impact of the valve seat material change was not identified until later. This lack of information was exacerbated by the incorrect vendor calculation that initially identified low or no margin versus the eventual determination of negative margin. These factors coupled with apparent physical degradation to the valve and material incompatibilities in valve material allowed the cause investigation team to be misled.
CORRECTIVE ACTIONS
Immediate:
No additional actions were required to restore 1 HP-5 to operability. When the negative valve actuator margin was determined, Oconee Unit 1 was in a refueling outage and not in a Mode of Applicability for TS 3.6.3. Prior to returning Oconee Unit 1 to service, an Engineering Change replaced the Bettis SR60 actuator spring with a stronger SR1 00 actuator spring on 1 HP-5 to restore margin.
Planned:
Cause analysis for the human performance aspects of this issue is not yet complete. The root cause evaluation results and corrective actions associated with the return to service of an inoperable containment isolation valve will be provided in a supplement to this report.
SAFETY ANALYSIS
With respect to the inoperability of the valve, 1 HP-5 is normally open during unit power operation to allow letdown flow from the Reactor Coolant System. The valve serves as the outside containment isolation valve for penetration number 6 and is automatically closed by an engineered safeguards (ES) signal. ES channel 2 automatically deenergizes a solenoid valve to close 1 HP-5. The valve has an instrument air operated piston actuator that goes to the closed position on loss of air or if the solenoid valve loses power. 1HP-5 also receives a close signal on high letdown temperature to terminate letdown flow; however, this function is provided to prevent damage to the Purification Demineralizers (equipment protection) rather than for nuclear safety.
The risk impact of 1HP-5 failing to close on demand was evaluated using the Oconee PRA model and determined to have a conditional core damage probability of less than 1 E-06 and a conditional large early release probability of less than 1E-07. Consequently, the inoperability of HP-5 on Oconee Unit 1 did not have a significant risk impact.
ADDITIONAL INFORMATION
This event is directly related to the failure of 1 HP-5 reported initially under LER 269/2011-02. That LER identified the technical cause of the valve failure whereas this event report is associated with an additional period of inoperability resulting from an incorrect cause determination. This event is also related to the failure of 2HP-5 and 3HP-5 reported under LER 270/2011-01. It was determined through the root cause extent of condition review that the condition described in this LER did not adversely affect any other similar valve and actuator combinations. Also, a search of Oconee's Corrective Action Program data base found no events similar to the modification quality issue during the previous five years of operation.
Energy Industry Identification System (EIIS) codes are identified in the test as [EIIS:XX]. The initial failure of 1 HP-5 was considered reportable under the Equipment Performance and Information Exchange (EPIX) program; however, this subsequent event is not an additional equipment concern and does not warrant reporting to EPIX. There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.