05000278/LER-2011-003, For Peach Bottom Unit 3, Regarding Automatic Depressurization System Safety Relief Valve Actuator Diaphragm Thread Seal Leak
| ML11325A383 | |
| Person / Time | |
|---|---|
| Site: | Peach Bottom  |
| Issue date: | 11/18/2011 |
| From: | Stathes G Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 11-003-00 | |
| Download: ML11325A383 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2782011003R00 - NRC Website | |
text
Exelon.
Exelon Nuclear www.exeloncorp.com Nuclear Peach Bottom Atomic Power Station 1848 Lay Rd.
Delta, PA 17314 10CFR 50.73 November 18, 2011 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Peach Bottom Atomic Power Station (PBAPS) Unit 3 Renewed Facility Operating License No. DPR-56 NRC Docket No. 50-278
Subject:
Licensee Event Report (LER) 3-11-03 Enclosed is a Licensee Event Report concerning a condition prohibited by Technical Specifications involving pneumatic leakage through the actuator diaphragm thread seal of the Automatic Depressurization System 'B' relief valve. In accordance with NEI 99-04, the regulatory commitment contained in this correspondence is to. restore compliance with the regulations.
The specific methods that are planned to restore and maintain compliance are discussed in the LER. If you have any questions or require additional information, please do not hesitate to contact us.
Sincerely, Garey L. Stathes Plant Manager Peach Bottom Atomic Power Station GLS/djf/IR 1268076 Attachment cc:
US NRC, Administrator, Region I US NRC, Senior Resident Inspector R. R. Janati, Commonwealth of Pennsylvania S. Grey, State of Maryland P. Steinhauer, PSE&G, Financial Controls and Co-owner Affairs INPO Records Center CCN: 11-94
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2013 (10-2010)
, the NRC may sfor each block) not conduct or sponsor, and a person is not required to respond to, the digits/characters
)information collection.
- 3. PAGE Peach Bottom Atomic Power Station (PBAPS) Unit 3 05000278 1 OF 4
- 4. TITLE Automatic Depressurization System Safety Relief Valve Actuator Diaphragm Thread Seal Leak
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR 05000 NNNTHDAY YEAR YEARNUMBER NO._I FACILITY NAME DOCKET NUMBER 09 25 2011 11 003 00 11 18 2011 05000
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) o 20.2201(b)
El 20.2203(a)(3)(i)
El 50.73(a)(2)(i)(C)
El 50.73(a)(2)(vii) 5 [1 20.2201(d)
El 20.2203(a)(3)(ii)
[I 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4) 0l 50.73(a)(2)(ii)(B)
El 50.73(a)(2)(viii)(B)
[I 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
[I 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL [I 20.2203(a)(2)(ii)
[I 50.36(c)(1)(ii)(A)
[I 50.73(a)(2)(iv)(A) 0l 50.73(a)(2)(x)
[E 20.2203(a)(2)(iii)
F-50.36(c)(2)
[I 50.73(a)(2)(v)(A)
[1 73.71 (a)(4) 0%
[1 20.2203(a)(2)(iv)
[I 50.46(a)(3)(ii)
[I 50.73(a)(2)(v)(B)
[1 73.71(a)(5)
[E 20.2203(a)(2)(v)
[I 50.73(a)(2)(i)(A)
[I 50.73(a)(2)(v)(C)
El OTHER [I 20.2203(a)(2)(vi) 0 50.73(a)(2)(i)(B)
[I 50.73(a)(2)(v)(D)
Specify in Abstract below or in Analysis of the Event, continued There were no actual safety consequences associated with this event. There were no design events that required the 71 B ADS Relief Valve to actuate during Cycle 18 operations.
There exists a total of 13 SRVs / Safety Valves (SVs) installed on the four Main Steam (EIIS:
SB) Lines. The 11 installed SRVs exhaust steam through discharge lines to a point below the minimum water level in the Suppression Pool. The two installed SVs discharge steam directly to the Drywell. The SRVs and SVs are located on the four Main Steam Lines within Primary Containment (i.e., Drywell). The SRVs are 'three-stage' valves consisting of a main valve disc and piston (third stage) operated by a second stage disc and piston displaced by either a first stage pressure-sensing pilot (for overpressure protection) or a pneumatically-operated mechanical push rod (for the ADS function or for remote-manual operation).
The pneumatic leak on the 71 B SRV would have had no impact on the ability of the SRVs / SVs to lift on an actual overpressure condition. The ADS system consists of 5 of the 11 SRVs (71 A, 71 B, 71 C, 71 G and 71 K). The ADS system is designed to provide depressurization of the reactor coolant system during a small break loss-of-coolant accident if the High Pressure Coolant Injection (HPCI) system is not able to maintain the required water level in the Reactor Pressure Vessel (RPV).
ADS SRV operation reduces the RPV pressure to within the operating pressure range of the low-pressure Emergency Core Cooling System (ECCS) subsystems so that the ECCS low-pressure subsystems can provide coolant inventory makeup.
In addition to the normal instrument nitrogen supply to each of the SRVs, the ADS SRVs are equipped with a nitrogen accumulator and isolation check valves from the non-safety related instrument nitrogen supply. In the event that normal instrument nitrogen pneumatic supply is lost during a design basis event, the accumulator provides the pneumatic power to actuate the valves to mitigate design basis events. Each ADS valve is provided with a short-term, safety grade, pneumatic supply by means of its associated accumulator to provide sufficient capacity to cycle the valve open five times at atmosphere pressure, twice at 70% of containment design pressure, or once at containment design pressure, all within a 4-hour time period. This capability is assured by maintaining a leak rate limit on the pneumatic system.
In order to perform the ADS system safety function, four of the five ADS SRVs are required to function.
In the unlikely event of a design basis event (including no credit for the normal nitrogen gas supply), the 71 B ADS SRV would not have been able to stroke multiple times as required by the licensing basis.
However, the 71A, 71C, 71G and 71K ADS SRVs were operable. Therefore, these other four ADS SRVs were capable of de-pressurizing the RPV for design basis events and the ADS safety function was maintained.
Based on the above, there was no significant nuclear safety concerns involved with this event.
Cause of the Event
The cause of the 71 B ADS SRV inoperability was attributed to a failure of the associated actuator diaphragm thread seal (EIIS: SEAL). When energized, the solenoid valve switches ports resulting in pneumatic pressure being applied to the SRV diaphragm. This results in the opening of the ADS SRV. The diaphragm thread seal leakage only occurred when the 71 B SRV was actuated and therefore, the leakage would not have been detectable during normal plant operations. When inspected by maintenance personnel, the thread seal had indications of being dry and brittle.
Subsequent review by Engineering personnel determined that the apparent cause was thermal degradation of the thread seal material.
A vendor technical evaluation report was issued in March 2011 and provides recommendations to upgrade the seal with a design that is more resistant to heat related failures.
The SRVs are manufactured by Target Rock Co. and are 'three-stage' relief valves. The thread seals are Target Rock part number 715-0004 containing Buna-N material.
Corrective Actions
The 71 B actuator diaphragm thread seal was replaced on 9/26/11.
As-left leak testing was performed and the valve was restored to an operable condition to support startup from the P3R18 Refueling Outage.
Based on March 2011 vendor technical evaluation report, upgrades to the diaphragm thread seal for ADS SRVs on Units 2 and 3 are planned.
Previous Similar Occurrences There were no previous similar LERs identified involving an ADS SRV inoperability due to failure of the actuator diaphragm thread seal.
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