05000390/LER-2015-001, Regarding Manual Reactor Trip Initiated Due to Rapid Loss of Main Condenser Vacuum
| ML15112A724 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  (NPF-090) |
| Issue date: | 04/22/2015 |
| From: | Walsh K Tennessee Valley Authority |
| To: | Document Control Desk, NRC/RGN-II |
| References | |
| LER 15-001-00 | |
| Download: ML15112A724 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| 3902015001R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 April 22,2015 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001
Subject:
10 cFR 50.73 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No.50-390 Licensee Event Report 390/2015-001, Manual Reactor Trip Due to Rapid Loss of Main Condenser Vacuum This submittal provides Licensee Event Report (LER) 390/2015-001. This LER provides details concerning a manual reactor trip due to rapid loss of main condenser vacuum at Watts Bar Nuclear Plant, Unit 1. This report is being submitted in accordance with 10 CFR s0.73(aX2)(ivXA).
There are no regulatory commitments in this letter. Please direct any questions concerning this matter to Gordon Arent, WBN Licensing Director, at (423) 365-2004.
Kevin T. Walsh Site Vice President Watts Bar Nuclear Plant
U.S= Nuclear Regulatory Commission Page 2 April 22,2015 Enclosure cc (Enclosure):
NRC Regional Administrator - Region ll NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Watts Bar Nuclear Plant
IRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION 02-2014)
LTGENSEE EVENT REPORT (LER)
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- 1. FACILIW NAME Watts Bar Nuclear Plant, Unit 1
- 2. DOCKET NUMBER 05000390
- 3. PAGE 1
OF 5
{. TITLE Manual Reactor Trip lnitiated Due to Rapid Loss of Main Condenser Vacuum
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FAGILITIES INVOLVED MONTHI OAV I veRn YEAR l t-*t'r=JJS' I X,="'
MONTH I DAY YEAR FACILITY NAME N/A DOCKET NUMBER N/A 02 I 21 12015 2015 - 001 - 00 04 22 2015 FACILITY NAME N/A DOCKET NUMBER N/A
- 9. OPERATING MODE t 1. THIS REPORT tS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR S: (Check all that apply) 1 tr zo.22o1(b) tr 2o.z2o3(aX3Xi) tr 50.73(ax2)(ixc) n 50.73(aX2Xvii) tr 20.2201(d) tr 2o.2zo3(aX3xii) tr s0.73(aX2XiiXA) tr 50.73(aX2)(viii)(A) tr 2o.22os(aX1) n 20.2203(aX4) tr 50.73(ax2xii)(B) n 50.73(a)(2XviitXB) tr zo.22o3(aX2)(i) tl 50.36(cxl xiXA) tr 50.73(ax2xiii) tr 50.73(aX2XixXA)
O. POWER LEVEL 100o/o tr 2o.22os(aX2Xii) tr 50.36(cxl XiiXA)
X 50.73(aX2XivXA) tr s0.73(aX2)(x) tr 20.2203(aX2)(iii) tr s0.36(c)(2) tl 50.73(aX2XvXA) tr rc.r1(aX4) n 2o.z2o3(aX2Xiv) tr s0.46(ax3)(ii) tr s0.73(aX2XvXB) tl rc.r1(a)(s) tr 2o.2zo3(aX2Xv) tr s0.73(aX2XiXA) n s0.73(aX2XvXc) tr orHER tr 2o.z2o3(aX2Xvi) tr 50.73(aX2XiXB) tr 50.73(aX2Xv)(D)
Specify in Abstract below or in
I
PLANT OPERATING CONDITIONS BEFORE THE EVENT
Watts Bar Nuclear Plant (WBN) Unit 1 was in Mode 1 at 100 percent raied thermal power (RTP). There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
DESCRIPTION OF EVENT
C.
Date Event On February 21,2015 at approximately 10:31 am Eastern Standard Time (EST), Watts Bar Nuclear Plant Unit 1 reactor [EllS: AC] was operating at normaloperating temperature and pressure when control room operators observed a rapid decrease in main condenser vacuum [EllS: SG]. Due to the loss of main condenser vacuum and rising levels in the condenser hotwell, control room operators entered the appropriate response procedures and initiated a manual reactor trip at approximately 10:32 am EST. Subsequent to the reactor trip, the Auxiliary Feedwater [EllS: BA]
system actuated as designed, in response to isolation of main feedwater flow [EllS: SJ]. Control and Shutdown rods fully inserted, and required safety systems responded as designed. The unit was stabilized in Mode 3, with decay heat removalvia Auxiliary Feedwater and the Atmospheric Dump Valves [EllS: Jl] (ADVs). The Main Steam lsolation Valves [EllS: SB]were closed and remained closed during the event, and the station was maintained in a normalshutdown electrical alignment.
Main Control Room personnel responded appropriately to the plant transient using abnormal operating instructions which address loss of main condenser vacuum and rapid load reduction.
Operations entered: 1-AOl-11, "Loss of Condenser Vacuum," 1-E-0, "Reactor Trip or Safety lnjection, ES-01, "Reactor Trip Response, and 1-GO-s, "Unit Shutdown From 30% Power to Hot Standby." Emergency and abnormal procedures were correctly followed, and the plant was placed in a stable condition in Mode 3.
Operations personnel confirmed that the plant response post trip was uncomplicated. Operations personnel, consistent with an uncomplicated shutdown, secured equipment including the following:
Condensate Circulating Pumps [EllS: SD] and Raw Water Cooling Pumps [EllS: NN].
This event is reportable under 10 CFR 50.73(aX2XivXA).
lnoperable Structures, Components, or Systems that Contributed to the Event No inoperable structures, components, or systems contributed to this event.
Dates and Approximate Times of Occurrences
Time Event
!t.
A.
B.
2006 2t21t15 2t21t15 N/A 10:31 am EST 10:31 am EST Replaced "C" condenser zone dog bone seal during scheduled refueling outage.
Operations entered 1-AOl-1 1 for loss of main condenser vacuum.
Turbine backpressure begins increasing in "C" zone of condenser, followed closely by "8" and "A" zones. Condenser Vacuum Lo/Lo-Lo alarm receaved in the main control room.
Date Time Event 2121115 10:32 am EST The plant was manually tripped due to loss of main condenser vacuum.
2121115 12:46 pm EST Event Notification 50839 was made to the NRC.
D. Manufacturer and Model Number of Components that Failed.
The "C" zone main condenser expansion joint boot seal failed at the splice joint. This seal was manufactured and installed by Keystone Rubber.
E. Other Systems or Secondary Functions Affected
There were no systems or secondary functions associated with this event.
F. Method of discovery of each Component or System Failure or Procedural Error
The failure of the main condenser boot seal resulted in a loss of main condenser vacuum, requiring a manual reactor trip. The Tennessee Valley Authority (TVA) investigation of the event revealed that the boot seal had failed on the "C" expansion zone. There were no procedural errors associated with this event.
G. Failure Mode and Effect of Each Failed Component Other than the main condenser expansion joint boot seal failure, there were no failed components associated with this event.
H. Operator Actions
This was an uncomplicated reactor trip. No special operator actions were required.
l. Automatically and Manually lnitiated Safety System Responses The reactor was tripped manually on decreasing main condenser vacuum. All automatic and manually initiated safety systems responded as expected
III. CAUSE OF THE EVENT
A. The cause of each component or system failure or personnel error, if known.
The expansion joint boot seal on the "C" condenser to low pressure turbine failed, resulting in loss of condenser vacuum which led to a manual trip. The failure of the seal was due to a non-optimal vulcanization process and inadequately overlapped application which significantly weakened the seal at the splice joint area. Additionally, seal water is supplied to the boot seals to minimize air inleakage through the expansion joint. During the failure analysis conducted to determine the seal failure, it was noted that water had wicked into the polyester fibers of the boot seal, further weakening the splice joint area.
B. The cause(s) and circumstances for each human performance related root cause.
An organizational driver for this event was an inadequate risk assessment process for critical maintenance. Specifically, in 2006 (prior refueling outage where the boot seal was replaced) the procedure for oversight of supplemental personnel screening criteria/process was inadequate to ensure proper risk categorization for critical maintenance. The work document used for installation
tv.
V.
Closing of the Main Condenser Shell and Hotwell," to include necessary inspection details and signoffs for the boot seal inspection by incorporating internal and external benchmarking.
Based on operating experience and the TVA's failure analysis, seal water supply to the boot seals to minimize air inleakage is not necessary and the system has been secured.
VI. ADDITIONAL INFORMATION
A. Previous similar events at the same plant On August 17, 2001, TVA submitted LER 50-390/2001-001, "Manual Reactor Trip Due to Reduced Circulating Water Flow." This LER describes an event where during normal operation at 100 percent power, the unit was manually tripped due to a rise in back-pressure in the condenser.
This was caused by reduced condenser circulating water flow, which resulted from cooling tower fill material obstructing the intake flume screens to the circulating water pumps. However, the root use was different; no approval was needed to deviate from design output documents, which allowed cooling tower fill repairs and/or replacement other than those issued in the design documents. This resulted in installed supports which failed, leading to this event.
B. Additional lnformation None.
C. Safety System Functional Failure Consideration This condition did not result in a safety system functionalfailure.
D. Scrams with Complications Consideration This reactor trip was not complicated.
VII. COMMITMENTS
None.