05000391/LER-2018-005, For Watts Bar Nuclear Plant, Unit 2, Automatic Reactor Trip Due to Turbine Control System Card Failure and Throttle Valve Closure
| ML18295A212 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 10/22/2018 |
| From: | Simmons P Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 2018-005-00 | |
| Download: ML18295A212 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3912018005R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 October 22,2018 10 cFR s0.73 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No.50-391 Subject: Licensee Event Report 391/2018-005-00, Automatic Reactor Trip Due to Turbine Control System Card Failure and Throttle Valve Closure This submittal provides Licensee Event Report (LER) 39112018-A05-00. This LER provides details concerning a plant trip as a result of a control system throttle valve manual up/down counter card failure. This condition is being reported in accordance with 10 CFR 50.73(aX2)(ivXA) as an automatic actuation of the Reactor Protection System and the Auxiliary Feedwater Systems.
There are no new regulatory commitments contained in this letter. Please direct any questions concerning this matter to Kim Hulvey, WBN Licensing Manager, at (423) 365-7720.
Respectfully, Paul Simmons Site Vice President Watts Bar Nuclear Plant Enclosure cc: see Page 2
U.S. Nuclear Regulatory Commission Page 2 October 22,2A18 cc (Enclosure):
NRC Regional Administrator - Region ll NRC Senior Resident Inspector - Watts Bar Nuclear Plant
NRC FORM 366 (04-2018)
- ltl'*BotO W
U.S. NUCLEAR REGULATORY COMMISSION LTCENSEE EVENT REPORT (LER)
APPROVED BY OMB: NO. 3150-0104 EXPTRES:03I31DAZA Estimated burden per response to comply with this mandatory collection request 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
Reported lessons leamed are incorporated into the licensing process and fed back to indusfy. Send comments regarding burden estimate to the lnforma$on Services Branch [f-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001,0r by e-mail to lnfocollecls. Resource@nrc.gov, and to the Desk fficer, Office of lnformation and Regulatory Affairs, NEOB-10202, (315&0104), Office of Managenrent and Budget, Washingrton, DC 20503. lf a means used to impose an information mllection does not dirylay a cunen{y valid OMB confol number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 1. Facility Name Watts Bar Nuclear Plant, Unit 2
- 2. Docket Number 05000391
- 3. Page 10F5
- 4. Title Automatic Reactor Trip Due to Turbine Control System Card Failure and Throttle Valve Closure
- 5. Event Date
- 6. LER Number
- 7. Repoft Date
- 8. Other Facilities lnvolved Monthl Day I Year year I S"Ouential I
Number Rev No.
Month I Day Year Facility Name I
DockCt NnmbCi NA losooo 08 I 22 12018 2018 -005
- - 00 10 22 2018 Facility Name I
DoCtaCf Nnmbe, NA losooo
- 9. Operating Mode 1
n 20.2201(b) tr 2o.z2a3(aX3Xi) n s0.73(aX2XiiXA) n 50.73(a)(2)(viii)(A) n 2a.2201(d) n 20.22a3(aX3Xii) n 50.73(aX2XiiXB) n s0.73(a)(2)(viii)(B) n 20.2203(a)(1) n 2a.2203(aX4) n s0.73(ax2xiii) n s0.73(aX2XixXA) tr 2o.z2as(a)(2Xi) n 50.36(cXlXiXA)
X s0.73(aX2XivXA) n 50.73(aX2Xx)
- 10. Power Level tr 2o.22os(aX2Xii) il 50.36(cxlXiiXA) n s0.73(aX2XvXA) n rcl1(a)(4) 100 n 20.2203(aX2Xiii) fI so 36(cX2) tl 50.73(a)(2)(v)(B) n rc.r1(a)(5) n 2o.2zat(aX2Xiv) n 50.46(ax3xii) n 50.73(aX2XvXc) n rc.Tr(a)(1) n zo.22o3(aX2)(v) tr 50.73(aX2XiXA) n 50.73(aX2XvXD) n fi.l7(ax2xi) n zo.zza3a)e (vi) n s0.73(a)(2XiXB) n 50.73(aX2Xvii) n ft.Tr(ax2xii) n 50.73(a)(2xixc) n OTHER Specify in Abstract betow or in
D. Manufacturer and model number of each component that failed during the event
Throttle valve closure was caused by a turbine control system throttle valve manual U/D counter card (Siemens part number'1851049-101) failure. Specifically, the DC/DC
+151+5 converter {EIIS:CNV} on the card had failed. The DC/DC converters were made by V-lnfinity, model PTK1 0-Q24-S5.
E. Other systems or secondary functions affected
Secondary systems functioned as expected
F. Method of discovery of each component or system failure or procedural error
The turbine control system throttle valve manual U/D counter card failure was identified after the plant trip as part of troubleshooting.
G. Failure mode, mechanism, and effect of each failed component The DC/DC +15/+5 converter on the turbine control system throttle valve manual U/D counter card failed. This failure caused all four throttle valves to close, which initiated a reactor trip with a follow-on turblne trip.
H. Operator actions
Operations personnel promptly stabilized the plant following the plant trip.
l. Automatically and manually initiated safety system responses
Reactor trip was in response to throttle valve closure. Safety systems responded as expected, including reactor trip and automatic initiation of AFW.
lll. Cause of the Event
A. Cause of each component or system failure or personnel error
The troubleshooting performed following the event identified the cause of the trip to be a turbine control system throttle vaive manual U/D counter card failure. Specifically, the DC/DC +15/+5 converter on the card had failed.
A review of the event identified opportunities where management of risk-based decisions could be improved. These opportunities for improvement do not represent human performance root causes.
NRC FORM 3664 {O4-2A18)
lV. Analysis of the Event On August 22,2018, while preparing to lower turbine load to remove the Unit 2 Standby Main Feed Pump (SBMFP) from service, operators placed the AEH controls in Turbine Manual.
Wthin seconds of placing the system in manual, high pressure turbine throttle valves closed which resulted in a Reactor Trip with a follow-on Turbine Trip. Following the trip, operators entered 2-E-4, Reactor Trip or Safety lnjection, and subsequently transitioned to 2-ES-0.1, Reactor Trip Response, to place Unit 2 in Mode 3 using procedure 2-GO-5, Unit Shutdown fiom 30 percent Reactor Power to Hot Standby. This reactor trip is considered uncomplicated.
V. Assessment of Safety Consequences
The automatic Unit 2 reactor trip that occurred on August 22,2018 is generally comparable to the Updated Final Safety Analysis Report (UFSAR) description of the Loss of External Electrical Load and/or Turbine Trip described in UFSAR Section 15.2.7. A probabilistic risk analysis performed for this event indicates the conditional core damage probability from this event is small.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The turbine controls system is not safety related. The failure of the turbine control system throttle valve manual U/D counter card resulted in a unit trip, but otherwise did not impact any plant safety functions.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shut down the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident Not applicable.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service Not applicable.
Vl. Corrective Actions This condition was entered into the TVA Corrective Action Program (CAP) and is being tracked under Condition Report (CR) 1441438.
NRC FORM 3664 (041018)
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A. lmmediate Corrective Actions Operations personnel promptly stabilized the plant in Mode 3. A field walkdown of the turbine control cards identified the throttle valve manual U/D counter card with no light indications, indicating it had lost power. Troubleshooting concluded that the turbine control system throttle valve manual U/D counter card had failed which was the cause of this event. The failed card was replaced with a card containing a Lambda DC/DC converter B. Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future To reduce the probability of a similar event occurring, three additional counter cards (setter, reference, and governor valve manual) with V-lnfinity DC/DC power converters were replaced with cards having Lambda converters.
The main turbine AEH control system is scheduled to be replaced with a modern fault tolerant digital turbine control system. lnstallation is scheduled for the Unit 2 Cycle 2 refueling outage in April 2019. Until the digital electro-hydraulic turbine control system is installed, a monitoring plan has been established that will be used when maneuvering the main turbine from Auto to Manual. The plan will identify a failed counter card prior to shifting to Manual.
Corrective actions to improve risk management are being performed under Root Cause Analysis (RCA) for CR 1415482, Unplanned Plant Trips since Commercial Operation.
Vll. Previous Similar Events at the Same Site LER 390/2016-004-00 describes a Unit 1 trip that occurred on March 22,2016 as a result of a turbine control system VPL U/D counter card failure, specifically the DC/DC converter on the card. The VPL counter card is the same part number as the turbine control system throttle valve manual U/D counter card, which failed on August 22,2018: however, there was no trend of failure at the time.
VIll. Additionallnformation There is no additional information.
lX. Commitments There are no new commitments.Page 5 of 5