05000391/LER-2018-003, Reactor Trip Due to Main Generator Differential Relay Actuation

Reactor Trip Due to Main Generator Differential Relay Actuation

ML18233A324
Person / Time
Site:	Watts Bar
Issue date:	08/21/2018
From:	Simmons P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 2018-003-00
Download: ML18233A324 (6)

LER-2018-003, Reactor Trip Due to Main Generator Differential Relay Actuation

Event date:
Report date:
3912018003R00 - NRC Website

text

Tennessee Valley Authority, Post Office Box 2000, Spring City, Tennessee 37381 August 21, 2018 10 cFR 50.73 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 3911201&-003-00, Reactor Trip Due to Main Generator Differential Relay Actuation This submittal provides Licensee Event Report (LER) 391t2018-OO3-00. This LER provides details concerning a plant trip as a result of an electrical issue with a main generator differential current relay circuit. This condition is being reported in accordance with 10 CFR 50.73(aX2XivXA) as an automatic actuation or 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, Site Vice President Watts Bar Nuclear Plant Enclosure cc: See Page 2

U.S. Nuclear Regulatory Commission Page 2 August 21, 2018 cc (Enclosure):

NRC Regional Administrator - Region ll NRC Senior Resident lnspector - Watts Bar Nuclear Ptant

NRC FORM 366 (02-2018)

O"o'"*"ra U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

APPROVED BY OMB: NO. 3100-0104 EstimaEd burden per Bsponse b comply wifrr this mandatory collection request 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.

ReporEd lessons learned are incorporabd into the licensing process and Ed back b indusfy. Send comnents regarding burden estimab to the lnformation Services Branch [t-2 F43), u.s. Nuchar Regulabry commission, washington, DC 205s$0001,0r by e-mail to lnfocollec8. Resource@nrc.gov, and b flre Desk fficer, ffice of lnformati-on ard Reguldory Afiairs, NE0&10202, (31s0-0104), office, of Management and Budget Washingbn, DC 20503. lf a means used h impose an information collection does not display a cunenty valid OMB oonfol number, flre NRC may not conduct on sponsor, and a person is notreguircd b respond b, fie infomation collection.

1. Facility Name Watts Bar Nuclear Plant, Unit 2

2. Docket Number 05000391

3. Page 1 0F4

4. Title Reactor Trip Due to Main Generator Differential Relay Actuation

5. Event Date

6. LER Number

7. Report Date

8. Other Facilities lnvolved Monthl Day I Year year I S.quential I

Number Rev No.

Month I Day Year Facility Name N/A losooo 06 22 I 2018 2019 -003

- 00 08 21 2018 Facility Name

9. Operating Mode I l. This Report is Submitted Pursuant to the Requirements of 10 CFR g: (Check a1 that apply) 1 n zo.2zo1(b) 2o.22os(aX3)(i) n 50.73(aX2XiiXA) tr 50.73(aX2XviiiXA) n 20.2201(d) tr 20.2203(aX3)(ii) n 50.73(aX2X.iXB) n 50.73(aX2XviiiXB) tl 2o.z2o3(a)(1) tr zo.22o3(aX4) tr 50.73(a)(2xiii) tr 50.73(aX2Xix)(A)

D 20.2203(aX2Xi) n 50.36(cXlXiXA)

X 50.73(aX2)(ivXA) n 50.73(a)(2Xx)

10. Power Level n 20.2203(aX2)(ii) n 50.36(cxl XiiXA) n 50.73(aX2XvXA) tr rc.r1(a)(4) 96 tr 20.2203(aX2)(i.i) tr 50.36(cX2) tr s0.73(aX2Xv)(B) n rs.r1(a)(s) n 2o.22os(aX2Xiv) tr 50.46(ax3)(ii) n 50.73(aX2)(v)(c) tl rc.72(a)(1) tr 20.2203(aX2Xv) n 50.73(aX2XiXA) tr 50.73(aX2)(vXD) n B.rz(a)(2xu tr 20.2203(aX2Xvi) tr 50.73(aX2)(iXB) tr 50.73(aX2Xvii) tr B.rr(a)(2xii) tr 50.73(aX2XiXc) n OTHER Specify in Abstract betow or in G. Failure mode, mechanism, and effeit of each faired component The likely failure mode of equipment associated with this event is an intermittent equipment failure between the neutral side current transformer of the main generator and the protectibn relays which led to actuation of the A phase main transformer differential current relay.

H. Operator actions

Operations personnel promptly stabilized the plant following the plant trip

l. Automatically and manually initiated safety system responses

The turbine trip resulted in a reactor trip. 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 did not definitively identify the cause of the trip.

The most probable ctuse of the event is an intermittent equipment fiilure inifre n phase signal loop between the generator neutral current transformer (251'CT loop) {ElIS:XCT} ind the digitat protection relays (2-RLY-244-S300G3/1 and 2-RLY-244-M3425N2ietiys) which ted to actuation of the A phase main transformer differential current relay (287T){EllS:8i}.' The source of the intermittent equipment failure will be further investigated during tire Spring 2019 Unit 2 refueling outage.

B. Cause(s) and circumstances for each human performance related root cause

No human performance root causes are attributed to this event.

lV. Analysis of the Event On June 22,2018, an intermittent electrical equipment issue of slightly over one cycle (16.7 milliseconds) was measured on the A phase of the main transformer which caused an actuation of the digital protective relays, resulting in a. generatgr trip. The automatic Unit 2 reactor trip that occurred on Junj22, )Ot A 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 t6.Z.l. Following the trif, Operations entered 2-E-O, Reactor Trip or Safety lnjection, and subsequenfly transitioned to Z-gS-0. t, Reactor Trip Response, to place Unit 2 in Mode 3 using procedure 2-GO-5, [Jnit Shutdown from 30 percent Reactor Power to Hot Standby. This reactor trip is considered uncomplicated.

V. Assessment of Safety Consequences

As described in the previous section, the trip that occurred was uncomplicated and is bounded by the loss of external electrical load event described in the UFSAR. A probabilistic risk analysis performed ior this event indicates the conditionalcore damage probability from this event is very small.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event Not applicable.

B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residuai 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 event was entered into the T_elnessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Report (CR) 1425291.

A. lmmediate Corrective Actions Operations personnel promptly stabilized the plant in Mode 3. A short term corrective action was taken to relocate the 287T differential trip signal to a different CT loop such that the one out of two taken twice logic and circuit protection is maintained. Future A phase current drop events on the 251 CT loop will be able to be monitored without causing a direct unit trip.

B. Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future The intermittent equipment failure between the neutral side current transformer (251 CT loop) of the main generator and the digital protection relays (2-RLY-244-S300G3/1 and 2-RLy-244-M3425N2 relays) will be fully investigated and corrected during the Unit 2 Spring 2019 refueling outage.

Vll. Previous Similar Events at the Same Site LER 391/2016-008 describes a unit trip that occurred on August 20,2016 as a result of an internal fault on the 2B Main Bank Transformer. This event was most likely caused by an inadequate internal bus clearance as a result of either a latent design issue or initial installation error. While the event is associated with the main transformers, the equipment causes are different than this event.

Vlll. Additionallnformation None.

lX. Commitments None.Page 4 of 4