05000390/LER-2017-006

LER-2017-006, Structural Degradation of 161 kV Line Pole Leads to a Condition Prohibited by Technical Specifications

Watts Bar Nuclear Plant, Unit 1
Event date:	06-01-2017
Report date:	07-31-2017
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3902017006R00 - NRC Website

LER 17-006-00 for Watts Bar, Unit 1, Regarding Structural Degradation of 161 kV Line Pole Leads to a Condition Prohibited by Technical Specifications

ML17212A373
Person / Time
Site:	Watts Bar
Issue date:	07/31/2017
From:	Simmons P Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 17-006-00
Download: ML17212A373 (7)

comments regarding burden estimate to the Information Services Branch (T-2 F43), U S. %Idea, Regulatory Commission.

Washington, DC 20555-0001, or by e-mail to Infocollects Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202. (3150-0104), Office of Management and Budget. Washington, DC 20503. If a means used to moose an information collection does not display a current y valid OMB controi number, the NRC may not conduct o

• sponsor. and a person is not required to respond to. the information collection

3. LER NUMBER

2017 - 00 006

I. PLANT OPERATING CONDITIONS BEFORE THE EVENT

Watts Bar Nuclear Plant (WBN) Unit 1 was at 100 percent rated thermal power (RTP). WBN Unit 2 was in Mode 5 and was not impacted by this event.

II. DESCRIPTION OF EVENT

A. Event Summary operations personnel declared the Watts Bar Hydro (WBH) line B offsite power source {EIIS:FK} to be inoperable in accordance with Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, Condition A. Pole 9 of this offsite power source had been found with a crack near its base by Transmission personnel such that it was not capable of meeting its designed ice and wind loading specified in the Updated Final Safety Analysis Report (UFSAR). The pole was replaced and the line returned to service on June 2, 2017.

This crack was determined to have been caused by an earlier line failure on May 27, 2017 when adjacent poles fell during a thunder storm, which also caused the plant to enter TS 3.8.1, Condition A. Based on evidence demonstrating the pole with the crack had not met requirements from May 27, 2017 until replaced on June 2, 2017, a condition prohibited by Technical Specification 3.8.1 occurred because the line was not functional for a period longer than the allowed outage time.

This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS.

B. Inoperable Structures, Components, or Systems that Contributed to the Event Degraded offsite power (OSP) transmission poles contributed to this event.

C. Dates and Approximate Times of Occurrences Date Time Event (EDT) 5/27/17 2220 WBN 1 Enters 0-A01-34, Loss of Offsite Power for loss of WBH Line B offsite power supply during a storm.

Enter TS LCO 3.8.1, Condition A.

5/29/17 1739 Exit TS LCO 3.8.1, Condition A after return of WBH line B.

6/1/17 1550 Entered TS LCO 3.8.1, Condition A when Pole 9 for WBH Line B is declared inoperable.

6/2/17 1551 Exited TS LCO 3.8.1, Condition A when WBH Line B is declared operable.

D. Manufacturer and Model Number of Components that Failed During the Event The OSP transmission supports were fabricated by Union Metal of COR-TEN steel.

Est mated burden per response to comply with the mandatory collection request. 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported Send comments regarding burden est mate to the Information Services Branch (T-2 F43). U.S. Nuclear Regulatory Commission. Washington, DC 20555-0001, or by e-mail to infocollects Resource@n

• c.gov, and to the Desk Officer. Office of Information and Regulatory Affairs, NEOB-10202. (3150-0104). Office of Management and Budget, Washington, DC 20503 If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor. and a person is not required to respond to, the information collecton

3. LER NUMBER

2017 - 00 006

E. Other Systems or Secondary Functions Affected

No other systems or secondary functions were affected .

F. Method of discovery of each Component or System Failure or Procedural Error The degraded condition WBH Line B pole 9 was discovered by TVA transmission personnel during investigation of the condition of the offsite power system serving WBN following a loss of the WBH Line B during a storm on May 27, 2017. The condition of the pole was promptly reported to site operations personnel.

G. Failure Mode and Effect of Each Failed Component Material degradation (corrosion).

H. Operator Actions

Upon discovering the degraded condition of the WBH Line B pole, operations personnel declared the B offsite power source to be inoperable.

I. Automatically and Manually Initiated Safety System Responses Not applicable.

III. CAUSE OF THE EVENT

A. The cause of each component or system failure or personnel error, if known.

The WBH Line B pole 9 described in this report had been subject to significant corrosion prior to the May 27, 2017 storm. The collapse of two adjacent poles on May 27, 2017 resulted in the partial circumferential crack near the base of pole 9.

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

No human performance causes have been attributed to this event.

IV. ANALYSIS OF THE EVENT

Two 161 kV transmission lines extend approximately 1.5 miles from Watts Bar Hydro Plant switchyard to the Watts Bar Nuclear Plant site to furnish preferred power to the nuclear plant. The transmission line structures for 161kV lines are designed for wind and heavy icing conditions in excess of those that would be expected to occur in this area. The 161 kV line poles are fabricated of COR-TEN, a steel alloy that is intended to create a protective oxide coating to prevent atmospheric corrosion.

A storm on May 27, 2017 resulted in multiple poles falling over, causing the loss of one of the two required preferred power sources (WBH Line B). It has been concluded the failure of these poles, coupled with existing wall thinning of pole 9, resulted in a significant load being applied to pole 9, cracking it. During comments regarding burden estimate to the !nformation Services Branch (T-2 F43), U S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-ma: to Infocollects Resoxce@nrc goy, and to the Desk Officer, Office of Information and Regulatory Affairs, NFOB-10202. (3150-0104), Office of Management and Budget, Washington. DC 20503. If a means used to impose an :nfornat on collection does not display a currently valid OMB control number, the NRC may not conduct or soonso

• . and a person is not required to respond to. the information collection.

3. LER NUMBER

2017 - 00 006 repair and investigation efforts for the May 27, 2017 event, the crack in pole 9 was not immediately identified because heavy vegetation was in place over the crack. When this vegetation was removed several days later by transmission personnel during further extent of condition review, the crack was revealed. The assessment of the crack and wall thinning of pole 9 due to corrosion by transmission personnel concluded that the pole could not support expected design ice and wind loads and should be promptly replaced. Transmission personnel brought this information to operations personnel on June 1, 2017 and the pole and associated offsite power source were declared inoperable.

V. ASSESSMENT OF SAFETY CONSEQUENCES

The offsite power system at WBN has historically been highly reliable. During the storm on May 27, 2017 that caused a failure of WBH line B, the other offsite power source remained operable. Following the initial restoration of WBH line B on May 29, 2017, all required TS surveillances were met for offsite power. WBN is also provided with onsite power in the event offsite power is lost. Based on the redundancy inherent in this design, the safety consequences of this event are considered to be low.

A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The WBH line A power source remained available during the time period described in this LER.

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 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 While the offsite power system is not specifically safety related, the WBH line B was returned to service the day following identification of pole 9 degradation.

VI. CORRECTIVE ACTIONS

This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Reports (CRs) 1302767 and 1301565.

A. Immediate Corrective Actions

Upon discovering the WBH line B pole 9 in a degraded state, the WBH line B offsite power source was declared out of service. The affected pole was replaced and the WBH line B offsite power source was returned to service the following day.

B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future comments regard.ng burden estimate to the Information Services Branch (T-2 F43), U S. Nuclear Regulatory Commission. Washington, DC 20555-0001, or by e-ma. to Infocollects Resou

• ce@nrc.gov. and to the Desk Officer. Office of Information and Regulatory Affars.

NEOB-10202. ;3150-0104). Office of Management and Budget, Washington, DC 20503. If a means used to impose an nformation collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor. and a person is not

• equ:red to respond to, the information collection

3. LER NUMBER

2017 - 00 006 TVA transmission personnel have performed additional investigations of the offsite power system lines serving WBN. Additional poles have been replaced, though none were in the condition identified for pole 9. Future actions are planned to replace all of the COR-TEN poles in WBH lines A and B with poles of a different design.

VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE

No previous similar occurrences associated with the offsite power system have occurred within the last three years.

VIII. ADDITIONAL INFORMATION

None.

IX. COMMITMENTS

None.