05000328/LER-2019-002, Loss of Heater Drain Tank Flow Causes Turbine Runback and Manual Reactor Trip

Loss of Heater Drain Tank Flow Causes Turbine Runback and Manual Reactor Trip

ML20038A107
Person / Time
Site:	Sequoyah
Issue date:	02/07/2020
From:	Rasmussen M Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 2019-002-00
Download: ML20038A107 (7)

LER-2019-002, Loss of Heater Drain Tank Flow Causes Turbine Runback and Manual Reactor Trip

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2) 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)(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)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i)
3282019002R00 - NRC Website

text

Tennessee Valley Authority, Sequoyah Nuclear Plant, P.O. Box 2000, Soddy Daisy, Tennessee 37384 February 7, 2020 10 CFR 50.73 ATTN:

Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Unit 2 Renewed Facility Operating License No. DPR-79 NRC Docket No. 50-328

Subject:

Licensee Event Report 50-328/2019-002-00, Loss of Heater Drain Tank Flow Causes Turbine Runback and Manual Reactor Trip The enclosed licensee event report provides details concerning a manual reactor trip and automatic safety system actuation. This report is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in actuation of the Reactor Protection system and Auxiliary Feed Water system.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr. Jeffrey Sowa, Site Licensing Manager, at (423) 843-8129.

Respectfully, Matthew Rasmussen Site Vice President Sequoyah Nuclear Plant Enclosure: Licensee Event Report 50-328/2019-002-00 cc:

NRC Regional Administrator - Region II NRC Senior Resident Inspector-Sequoyah Nuclear Plant printed on recycled paper

NRC FORM 366 (04-2018) s)>>nH*U, U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

(SeePage2for required number ofdigits/characters for eachblock)

(SeeNUREG-1022, R.3 for instruction andguidance forcompleting this form hto://www.nro.Qov/readinq-m^^

APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020

, the NRC may not conduct orsponsor, and a person isnot required torespond to.theinformation collection.

1. Facility Name Sequoyah Nuclear Plant, Unit 2

2. Docket Number 05000328

3. Page 1 OF6

4. Title Loss of Heater Drain Tank Flow Causes Turbine Runback and Manual Reactor Trip

5. Event Date

6. LER Number

7. Report Date Month Day Year 12 12 2019 a OperatingMode 1

10. Power Level 100 Licensee Contact [Andrew McNeil

Cause

System SN Year 2019 Sequential Number 002 Rev No.

00 Month 02 Day Year 07 2020 Facility Name NA Facility Name NA

8. Other Facilities Involved Docket Number 05000 Docket Number 05000 20.2201(b) llTrtte Report is SubmittBd Pursuantto the Requiieme^

20.2203(a)(3)(i) 50.73(a)(2)(H)(A) 50.73(a)(2)(viii)(A) 20.2201(d) 20.2203(a)(3)(H) 50.73(a)(2)(ii)(B) 50.73(a)(2)(viii)(B) 20.2203(a)(1) 20.2203(a)(4) 50.73(a)(2)(iii) 50.73(a)(2)(ix)(A) 20.2203(a)(2)(i) 50.36(c)(1 )(i)(A)

SI 50.73(a)(2)(iv)(A) 50.73(a)(2)(x) 20.2203(a)(2)(H) 50.36(c)(1)(ii)(A) 50.73(a)(2)(v)(A) 73.71(a)(4) 20.2203(a)(2)(iii) 50.36(c)(2) 50.73(a)(2)(v)(B) 73.71(a)(5) 20.2203(a)(2)(iv) 50.46(a)(3)(H) 50.73(a)(2)(v)(C) 73.77(a)(1) 20.2203(a)(2)(v) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(D) 73.77(a)(2)(H) 20.2203(a)(2)(vi) 50.73(a)(2)(i)(B) 50.73(a)(2)(vii) 73.77(a)(2)(Hi) 50.73(a)(2)(i)(C)

D Other (Specify inAbstract below orinNRC Form 366A)

12. Licensee Contact for this LER Component LCV Telephone Number (IncludeArea Code)

[423-843-8098 ia CompleteOne Lineforeach(^pctnefrt Failure Descried inthis Report Manufacturer Reportable To ICES

Cause

System Component Manufacturer Reportable To ICES M430

14. Supplemental Report Expected Month

15. Expected Submission Date Day Year Yes (If yes, complete 15. Expected Submission Date) £3 No Abstract (Limit to 1400spaces, i.e., approximately 14single-spacedtypewritten lines)

On December 12,2019, at 0433 Eastern Standard Time (EST), Sequoyah Nuclear Plant Unit 2 was manually tripped as a result of a secondary system level control valve positioner failure. The failure of the valve led to reduced secondary system Heater Drain Tank flow and a unitdown power. During the down power, manual reactor trip criteriawas reached. All safety-related equipment operated as designed, all controls rods fully inserted as required, and the Auxiliary Feedwater system automatically initiated as expected.

The failed positionerwas replaced with a newer model. Investigation revealed that a fine adjust screw had loosened, preventing the valve positioner from correctly responding to control signals. An extent-of-condition was performed to identify other susceptible components, and actions have been taken to replace a necessary component with a newer model.

NRC FORM 366 (04-2018)

I.

Plant Operating Conditions Before the Event

At the time of the event, Sequoyah Nuclear Plant (SQN) Unit2 was operating at 100-percent rated thermal power.

II.

Description of Event

A.

Event Summary

On December 12, 2019, at 0429 EST, SQN Unit 2 Control Room received an annunciator alarm "No. 3 HTR DR TK BYPASS TO COND C OPEN". Level Control Valve 2-LCV-6-1Q5A [ENS: LCV] failed in a full OPEN position while controlling #3 Heater Drain Tank (HDT)

[EMS: TK] level, which resulted in the HDT contents discharging directly to the main condenser and resulted in a lowering tank level. The lowering #3 HDT level triggered protective trips for the associated HDT pumps [EMS: P] 2A, 2B, and 2C. 2C HDT Pump tripped first at 0432, triggering a designed secondary system runback, followed by the trip of 2B and 2A HDT Pumps. Main Control Room (MCR) Operators entered the appropriate Abnormal Operating Procedure (AOP) for the loss of HDT flow in parallel with monitoring the runback and dispatching field operators to investigate. Operators manually tripped the reactor at 0433 EST in accordance with (IAW) the AOP guidance after the third #3 HDT pump automatically tripped on low HDT level, resulting in no operating #3 HDT pumps.

Following the reactor trip, all safety related equipment operated as designed. No complications were experienced. The Auxiliary Feedwater System (AFW) [ENS: BA] automatically initiated post-trip as designed and restored steam generator [EMS: SG] water levels.

The Unit 2 reactor trip is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in an actuation of the Reactor Protection System (RPS) [EMS: JC] and the Auxiliary Feedwater System.

B.

Status of structures, components, or systems that were inoperable at the start of the event and contributed to the event:

No inoperable structures, components, or systems (SSCs) contributed to this event.

05000-328 YEAR 2019 SEQUENTIAL NUMBER 002 REV NO.

- 00 Page 2 of 6(04-2018)

'if)

U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020

, the NRC may not conduct or sponsor, anda personis notrequired to respondto,the infbrmationcollection.

Sequoyah Nuclear Plant, Unit 2 05000-328 C.

Dates and approximate times of occurrences

YEAR 2019 3.LBRNUMBBR SEQUENTIAL NUMBER 002 REV NO.

- 00 Date/Time (EST)

Description

December 12, 2019 at 0429 2-LCV-6-105A failed in the FULLOPEN position, discharging #3 HDT contents into the main condenser.

December 12, 2019 at 0429

1. 3 HDT Low Level alarm was received in MCR.

December 12, 2019 at 0432 2C HDT Pump tripped, followed by 2B HDT Pump trip. A Main Turbine Runback automatically initiated. Operators entered AOP-S.04 (Condensate or Heater Drains Malfunction).

December 12, 2019 at 0433 2A HDT Pump tripped, leaving no #3 HDT pumps operating. The Unit Supervisor directed a manual reactor trip IAW Abnormal Operating Procedure guidance, followed by entry into procedure E-0 (Reactor Trip).

December 12, 2019 at 0433 The AFW system automatically initiated as designed in response to the reactor trip.

December 12, 2019 at 0443 Operators transitioned from E-0 (Reactor Trip) to ES-0.1 (Reactor Trip Response) and Mode 3 operations.

D.

Manufacturer and model number of each component that failed during the event

Moore Model 750P Pneumatic Valve Positioner.

E.

Other systems or secondary functions affected

There were no other systems or secondary functions affected by this event.

F.

Method of discovery of each component or system failure or procedural error

Main Control Room alarms and annunciators provided indication to the operators.

G.

Failure mode, mechanism, and effect of each failed component:

The affected level control valve failed in the open position due to a failed valve positioner. A locking nut backed off an adjustment screw on the positioner, resulting in a lack of proper spring tension on the positioner pilot assembly. When the valve received a demand bias to Page 3 of6(04-2018)

'if)

U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020 Estimated burden perresponse to comply with this mandatory collection request80 hours. Reported lessons learned are incorporated into the licensing process and ted backto industry. Sendcomments regarding burden estimate to the Information Services Branch (T-2 F43), U. S. Nuclear Regulatory Commission, Washington, DC 20555-0001,or bye-mail to lrrfbcollects.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 impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, anda personis notrequired to respondto,the informationcollection.

3. LB* NUMBER Sequoyah Nuclear Plant, Unit 2 05000-328 YEAR 2019 SEQUENTIAL NUMBER 002 REV NO.

- 00 modulate for tank level control, the valve traveled to the full open position due to a lack of balanced spring tension.

H.

Operator actions

The MCR Operators responded to loss of HDT Pump flow, runback, and reactor trip as required. They promptly identified plant conditions and took appropriate actions.

I.

Automatically and manually initiated safety system responses

The RPS manual actuation, and automatic AFW System initiation, responded as designed.

III.

Cause of the Event

A.

Cause of each component or system failure or personnel error

Although the exact cause of the valve positioner failure is unknown, possible causes of the fine zero lock nut backing off the screw include wear of the threads on the lock nut or screw, vibration, poor design of locking mechanism, and lack of vendor guidance for torque values.

B.

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

There was no identified human performance related root cause.

IV.

Analysis of the Event

The SQN Heater Drain system [EMS: SN] is a secondary system designed to remove and dispose of all drainage from the moisture seperators, reheaters, feedwater heaters, main feed pump turbine condensers and gland steam condensers during all modes of unit operation by returning the condensed water back to the Condensate-Feedwater System.

Condensed water from high pressure secondary components flow into the #3 HDT, and that water is then pumped forward into the condensate cycle by the #3 HDT Pumps, consisting of three pumps in parallel. Level in the #3 HDT is maintained within the proper range by modulating level control valves at the discharge of the #3 HDT Pumps. High level in the #3 HDT initiates opening of modulating bypass-to-condenser valves. Low level in the HDT results in a trip of all operating #3 HDT Pumps. If the discharge flowrate of the #3 HDT Pumps is reduced below a predefined setpoint of 5500 gallons-per-minute with unit power greater than approximately 82 percent, a main turbine runback is initiated in order to ensure sufficient condensate-feedwater flow.

The plant safety system responses during and after the reactor trip were bounded by the responses described in the Updated Final Safety Analysis Report (UFSAR) for a total loss of normal feedwater.

Page 4 of6(04-2018)

U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020 Estimated burden perresponse to comply with this mandatory collection request80 hours. Reported lessons learned are incorporated into the licensing process andfed backto industry. Sendcomments regarding burden estimate to the Information Services Branch (T-2 F43), U. S. Nuclear Regulatory Commission, Washington, DC 20555-0001,or bye-mail to lrrfocollects.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 impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor,anda personis notrequired to respondto,the informationcollection.

& LER NUMBER Sequoyah Nuclear Plant, Unit 2 05000-328 YEAR 2019 SEQUENTIAL NUMBER

- 002 REV NO.

- 00 Therefore, this event did not adversely affect the health and safety of plant personnel or the general public.

V.

Assessment of Safety Consequences

There were no actual safety consequences as a result of this event.

A.

Availabilityof systems or components that could have performed the same function as the components and systems that failed during the event:

The event occurred with the plant at power with no other coincident transients or events.

Had a total loss of main feed water occurred, decay heat removal capability existed in the form of AFW pumps and steam reliefs to maintain sufficient heat removal as described in the UFSAR.

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:

The event did not occur when the reactor was shut down.

C.

For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:

This event did not render a safety system inoperable.

VI.

Corrective Actions

The reactor trip event was entered into the Tennessee Valley Authority Corrective Action Program (CAP) under Condition Report #1572651.

A.

Immediate Corrective Actions

The affected positioner was replaced with a newer model. The newer model provides equivalent operation but includes a more robust mechanism for adjusting calibration, which includes a cam locking nut and a cam locking screw to prevent the fine zero lock nut from inadvertent movement.

B.

Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future:

An extent of condition evaluation was performed to determine ifany similar positioners were Page 5 of6(04-2018)

U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET

• 7 APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020 Estimated burden perresponse 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 learned are incorporated intothe licensing process and fed backto industry. Send comments regarding burden estimate to the Information Services Branch (T-2 F43), U. S. Nuclear Regulatory Commission, Washington, DC 20555-0001,or bye-mail to lnfbcollects.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 impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor,anda personis notrequired to respondto, the informationcollection.

3. LER NUMBER Sequoyah Nuclear Plant, Unit 2 05000-328 YEAR 2019 SEQUENTIAL NUMBER 002 REV NO.

- 00 installed in either Unit 1 or Unit 2. As a result corrective actions have been taken to replace one other positioner on Unit 1. The positioner classifications have been upgraded to a critical component. Actions have been implemented to ensure preventative maintenance items are generated as needed to align with the fleet maintenance strategy.

VII.

Previous Similar Events at the Same Site

On October 24, 2019, the #3 HDT condenser bypass valve (2-LCV-006-105A) spuriously throttled open with SQN Unit 2 at power. Operations identified the condition and lowered power to 95 percent. While stabilizing power for investigation, further spurious movement of the level control valve reduced forward flow of the #3 HDT Pump which initiated the secondary runback.

The runback was terminated and plant power was stabilized at 74 percent RTP. Investigation revealed that the positioner for the level control valve was faulty. The positioner was replaced, and the issue documented in TVA CAP Condition Report #1559882.

VIII.

Additional Information

There is no additional information.

IX.

Commitments

There are no commitments.