05000328/LER-2020-001, Ice Bed Inoperable Due to Exceeding Maximum Allowed Ice Bed Temperature

Ice Bed Inoperable Due to Exceeding Maximum Allowed Ice Bed Temperature

ML20262G938
Person / Time
Site:	Sequoyah
Issue date:	09/16/2020
From:	Rasmussen M Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 2020-001-00
Download: ML20262G938 (7)

LER-2020-001, Ice Bed Inoperable Due to Exceeding Maximum Allowed Ice Bed Temperature

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v), Loss of Safety Function
3282020001R00 - NRC Website

text

Tennessee Valley Authority, Sequoyah Nuclear Plant, P.O. Box 2000, Soddy Daisy, Tennessee 37384 September 16, 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 50-328/2020-001-00, Ice Bed Inoperable Due to Exceeding Maximum Allowed Ice Bed Temperature The enclosed licensee event report provides details concerning an inoperable ice bed due to exceeding the maximum temperature allowed by technical specifications. This event is being reported, in accordance with 10 CFR 50.73(a)(2)(v), as an event or condition that could have prevented the fulfillment of a safety function of structures or systems that are needed to:

(C) control the release of radioactive material and (D) mitigate the consequences of an accident.

There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr. Andrew McNeil, Site Licensing Manager (Acting),

at (423) 843-8098.

Respectfully, Matthew Rasmussen Site Vice President Sequoyah Nuclear Plant Enclosure: Licensee Event Report 50-328/2020-001-00 cc:

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Sequoyah Nuclear Plant nth

Abstract

On July 23, 2020, at 1920 eastern daylight time, the SQN Unit 2 Main Control Room received an unexpected glycol expansion tank low level alarm, followed by a low-low level alarm at 1928. An auxiliary unit operator was dispatched to investigate and discovered the Auxiliary Glycol Cooling System (AGCS) skid was leaking glycol through a flanged connection. The AGCS was aligned for Unit 2 ice bed cooling, in lieu of plant installed chillers, as part of an engineering design change to replace the ice bed glycol chillers. Upon discovery, Operations took action to secure and isolate the AGCS from the Unit 2 ice bed cooling loop and aligned the plant installed chillers to the Unit 2 ice bed. However, at 0105, on July 24, the Unit 2 ice bed was declared inoperable due to exceeding the maximum ice bed temperature of </= 27 degrees Fahrenheit (F) for Technical Specification (TS) Surveillance Requirement (SR) 3.6.12.1. At 0833, the Unit 2 ice bed maximum temperature decreased below 27 degrees F. Unit 2 exited TS 3.6.12, Condition A for the inoperable ice bed.

The cause of the event was the failure of a gasket on vendor supplied equipment at the flanged connection on the AGCS skid. The corrective action for this event was to repair the AGCS skid.

I.

Plant Operating Conditions Before the Event

At the time of the event, Sequoyah Nuclear Plant (SQN) Unit 2 was in Mode 1 at 100 percent rated thermal power.

II.

Description of Event

A.

Event Summary

On July 23, 2020, at 1920 eastern daylight time (EDT), the SQN Unit 2 Main Control Room (MCR) received an unexpected glycol expansion tank low level alarm [EIIS: LA], followed by a low-low level alarm at 1928. The low-low level alarm was followed by a containment isolation [EIIS: JM] signal to close two ice condenser system [EIIS: BC] valves [EIIS: ISV] (a designed response). One valve automatically closed while the other valve required closure from the MCR. An auxiliary unit operator (AUO) was dispatched to investigate and discovered the Auxiliary Glycol Cooling System (AGCS), implemented by a temporary modification, was leaking glycol through a flanged connection. The AGCS was aligned for Unit 2 ice bed cooling, in lieu of plant installed chillers, as part of an engineering design change to replace the ice bed glycol chillers. Upon discovery, Operations took action to secure and isolate the AGCS from the Unit 2 ice bed cooling loop and aligned the plant installed chillers to the Unit 2 ice bed.

However, at 0105, on July 24, the Unit 2 ice bed was declared inoperable due to exceeding the maximum ice bed temperature of </= 27 degrees Fahrenheit (F) for Technical Specification (TS) Surveillance Requirement (SR) 3.6.12.1.

At 0833, the Unit 2 ice bed maximum temperature decreased below 27 degrees F. Unit 2 exited TS 3.6.12, Condition A for the inoperable ice bed.

At 0900, an 8-hour non-emergency event notification (EN 54800) was made to the NRC in accordance with 10 CFR 50.72(b)(3)(v), as an event or condition that could have prevented the fulfillment of a safety function of structures or systems that are needed to: (C) control the release of radioactive material and (D) mitigate the consequences of an accident. This LER is submitted based on NUREG-1022, Revision 3, Section 3.2.7 guidance that identifies that the requirements of 10 CFR 50.73(a)(2)(v) apply when a system that is within the scope of the criterion is declared inoperable and no redundant system or equipment could be declared operable (the ice bed is a single train system).

The highest temperature identified during the event was 27.65 degrees F. This value is well below the melting point of the stored ice (31.2 degrees F). The Unit 2 ice bed remained capable of performing its required safety function to provide a heat sink during a Design Basis Accident (DBA) in containment. Therefore, no loss of safety function occurred.

Page 3 of 6 (08-2020)

U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)

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

2020

- 001

- 00 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 contributed to this event.

C. Dates and approximate times of occurrences

Date/Time (EDT)

Description

07/23/20, 1920 The MCR received an unexpected alarm annunciator for glycol expansion tank level low.

1928 The MCR received a glycol expansion tank low-low level alarm and two ice condenser system containment isolation valves received an auto-close signal, as designed; however, one of the valves had to be closed manually.

1937 An AUO was dispatched and reported a 10 gallon per minute glycol leak at the AGCS skid.

2215 A plan was developed to realign the Unit 2 ice bed cooling from the AGCS skid to plant installed chillers, reopen the isolation valves, and repair the leaking gasket.

07/24/20, 0050 The failed gasket was replaced and the remaining gasket connections were torque checked.

0105 Unit 2 entered TS 3.6.12, Condition A due to exceeding the maximum ice bed temperature for SR 3.6.12.1.

0114 Ice bed chillers were aligned to Unit 2 and placed in service.

0833 Unit 2 ice bed declared operable and TS 3.6.12, Condition A exited.

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

Garlock Multiswell 3760 gasket.

E.

Other systems or secondary functions affected

The glycol expansion tank low-low level alarm caused two containment isolation valves associated with the ice condenser system to be closed.

F.

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

MCR alarms annunciated and an AUO was dispatched into the field to investigate.

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U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)

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

2020

- 001

- 00 G. Failure mode, mechanism, and effect of each failed component:

The failure mode of the AGCS flange gasket was a compromised pressure boundary. The failure mechanism was material fracture or tearing.

H. Operator actions

Operations personnel responded to the annunciators as required and dispatched an AUO to investigate.

I.

Automatically and manually initiated safety system responses

The glycol expansion tank low-low level alarm caused the containment isolation valves associated with the ice condenser system to be closed.

III.

Cause of the Event

A.

Cause of each component or system failure or personnel error

The cause of the AGCS leak and subsequent ice bed inoperability was the failure of a gasket on vendor supplied equipment at the flanged connection on the AGCS skid. This resulted in a leak that led to isolation of the AGCS skid and caused a loss of cooling for the Unit 2 ice bed.

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 ice bed consists of a minimum of 1,916,000 pounds of ice stored within the ice condenser. The primary purpose of the ice bed is to provide a large heat sink in the event of a release of energy from a DBA in containment. The ice would absorb energy and limit containment peak pressure and temperature during the accident transient. Limiting the pressure and temperature reduces the release of fission product radioactivity from containment to the environment in the event of a DBA.

According to the Updated Final Safety Analysis the melting point of the ice solution is 31.2 degrees F. The highest temperature identified during the event was 27.65 degrees F. The Unit 2 ice bed remained capable of performing its required safety function to provide a heat sink during a DBA in containment.

V.

Assessment of Safety Consequences

There were no actual safety consequences as a result of the ice bed inoperability. No actual loss of safety function occurred. The ice mass required for a DBA in containment was available.

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U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)

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

2020

- 001

- 00 A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:

None.

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:

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:

The ice bed is a single train safety system that was rendered inoperable for approximately 7.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.

VI.

Corrective Actions

The event was entered into the Tennessee Valley Authority Corrective Action Program (CAP) under CR 1625372.

A.

Immediate Corrective Actions

Operations personnel placed plant installed glycol chillers into service and observed a decrease in ice bed temperature after approximately 2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.

B.

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

The corrective action for this event was to repair the AGCS skid.

Additional actions include:

Initiating a work order to repair the ice condenser system containment isolation valve that failed to close on the low-low level alarm.

VII. Previous Similar Events at the Same Site

There were no previous similar events occurring at SQN within the last three years.

VIII. Additional Information

There is no additional information.

Page 6 of 6 (08-2020)

U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023 LICENSEE EVENT REPORT (LER)

CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form https://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)

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

2020

- 001

- 00 IX.

Commitments

There are no commitments.