05000328/LER-2021-001, Ice Bed Inoperable Due to Exceeding Surveillance Requirement Frequency
| ML21350A186 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 09/22/2021 |
| From: | Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML21350A185 | List: |
| References | |
| LER 2021-001-00 | |
| Download: ML21350A186 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3282021001R00 - NRC Website | |
text
Abstract
On July 25, 2021, at 0544 eastern daylight time (EDT), the SQN Unit 2 ice bed temperature monitoring system stopped providing accurate data for the ice bed. The data isused to complete a surveillance instruction (SI) to verify the ice bed temperature does not exceed 27 degrees Fahrenheit (F) as required by Technical Specification (TS) Surveillance Requirement (SR) 3.6.12.1 at a Frequency of every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Due to the failure, Main Control Room operators authorized performance of 2-SI-IXX-061-138.0, Backup Ice Condenser Temperature Monitoring. At 1258, SR 3.6.12.1 was successfully performed; however, the completion time exceeded the specified Frequency plus the 25 percent extension allowed by SR 3.0.2 by 20 minutes (SR 3.6.12.1 had last been completed at 2138 on July 24). Therefore, the Unit 2 ice bed was declared inoperable for 20 minutes from 1238 until 1258.
The cause of the event was the failure of the ice bed temperature monitoring system remote scanner due to an internal power supply failure. Corrective actions for this event includedeveloping procedural guidance for the response required for a nonfunctionalice bed temperature monitoring system and replacement of obsolete instrumentation in the ice bed temperature monitoring system.
I.Plant Operating Conditions Before the Event
At the time of the event, Sequoyah Nuclear Plant (SQN) Unit2 was in Mode1 at 100 percent rated thermal power.
II.Description of Event
A.Event Summary:
On July25, 2021, at 0544 eastern daylight time (EDT), the SQN Unit2 ice bed[EIIS: BC]
temperature monitoring system stopped providing accurate data for the ice beddue to the failure of a remote scanner[EIIS:MPX]in the system. Thedata isused to complete a surveillance instruction(SI)to verify the ice bed temperature doesnot exceed 27degrees Fahrenheit(F) as required by Technical Specification (TS) Surveillance Requirement (SR)3.6.12.1at a Frequency of every 12hours. Due to the failure, Main Control Room (MCR) operators authorized performance of 2-SI-IXX-061-138.0, Backup Ice Condenser Temperature Monitoring, which requires calling in instrumentation and controls technicians to take resistance temperature detector readings on individual temperature elements inside containment.
At 1258, SR3.6.12.1 was successfully performed; however, the completion timeexceeded the specified Frequency plus the 25 percent extensionallowed by SR3.0.2 by 20minutes (SR3.6.12.1 had last beencompleted at 2138 on July24). SR3.0.1 states, in part, [f]ailure to perform a Surveillance within the specified Frequency shall be failure to meet the LCO [limiting condition for operation]. Therefore, the Unit2 ice bed was declared inoperable for 20minutes from 1238 until 1258.
At 1600, an 8-hour non-emergency event notification (EN 55379) 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, Revision3, Section3.2.7 guidance that identifies that the requirementsof 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).
Although the Frequency of SR3.6.12.1 was exceeded andrequired the Unit2 ice bed to be declared inoperable, the Unit2 ice bed remainedbelow 27 degreesF andcapable 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.NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB:NO.3150-0104 EXPIRES:08/31/2023 (08-2020)
YEARSEQUENTIAL REV Sequoyah Nuclear Plant Unit205000-328 NUMBER NO.
2021-001-00 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 07/24/21, 2138SR3.6.12.1 successfully performed.
07/25/21, 0538The Plant Engineering Data System indicated the maximum ice bed temperature was 20.93degrees.
0544MCRoperators recognized the Ice Bed Temperature Monitoring System was nonfunctional.
Operationspersonnel authorized a conditional performance of 1030 2-SI-IXX-061-138.0, Backup Ice Condenser Temperature Monitoring.
SR3.6.12.1 declared not met due to exceeding the specified 1238 Frequency plus the 25percent extensionallowed by SR3.0.2 (15hours total). Unit2 entered TS3.6.12, ConditionA in accordance with SR3.0.1.
1258Conditional performance of SR3.6.12.1 successfully completed. Unit2 ice bed declared operable and TS3.6.12, ConditionA exited.
D.Manufacturer and model number of each component that failed during the event:
SQN-2-XS-061-0138B is an ice bed temperature monitoring system remote scanner manufactured by Yokogawa Corporation of America, model number38826-232-12/NS-SR.
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:
The component failure was identified by operator walkdown.NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB:NO.3150-0104 EXPIRES:08/31/2023 (08-2020)
YEARSEQUENTIAL REV Sequoyah Nuclear Plant Unit205000-328 NUMBER NO.
2021-001-00 G.Failure mode, mechanism, and effect of each failed component:
The failure mode and mechanism of the componentare unknown. The failure of the component caused the ice bed temperature monitoring system to become nonfunctional.
H.Operator actions:
Operationspersonnel authorized a conditional performance of 2-SI-IXX-061-138.0, Backup Ice Condenser Temperature Monitoring.
I.Automatically and manually initiated safety system responses:
There were no automatic or manually initiated safety system responses associated with this event.
III.Cause of the Event
A.Cause of each component or system failure or personnel error:
The cause of the event was the failure of the ice bed temperature monitoring system remote scanner due to an internal power supply failure.
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.
The Unit2 ice bed remained below 27 degreesF and 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.NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB:NO.3150-0104 EXPIRES:08/31/2023 (08-2020)
YEARSEQUENTIAL REV Sequoyah Nuclear Plant Unit205000-328 NUMBER NO.
2021-001-00 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 20minutes.
VI.Corrective Actions
The event was entered into the Tennessee Valley Authority Corrective Action Program (CAP) under condition reports (CRs)1709839 and 1709881.
A.Immediate Corrective Actions:
Operations personnel authorized the conditional performance2-SI-IXX-061-138.0, Backup Ice Condenser Temperature Monitoring, so the ice bed temperatures could be obtained.
B.Corrective Actions to Prevent Recurrence or to reduce probability of similar events occurring in the future:
Corrective actions for this event include developing procedural guidance for the response required for a nonfunctional ice bed temperature monitoring system and replacement of obsolete instrumentation in the ice bed temperature monitoring system.
VII.Previous Similar Events at the Same Site:
LER2-2020-001-00 was submitted for an inoperable Unit2 ice bed. The cause of the event was the failure of a gasket on vendor supplied equipment at the flanged connection of the Auxiliary Glycol Cooling System skid. The corrective action was to repair the skid.
LER1-2021-002-00 was submitted for an inoperable Unit1 ice bed (although reported in 2021, the event occurred in 2018). The cause of the event was the failure of the Unit1 ice bed temperature monitoring system temperature recorder. The corrective action was to replace the ice bed temperature monitoring system temperature recorder.NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB:NO.3150-0104 EXPIRES:08/31/2023 (08-2020)
YEARSEQUENTIAL REV Sequoyah Nuclear Plant Unit205000-328 NUMBER NO.
2021-001-00 VIII.Additional Information
There is no additional information.
IX.Commitments:
There are no commitments.